SE Minnesota Karst

iii

MINNESOTA GEOLOGICAL SURVEYV.W. Chandler, Interim Director

HYDROGEOLOGY OF THE PALEOZOIC BEDROCK

IN SOUTHEASTERN MINNESOTA

Anthony C. Runkel Robert G. TippingMinnesota Geological Survey Minnesota Geological Survey

E. Calvin Alexander, Jr. Jeffrey A. GreenDepartment of Geology and Geophysics, Minnesota Department of Natural Resources,

University of Minnesota Rochester

John H. Mossler Scott C. AlexanderMinnesota Geological Survey Department of Geology and Geophysics,

University of Minnesota

Report of Investigations 61ISSN 0076-9177

Saint Paul — 2003

Map plates 1 and 2 are provided as separate pdf files in the same folder on the FTP site. They are labeled plate1.pdf and plate2.pdf respectively.

iv

HYDROGEOLOGY OF THE PALEOZOIC BEDROCKIN SOUTHEASTERN MINNESOTA

v

Minnesota Geological Survey2642 University Avenue WestSaint Paul, Minnesota 55114-1057

Telephone: 612-627-4780Fax: 612-627-4778E-mail address: [email protected] site: http://www.geo.umn.edu/mgs

©2003 by the Regents of the University of Minnesota

All rights reserved.

ISSN 0076-9177

The University of Minnesota is committed to the policy that all persons shall have equal access to itsprograms, facilities, and employment without regard to race, color, creed, religion, national origin,sex, age, marital status, disability, public assistance status, veteran status, or sexual orientation.

Recommended citationRunkel, A.C., Tipping, R.G., Alexander, E.C., Jr., Green, J.A., Mossler, J.H., and Alexander, S.C., 2003,Hydrogeology of the Paleozoic bedrock in southeastern Minnesota: Minnesota Geological SurveyReport of Investigations 61, 105 p., 2 pls.

This publication is accessible from the home page of the Minnesota Geological Survey(http://www.geo.umn.edu/mgs) as a PDF file readable with Acrobat Reader 4.0.

Date of release: February, 2003

vi

CONTENTS page

ABSTRACT ............................................................................................................................................ 1INTRODUCTION ................................................................................................................................ 3STRATIGRAPHY AND STUDY AREA ........................................................................................... 3DATA AND METHODS ..................................................................................................................... 3

Hydrostratigraphic analyses .................................................................................................. 3Hydraulic analyses .................................................................................................................. 10

OVERVIEW OF HYDROSTRATIGRAPHIC, HYDRAULIC, AND HYDROGEOLOGIC ATTRIBUTES ............................................................................................. 14

Hydrostratigraphy ................................................................................................................... 14Matrix porosity and permeability ............................................................................ 14Secondary porosity: fractures and dissolution features ........................................ 16

Hydraulic character ................................................................................................................. 22Hydrogeologic framework ..................................................................................................... 26

HYDROGEOLOGIC ATTRIBUTES OF INDIVIDUAL LITHOSTRATIGRAPHIC UNITS ................................................................................................ 28MT. SIMON SANDSTONE ................................................................................................................ 28

Hydrostratigraphic attributes ................................................................................................ 28Matrix porosity ........................................................................................................... 28Secondary porosity ..................................................................................................... 28

Hydraulic attributes ................................................................................................................ 29Hydrogeologic synthesis ........................................................................................................ 37

EAU CLAIRE FORMATION .............................................................................................................. 38Hydrostratigraphic attributes ................................................................................................ 38

Matrix porosity ........................................................................................................... 38Secondary porosity ..................................................................................................... 38


IRONTON AND GALESVILLE SANDSTONES .......................................................................... 41Hydrostratigraphic attributes ................................................................................................ 41



FRANCONIA FORMATION ............................................................................................................. 45Hydrostratigraphic attributes ................................................................................................ 45



vii

ST. LAWRENCE FORMATION ......................................................................................................... 55Hydrostratigraphic attributes ................................................................................................ 55



JORDAN SANDSTONE ..................................................................................................................... 59Hydrostratigraphic attributes ................................................................................................ 59



PRAIRIE DU CHIEN GROUP ........................................................................................................... 65Hydrostratigraphic attributes ................................................................................................ 65



ST. PETER SANDSTONE ................................................................................................................... 77Hydrostratigraphic attributes ................................................................................................ 77



GLENWOOD FORMATION .............................................................................................................. 81Hydrostratigraphic attributes ................................................................................................ 81



PLATTEVILLE FORMATION ............................................................................................................ 81Hydrostratigraphic attributes ................................................................................................ 81



DECORAH SHALE .............................................................................................................................. 84Hydrostratigraphic attributes ................................................................................................ 84



viii

GALENA THROUGH CEDAR VALLEY GROUPS ...................................................................... 87Hydrostratigraphic attributes ................................................................................................ 87



DISCUSSION: CLASSIFICATION OF AQUIFERS AND CONFINING UNITS ................... 95SUMMARY ............................................................................................................................................ 96RECOMMENDATIONS ..................................................................................................................... 97ACKNOWLEDGMENTS .................................................................................................................... 98REFERENCES ........................................................................................................................................ 98

Plates 1 and 2 are located in the back pocket of this report.

ix

NOTE ON MEASUREMENTS USED INTHIS REPORT

Although the metric system is preferred inscientific writing, certain measurements are stillroutinely made in English customary units; forexample, distances on land are measured in miles anddepths in drill holes are measured in feet. Preferencewas given in this report to retaining the units in whichmeasurements were made. To assist readers,conversion factors for some of the common units ofmeasure are provided below.

English units to metric units:

To convert from to multiply by

inch millimeter 25.40inch centimeter 2.450foot meter 0.3048mile kilometer 1.6093

Metric units to English units:

To convert from to multiply by

millimeter inch 0.03937centimeter inch 0.3937meter foot 3.2808kilometer mile 0.6214

1

HYDROGEOLOGY OF THE PALEOZOIC BEDROCK INSOUTHEASTERN MINNESOTA

Anthony C. Runkel, Robert G. Tipping, E. Calvin Alexander, Jr., Jeffrey A. Green,John H. Mossler, and Scott C. Alexander

ABSTRACT

The Paleozoic bedrock of southeastern Minnesota contains some of the most heavily usedaquifers in Minnesota. In this report we characterize the hydrogeologic attributes of thesestrata by compiling and interpreting a large volume of hydrostratigraphic and hydraulic data.The result is a hydrogeologic framework for southeastern Minnesota that can be used to formulatemore effective ground-water management strategies, and in particular it improves our abilityto predict aquifer productivity and contaminant transport paths.

This report describes the hydrostratigraphic heterogeneity within individual Paleozoiclithostratigraphic units in detail for the first time. Our hydrostratigraphic analysis is basedchiefly on plug tests of rock samples, outcrop and core observations of secondary pores, anda number of borehole geophysical techniques. Collectively, this information allows us to define"hydrostratigraphic units"—bodies of rock defined on the basis of their characteristic porosityand permeability—without regard for traditional lithostratigraphic boundaries (Seaber, 1988).Our hydrostratigraphic characterization provides a depiction of the spatial distribution of matrixand secondary porosity in a spectrum of geologic settings across southeastern Minnesota. Ofparticular importance is our effort to fully integrate the distribution and abundance of fracturesand dissolution cavities into the hydrostratigraphic characterization.

The Paleozoic bedrock of southeastern Minnesota can be divided into three principal matrixhydrostratigraphic components: coarse clastic rock of high porosity and permeability; fine clasticrock of low porosity and permeability; and carbonate rock, also of low porosity and permeability.All three of these matrix components contain secondary pores such as systematic fractures,dissolution features, and nonsystematic fractures, but they are most abundant in "shallow" bedrockconditions—areas where Paleozoic strata are within about 200 feet of the bedrock surface.In deeper bedrock conditions, secondary pores such as systematic and bedding-plane fracturesare known to occur, but their distribution and abundance is poorly understood. They appearto be concentrated along a few discrete stratigraphic intervals, separated from one another bystrata with few secondary pores.

Hydraulic analyses of Paleozoic strata provide information on the manner in which groundwater travels through matrix and secondary pores, and is evaluated in this report based chieflyon interpretation of pump tests, dye-trace studies, borehole flowmeter logs, water chemistry,and potentiometric data within the context of our hydrostratigraphic framework. The ground-water system appears to be relatively simple and predictable in conditions of deep burial byyounger bedrock. Under these conditions, coarse clastic strata are of relatively high hydraulicconductivity, typically ranging from a few feet per day to a few tens of feet per day, presumablyreflecting flow through large, well-connected intergranular pore spaces. In contrast, the matrixconductivity of the fine clastic and carbonate rock components is low enough in a verticaldirection (10-7 to 10-3 foot per day) that intervals dominated by these components can providehydraulic confinement. Intervals of carbonate rock containing abundant dissolution featureshave hydraulic conductivity values commonly as high as hundreds of feet per day, and in locallydeep bedrock settings, have flow speeds so rapid that they are measured in miles per day alongdiscrete intervals where well-developed conduit systems are present.

The enhanced development of secondary pores in shallow bedrock conditions correspondsto a measurable increase in hydraulic conductivity for the Paleozoic bedrock of southeasternMinnesota. Individual layers composed of coarse clastic, fine clastic, or carbonate componentsin relatively shallow bedrock conditions are very different hydrogeologically from the samelayers in relatively deep bedrock conditions because secondary porosity is vastly different.

2

In shallow settings they have higher bulk conductivity, greater range in conductivity, and arelikely to transmit the greatest volumes of ground water through conduit networks.

Our new hydrogeologic framework for southeastern Minnesota is based on hydraulic datainterpreted within the context of the hydrostratigraphic attributes. It differs considerably frompreviously published frameworks in its classification of regionally extensive aquifers and confiningunits, and because it places greater emphasis on the importance of flow through secondarypores. Eleven regional aquifers separated by ten confining units are recognized in the bedrockof southeastern Minnesota. The "major" confining units are regionally extensive, relativelythick intervals of fine clastic and carbonate rock that have been demonstrated to be of sufficientlylow bulk vertical conductivity to provide confinement under particular conditions of hydraulicstress, and where they are not breached by vertical fractures. The aquifers we define are thebodies of rock dominated by coarse clastic strata or relatively thick intervals of carbonate rockwith abundant secondary pores that are known to yield moderate to large volumes of water indeep bedrock settings. The coarse clastic aquifers typically have a bulk horizontal conductivitybetween 5 and 60 feet per day in deep bedrock conditions. The carbonate rock aquifers aremuch more variable in hydraulic conductivity, and typically consist internally of relativelynarrow intervals of high to very high conductivity (tens to thousands of feet per day) separatedby thick intervals of tight carbonate rock that is orders of magnitude lower in conductivity.

Our hydrogeologic framework also delineates three major "karst systems," based largelyon the work of Alexander and Lively (1995), Alexander and others (1996), and Green andothers (1997). A karst system is an integrated mass-transfer system in soluble rocks with apermeability structure dominated by conduits dissolved from the rock and organized to facilitatethe circulation of fluid (Klimchouk and Ford, 2000). Southeastern Minnesota karst systemsare composed of carbonate-dominated strata where they lie in shallow bedrock conditions.Each karst system is characterized by relatively abundant secondary pores that include largecavities and dissolution-enlarged systematic and nonsystematic fractures, and rapid, directconnections between surface and ground water. The karst aquifers are of particular importanceto ground-water management because the ground-water movement through conduits can berapid and difficult to predict.

Our synthesis of the hydrogeologic attributes of Paleozoic bedrock in southeastern Minnesotahighlights the need for a better understanding of ground-water flow through secondary pores.Most models of ground-water flow in southeastern Minnesota do not adequately account forthe importance of flow through secondary pores in both the aquifers and confining units. Inshallow bedrock conditions, the ground-water system may be dominated by relatively rapidmovement of water through interconnected networks of secondary pores. The ability of confiningunits to protect underlying aquifers in such settings has not been carefully evaluated. Furthermore,flow paths and travel times in such conditions are less predictable than commonly depictedin models formulated under the assumption that intergranular flow is dominant. Limitedhydrogeologic data for deep bedrock conditions are also not entirely compatible with simple,intergranular flow interpretation. Regional-scale connectivity of secondary pores in deep bedrocksettings may provide an enhanced large-scale conductivity to the aquifers and confining bedsin southeastern Minnesota that has not been measured by the standard hydraulic tests performedthus far. Researchers are encouraged to analyze both new and existing data in the context ofour new hydrogeologic framework to further its development by addressing these and otherproblems.

INTRODUCTION

The Paleozoic bedrock of southeastern Minnesota (Figs. 1, 2) contains some of the mostheavily used aquifers in Minnesota. Over one-half of the wells in this part of the state drawwater from Paleozoic bedrock, and most municipalities rely entirely on these strata for theirpotable water supply (County Well Index database maintained by the Minnesota GeologicalSurvey and Minnesota Department of Health). Despite their importance as a source of groundwater, the hydrogeologic attributes of these strata have not been comprehensively characterizedin a scientifically consistent manner that considers substantial variations in porosity and

3

permeability. In this report we provide such acharacterization based on the compilation of the resultsof a number of studies conducted largely over the pasttwenty years. The result is a hydrogeologic frameworkfor southeastern Minnesota that is important toenvironmental managers and scientific investigationsbecause it increases the accuracy and usefulness ofground-water protection plans, and improves our abilityto predict aquifer productivity and contaminanttransport.

In this report, hydrostratigraphic heterogeneitywithin individual Paleozoic lithostratigraphic units isdescribed in detail for the first time. We define"hydrostratigraphic units"—bodies of rock defined onthe basis of their characteristic porosity andpermeability—without regard for traditionallithostratigraphic boundaries (Seaber, 1988). Ourhydrostratigraphic characterization provides a depictionof the spatial distribution of matrix and secondaryporosity in a spectrum of geologic settings acrosssoutheastern Minnesota. Of particular importance isour effort to fully integrate the distribution andabundance of fractures and dissolution cavities into thehydrostratigraphic characterization. Our newhydrogeologic framework for southeastern Minnesota(Plates 1, 2; back pocket) is based on hydraulic datasuch as potentiometric levels, water chemistry, andpump tests, interpreted within the context of thehydrostratigraphic attributes. It differs considerablyfrom previous frameworks in its classification ofregionally extensive aquifers and confining units, andin the relatively great importance of flow throughsecondary pores.

STRATIGRAPHY AND STUDY AREA

This report synthesizes the results of a large numberof studies that collectively provide a depiction ofhydrogeologic attributes of the entire Paleozoicstratigraphic section in a variety of geologic settingsacross southeastern Minnesota (Figs. 1, 2). Thethickness and distribution of individual Paleozoiclithostratigraphic units in southeastern Minnesota (Fig.2; Mossler, 1987, 1998) are shown on bedrock geologicmaps constructed by the Minnesota Geological Surveyat scales ranging from 1:24,000 to 1:250,000 (Sloanand Austin, 1966; Olsen, 1982, 1988a; Mossler andBook, 1984; Olsen and Bloomgren, 1989; Mossler,1990, 1995a, b, 1998, 2001; Mossler and Bloomgren,1990, 1992; Runkel, 1996a, b, 1998; Mossler andTipping, 2000). In addition, lithostratigraphic units aredelineated for individual water-well sites in the CountyWell Index database available at the MinnesotaGeological Survey.

DATA AND METHODS

The investigative methods and data synthesized inthis report are grouped into one of two major categories:hydrostratigraphic analyses and hydraulic analyses.Hydrostratigraphic analyses provide information aboutthe distribution of porosity and permeability, chieflythrough plug tests of rock samples, outcrop and coreobservations of secondary pores, and a number ofborehole geophysical techniques. Hydraulic analysesof the Paleozoic strata provide information on themanner in which ground water travels through pores,and is evaluated in this report based chiefly oninterpretation of pump tests, dye-trace studies, boreholeflowmeter logs, water chemistry, and potentiometric datawithin the context of our hydrostratigraphic framework.These methods are described in greater detail below.

Hydrostratigraphic analysesCore analysis—Paleozoic bedrock cores provide

information on the character and distribution ofhydrostratigraphic units at varying depths of burialbeneath the bedrock surface. Porosity andpermeability in these cores are described in fourmanners (Fig. 3, for example):

1. Plug porosity is the measurement of pore spacethat can be filled with air or water in a rock.Porosity is measured in a laboratory using a smallsample of core, typically a one-inch diametercylinder called a "plug." The porosity value isthe percentage of the plug volume that is porespace. Values of porosity typically range from5 to 30 percent.

2. Plug permeability is a measurement of the abilityof a rock to transmit fluid. It is measured in alaboratory using a small sample of core, suchas a plug sample. Permeability values providequantitative calculation of the ability to transmitwater through intergranular pore spaces. Verticalpermeability measures the ability to transmitfluid in a direction perpendicular to bedding,whereas horizontal permeability reflects theability to transmit fluid in a direction parallelto bedding. Reservoir geologists consider valuesless than 5 millidarcies (md) to be very low,representative of "tight" strata. Values greaterthan 100 md are considered relatively high(Levorsen, 1967).

The bulk of our plug-scale porosity andpermeability data were synthesized fromunpublished reports by the Minnesota GasCompany (Minnegasco), which in the 1970sconducted a subsurface study to assess thefeasibility of underground natural gas storage in

4

Comprehensive investigations

Cores

Borehole geophysical studies, including flowmeter logs

Cedar Valley and Wapsipinicon Groups

Lithograph City Formation, Coralville Formation, andHinkle and Eagle Center Members of the Little Cedar Formation

Chickasaw Member of Little Cedar Formation

Bassett Member of Little Cedar Formation and Pinicon Ridge and Spillville Formations

Maquoketa and Dubuque Formations and Galena Group

Decorah Shale, St. Peter Sandstone, and Platteville and Glenwood Formations

Decorah Shale, mapped where possible

Prairie du Chien Group

Cambrian—Mt. Simon, Ironton–Galesville, and Jordan Sandstones, and St. Lawrence, Franconia, and Eau Claire Formations

ANOKA

ISANTISHERBURNE

WRIGHT

HENNEPIN

RA

MS

EY

WA

SH

ING

TON

DAKOTA

CHISAGO

CARVER

SCOTT

MCLEOD

SIBLEY

NICOLLET LESEUR RICE GOODHUE

WABASHA

BLUE EARTH WASECA STEELE DODGE OLMSTED WINONA

FREEBORNFARIBAULT

MOWER

FILLMORE HOUSTONMARTIN

MEEKER

STEARNS

BENTON

MIL

LE L

AC

S

KANABEC PINE

WAT

ON

WA

NB

RO

WN

Northfield

Faribault

H-1

Waseca–Waterville

BricelynAustin

Spring Valley

Winona

Redwing

Hastings

Prior Lake

Savage

Minneapolis &St. Louis Park

ATES Lakeland

Cottage Grove

Washington County Site (unique number 227031)

New Brighton

Anoka County Site (unique number 165573)

Oronoco

Rochester

43°30'

44°

45°

94°

93°

92°

FLOYD, IOWA

MITCHELL, IOWA

Enlargedarea

A

A'B B'

Bedrock of Precambrian and Cretaceous age

44°30'

45°30'

IOWA

ILLINOIS

WISCONSIN

MINNESOTA

Mississ

ippi R iv er

Min

nes

ota

Riv

er

Mississippi River

St. CroixR

iver

5

Paleozoic bedrock of southeastern Minnesota.The raw data collected as part of that work arestored at the Minnesota Geological Survey andcited as "Minnegasco Underground Gas StorageProject," or MUGSP (1980).

3. Visual porosity logs of core establish thestratigraphic position and relative degree ofdevelopment of cavities and open fractures,which can be the principal ground-water conduitsin bedrock. Such logs account for the porespaces that are larger than those measured inmost plug samples (plug samples are typicallycollected from intervals without visible fracturesand cavities). Values of nearly 100 percentcorrespond to well developed bedding-planefractures/dissolution features in core. Lesservalues of porosity represent the percent of corethat consists of open cavities based on a visualestimate (Fig. 3, for example). Althoughpermeability is a feature that cannot be visuallyestimated, ground-water conduits with relativelyhigh hydraulic conductivity should be expectedto have a high visual porosity if they areintersected by core.

Visual porosity in cores was estimated insuch a manner that the logs probably under-represent the abundance of secondary pores,especially large cavities. Intervals of core lossand breaks between cores, which may correspondto such features, were recorded as secondary

pores only if the core ends showed clear evidenceof dissolution or mineralization, indicating thepresence of a cavity. Furthermore, the abundanceof vertical, systematic fractures in the deepsubsurface (described later in this report), whichmay be hydraulically important features at somescale, is probably underestimated becauseindividual vertical cores and boreholes have asmall probability of intersecting such features.

4. Vertical fracture abundance is a visual estimateof the number of subvertical fractures per footof core. These are mostly "mesoscopic" fractures(Price and Cosgrove, 1990)—irregular, sinuousfractures that typically cannot be traced morethan a few inches. The apertures are largelyhealed or open only a fraction of a millimeter,and their hydraulic significance is not known.However, such fractures can be preferentialpathways for relatively slow-moving groundwater in strata that are otherwise of negligiblepermeability (Watts, 1983; Lorenz and Finley,1991). Additionally, these narrow fractures maybe preferentially opened compared to non-fractured rock when subjected to stress-releaseand weathering in near-surface conditions.Therefore the stratigraphic position ofmesoscopic fractures in deep core maycorrespond to intervals where secondary poresare preferentially developed in near-surfacebedrock conditions.

Figure 1. Map of southeastern Minnesota showing the distribution of Paleozoic lithostratigraphic units wherethey occur as the uppermost bedrock and locations of cores, borehole flowmeter studies, and sites of comprehensivehydrogeologic studies referenced in this report. The comprehensive hydrogeologic studies utilize a number oftechniques and include formally published aquifer characterization of the Eau Claire through lower St. LawrenceFormations as part of the Aquifer Thermal Energy Storage Project (ATES Project; Miller and Delin, 1993),and of the Galena through Cedar Valley Groups in Floyd and Mitchell Counties, northern Iowa (Libra andHallberg, 1985; Witzke and Bunker, 1985). Informally published site remediation investigations include workfocused on the Ironton–Galesville Sandstone and Franconia Formation near Lakeland, Minnesota (Braun Intertec,1992; Delta Environmental Consultants, Inc., 1992); the Jordan Sandstone and Prairie du Chien Group at anabandoned landfill near Oronoco (Alexander, 1990; Donahue and Associates, Inc., 1991; RMT, Inc., 1992),and in the Arden Hills–New Brighton area (Camp, Dresser and McKee, 1991); the Platteville Formation in St.Louis Park (for example ENSR International, 1991) and northeast Minneapolis (for example Barr Engineering,1991); and the Galena Group and Dubuque and Maquoketa Formations at the Spring Valley Amoco terminal(Delta Environmental Consultants, Inc., 1995). The Ironton–Galesville Sandstone and Franconia Formationwere also studied at a proposed expansion of an ash disposal site near Red Wing (Wenck and Associates, Inc.,1997). The heavy lines outline the seven-county Twin Cities Metropolitan area, and the locations of cross-sections on Plates 1 and 2 also appear.

6

Fe Fe

Ph PhPh

Fe Fe

ProsserLimestone

Cummings-ville

Formation

DecorahShale

PlattevilleFormationGlenwoodFormation

St. PeterSandstone

Ogsv

Odcr

Ostp

70-8

045

-50

70-7

540

-50

Upp

er O

rdov

icia

n (4

58–4

44 m

.y.)

Gal

ena

Era

Ser

ies Group,

Formation,Member La

bel

Lithology

Natural gamma log

API-G units

0 100

Increasing count

Thi

ckne

ss (

in fe

et)

DcuuLithograph

CityFormation

CoralvilleFormation

Ced

ar V

alle

y G

roup

Mid

dle

Dev

onia

n (3

87–3

74 m

.y.)

Wap

sipi

nico

n G

roup

SpillvilleFormation

PiniconRidge

Formation

Littl

e C

edar

For

mat

ion

BassettMember

ChickasawMember

Hinkle &Eagle

Center Mbrs

Dcum

Dclc

Dclp

Dspl

OmaqMaquoketaFormation

OdubDubuqueFormation

StewartvilleFormation

Gro

up

Up

to 5

057

-61

25-3

515

-43

40-7

020

-47

21-8

436

-85

23-4

075

-85

20-3

05-

6

Ogpr

Ogcm

Opvl

Ogwd

PA

LEO

ZO

IC

Mt. SimonSandstone

200

or le

ss

JordanSandstone

St. LawrenceFormation

Fra

ncon

ia F

orm

atio

n

Ironton and Galesville

Sandstones

Eau ClaireFormation

Upp

er C

ambr

ian

(523

–505

m.y

.)

65-7

011

0-12

015

5-16

040

-45

200

Cstl

Reducedscale50%

G

G

G

G

G

G G

G

G

G

G

G

G

G

GG

G

GG

GG G

Pra

irie

du C

hien

Gro

up

Low

er O

rdov

icia

n (5

05–4

78 m

.y.)

PA

LEO

ZO

IC

Opsh

320-

340

G

G

G

G

ShakopeeFormation

OneotaDolomite

Coon ValleyMember

BirkmoseMember

Renoand

TomahMembers

Opod

Cfrn

Cigl

Cecr

Cmts

Continued above right

Figure 2. Standard bedrock stratigraphic columnshowing Paleozoic lithostratigraphic units ofsoutheastern Minnesota and typical gamma log.Modified from Mossler (1987, 1998).Hydrostratigraphic components are depicted in Plates1 and 2. Figure explanation is on the following page.

7

Field observations—A number of outcrop-basedstratigraphic and sedimentologic investigationsconducted over the past 50 years delineate faciesthat are now known to differ considerably from oneanother in intergranular porosity and permeability(for example Berg, 1954; Nelson, 1956; Setterholmand others, 1991; Runkel, 1996a, b, 1999, 2000;Runkel and Tipping, 1998; Runkel and others,1999). Those investigations, supplemented withfield work conducted as part of recent MinnesotaGeological Survey mapping in southeasternMinnesota (for example Mossler and Book, 1984;Mossler, 1990, 1995a, b, 1998, 2001; Mossler andBloomgren, 1990, 1992; Runkel, 1996a, b, 1998;Mossler and Tipping, 2000), allow us to delineateindividual hydrostratigraphic units within mappedlithostratigraphic units across the outcrop belt ofPaleozoic bedrock where cores are generally scarce.Additionally, outcrops provide an opportunity toexamine secondary pores in Paleozoic bedrock, andtheir interaction with surface waters. Theabundance, size, and stratigraphic distribution offractures and dissolution features were describedfor much of the Paleozoic section in representativelarge outcrops, quarries, and road cuts along theMississippi River and its tributaries. An intervalof strata in which secondary porosity ispreferentially developed in outcrop (Fig. 4) can bean important ground-water conduit in saturatedsubsurface conditions (for example Gianniny andothers, 1996). The distribution of springs andsinkholes (for example Alexander and others, 1996;Witthuhn and Alexander, 1996) also providesinsight into stratigraphic control of hydraulically

important fractures and dissolution cavities in near-surface bedrock settings.

Borehole logs—Natural gamma logs have been usedextensively by Minnesota Geological Surveyscientists to distinguish hydrostratigraphiccomponents that differ from one another inintergranular porosity and permeability (forexample Setterholm and others, 1991; Runkel,1996b). A slimline probe measures gamma raysnaturally emitted by rocks as it is slowly raisedin a borehole. In the Paleozoic strata ofsoutheastern Minnesota, fine-grained siliciclasticrocks with low intergranular permeability containpotassium in sufficient abundance to emit relativelyhigh levels of gamma rays, and therefore causestrong positive deflection on gamma logs (Fig. 2).Coarse-grained siliciclastic rocks with higherpermeability have low potassium content andtherefore correspond to low readings on the gammalogs. Carbonate strata most commonly havereadings between those of fine and coarsesiliciclastic rocks.

Borehole video, borehole televiewer (BHTV),and caliper logs provide information similar to thatof rock cores in that they are used to documentthe size, shape, and stratigraphic position offractures and dissolution features. Such logs areavailable at several state agencies, including theMinnesota Department of Health, MinnesotaPollution Control Agency, and the MinnesotaGeological Survey.

Borehole cuttings—High-quality sets of cuttings wereused in conjunction with outcrop study todemonstrate a correspondence between gamma log

Fe

G

Cavities (commonly filled with coarse calcite)

Chert

K-bentonite bed (altered volcanic ash bed

Oolites

Glauconite

Iron stain

Phosphate pellets

Algal mats

Algal domes; stromatolites

Fossiliferous; fossils (symbols notused in limestone or dolostoneunits)

Siltstone

Shale

Limestone

Dolostone

Sandstone

Sandy

Very fine- to fine-grained

Medium- to coarse-grained

Shaly

Fine- to medium-grained

Ph

Worm bored

Pebbles (gravel inunconsolidated units)

Flat-pebble conglomerate

Cross-bedded (festoon)

Cross-bedded (planar totangential)

Ripple cross-laminations

Dolomitic

Calcareous

Contact marks a major erosionalsurface

Facies change

EXPLANATION

8

Figure 3. Example of a presentation of plug porosity andpermeability values, and logs of visible porosity for an8-foot core of fine clastic and carbonate rock in the St.Lawrence Formation collected from the Waseca–Watervillearea. Lines are drawn from dissolution features in thecore to corresponding tick marks on the visual porositylog.

Mesoscopicfracture

0 100% 0 5 0 40% 100 md

Plu

g ve

rtic

alpe

rmea

bilit

y

Vis

ual

poro

sity

Frac

ture

spe

r fo

ot

Plu

gpo

rosi

ty

1 foot

10-6

Pos

ition

of

sam

pled

plu

g

9

Fracture

Figure 4. Large, interconnected dissolution cavities parallel to bedding in the carbonate strata along the ShakopeeFormation–Oneota Dolomite contact at a quarry near Red Wing in Goodhue County. Note the vertical fracturewith a large aperture in the quarry wall on the left side of the photo. Water commonly travels rapidly downwardthrough such fractures and subsequently travels laterally along bedding-plane parallel conduits such as theinterconnected dissolution cavities shown here (marked by arrows). The short vertical line below the cavitiesis approximately 5 feet tall.

signatures and hydrostratigraphic units (for exampleSetterholm and others, 1991). This correspondencewas successfully used to delineate subsurfacehydro-stratigraphic units in the Rochester area(Runkel, 1996b) and in Houston and GoodhueCounties (Runkel, 1996a, 1998). Cuttings alonetypically cannot be used to determine the precisethickness of hydrostratigraphic units because thesample stream from the drilling process has inherentinaccuracies related to poor collection methods andrecirculation problems.

Hydraulic analysesPump and slug tests—A large database of hydraulic

conductivity values is based on a compilation ofpump and slug tests conducted on Paleozoicbedrock in southeastern Minnesota and adjacentstates. They can be grouped into three principalcategories based on the quality of the test andamount of associated supplementary informationon borehole construction, testing procedures, andgeologic setting.

1. Discrete interval tests—Hydraulic data availablefrom comprehensive hydrogeologic reports thatdescribe controlled pump tests as well as detailed

stratigraphic and well-construction informationare considered to be of the highest quality usedin this report. They can be used to calculate thehydraulic conductivity of individualhydrostratigraphic units with confidence.Frequently cited examples of these kinds ofstudies include discrete-interval packer testingof Cambrian siliciclastic strata by Nicholas andothers (1987) and Miller and Delin (1993), andof Ordovician and Devonian age, carbonatedominated strata by Libra and Hallberg (1985),Nicholas and others (1987), Graese and others(1988), Donahue and Associates, Inc. (1991), andDelta Environmental Consultants, Inc. (1995).

2. Specific capacity tests—Specific capacity dataobtained from water well construction reportsin the County Well Index database were usedto calculate hydraulic conductivity following anapproach described by Bradbury and Rothschild(1985). Specific capacity values are correctedfor the effects of partial penetration, well loss,and borehole diameter. The hydraulicconductivity values calculated in this manner are

10

believed to be a more accurate measure of aquiferperformance than specific capacity values alone.

Hydraulic conductivity has been calculatedfor 8,626 wells that draw water from the Paleozoicstrata of southeastern Minnesota. Runkel (2000)demonstrated that a large database of such valuescan be used to recognize relative differences inaquifer performance that are consistent with theresults of higher quality, controlled pump testsand therefore can provide information aboutgeologic controls on aquifer performance. Exceptwhere otherwise noted, our database excludeswells constructed to draw water from more thanone of the eleven aquifers defined in this report.These data are summarized in this report chieflyas scatter plots, and as box plots that show medianvalues and statistically acceptable ranges.

Conductivity values calculated from specificcapacity tests may be less indicative of hydraulicperformance than high quality, discrete-intervalpump and slug test data because pumping ratesand drawdown measurements are typicallycollected in a less rigorous fashion, and becausethe tests are usually of short duration.Additionally, the database from which thesehydraulic conductivity values have beencalculated consists of tests of water wellsconstructed expressly for the purpose of extractingeconomic quantities of water. The values ofconductivity are therefore chiefly representativeof the most productive intervals of Paleozoic stratain a given geologic setting, and do not includea large sample of values representative of stratawith relatively low conductivity.

3. Standard aquifer tests—A large number ofhydraulic conductivity values for individualhydrostratigraphic units are based on aquifer testsconducted by private consultants and by stafffrom state and federal agencies that include theU.S. Geological Survey, the Division of Watersof the Minnesota Department of NaturalResources, the Minnesota Department of Health,and the Minnesota Pollution Control Agency.These are cited as "standard aquifer tests" in thisreport to distinguish them from specific capacitytests and discrete interval tests accompanied byhigher quality ancillary information describedabove. Standard aquifer pump test results aretypically not accompanied by reports in whichthe raw pump test data nor pumping proceduresare provided, and detailed hydrostratigraphiccontext is not available for the wells in thisdatabase. However, we have used drilling

records and natural gamma logs (where available)to roughly determine the hydrostratigraphiccomponents exposed in the open-hole intervalfor each of the wells in the database.

Our evaluation of the results of these aquifertests indicates that they commonly yield hydraulicconductivity values that are higher than thosecalculated on the basis of discrete interval andspecific capacity tests of the samehydrostratigraphic material. A possibleexplanation is that standard aquifer tests insoutheastern Minnesota have most commonlybeen performed on large diameter industrial andmunicipal wells that are developed to increaseproductivity through methods such as blasting.Large, high capacity wells such as these may bebetter connected to secondary pore networkscompared to narrower diameter, undevelopedboreholes subjected to packer tests, and to smalldiameter domestic wells that compose themajority of our specific capacity database.

Borehole geophysical and video logs—Vertical ground-water flow within a borehole in saturatedstratigraphic intervals can be detected byelectromagnetic and heat pulse flowmeters.Flowmeter logs collected under ambient conditionsare used to recognize the hydraulically dominantintervals of matrix and secondary pores in anindividual borehole, and the confining unit(s) thatseparates them (Fig. 5). Some boreholes are alsoflowmeter logged during stressed conditions createdby pumping from, or injecting water into, aborehole. Flowmeter logs collected under stressedconditions allow the hydraulic properties of discreteintervals to be quantified when compared toambient flowmeter measurements and accompaniedby ancillary information such as the change inpotentiometric level of the borehole. Thesetechniques are explained in greater detail in Pailletand others (2000).

A borehole video camera and a multi-parameterprobe that measures temperature, pH, and chlorideprovide information similar to that of flowmeters,but in a more qualitative and inconsistent fashion.Video logs can be used to identify seeps andcascading water along discrete intervals in openboreholes above the static water level. Waterentering or exiting a borehole along discreteconduits in saturated conditions can be recognizedon video logs by the movement of well sedimentheld in suspension and by shifts in temperature,pH, and chloride content measured by the multi-parameter probe.

11

Dye tracing—Dye-trace investigations have beensuccessful in providing quantitative measures ofground-water flow speeds, and the degree of verticalconnectivity across adjacent hydrostratigraphic unitsin near-surface bedrock conditions where flowalong secondary pores is of particular importance(for example Wheeler, 1993; Alexander and others,1996). Flow speeds are typically expressed asnominal flow rates, in feet per day or miles perday, and are lower limits on the true flow velocities.

Water chemistry—Chemical constituents such astritium, nitrates, and chlorides have commonly beenused to determine flow paths and hydraulicconnection between water-bearing bodies of rock.A large volume of ground-water chemistry data forsoutheastern Minnesota are scattered among severalstate agencies and private consultants, and in anumber of publications. In this report we focuson ground-water chemistry data collected andinterpreted as part of site-specific studies in whichthe geologic setting, well construction, andhydrostratigraphic attributes are well understood.

Potentiometric data—This report incorporatespotentiometric data compiled from the results ofsite-specific studies that include the informationnecessary to interpret the data within ourhydrostratigraphic framework. We also citepublished county and larger-scale potentiometricmaps that provide water level information that canbe used in the context of our hydrostratigraphicframework. We use a difference in static waterlevels (heads) above and below a low permeabilityhydro-stratigraphic unit as one line of evidence thatthe unit provides confinement.

Previous hydrogeologic investigations insoutheastern Minnesota have struggled with thequestion of what constitutes a significant differencein head for the purpose of recognizing discretehydrogeologic units. For example, is a five-footdifference significant at the scale of a site-specificstudy? Is it significant at a county or regional scale?Historically, hydrogeologic units have been definedregionally, using potentiometric data from waterwells, with elevations determined using 7.5-minutetopographic maps. Under these conditions, a five-foot head difference is smaller than the errorassociated with a well elevation. Using thesemethods, individual hydrogeologic units cannot bedistinguished. Unfortunately, regionally definedhydro-geologic units have been applied to site-specific studies, where a five-foot head differenceis important in distinguishing ground-water flow

paths.Head differences across a confining unit occur

under conditions of stress: either natural due toground-water flow patterns, or induced by pumping.Certain conditions of stress can cause aquifersseparated by a confining unit to have similar headseven though they are not hydraulically well-connected. For example, aquifers that are rechargedand discharge near the same elevation may not showlarge head differences between zones of rechargeand discharge. In this way, confining characteristicsof individual hydrostratigraphic units are notrevealed using potentiometric data alone. We inferthat a hydrostratigraphic unit that providesconfinement at an individual site has the ability toprovide confinement elsewhere, because bydefinition it has more or less consistent propertiesof porosity and permeability across its extent.Whether or not its confining properties are breachedby fractures is a question that potentiometric datacan help answer on a site-by-site basis.

OVERVIEW OFHYDROSTRATIGRAPHIC, HYDRAULIC,AND HYDROGEOLOGIC ATTRIBUTES

Hydrostratigraphy

Matrix porosity and permeabilityThe Paleozoic strata of southeastern Minnesota can

be generally divided into three distincthydrostratigraphic components based entirely on matrixcharacteristics (Runkel and Tipping, 1998; Runkel,1999). The components are: coarse clastic, fine clastic,and carbonate rock (Fig. 6; Plates 1, 2). The valuesfor matrix porosity and permeability of these threecomponents where they occur in settings with relativelyminor development of secondary porosity (fractures anddissolution features) have been determined at thesmallest scale through laboratory testing of plug samples(Norvitch and others, 1973; MUGSP, 1980; Setterholmand others, 1991; Walton and others, 1991; Wenck andAssociates, Inc., 1997).

The coarse clastic component is a poorly cemented,moderately to well-sorted, fine- to coarse-grainedsandstone composed of about 98 percent quartz. Plug-sample tests indicate it has a high porosity and verticalpermeability, commonly more than 20 percent and 1,000md, respectively, due to relatively large, well-connectedintergranular pore spaces. Horizontal permeabilitytypically is equal to, or as much as an order ofmagnitude greater than vertical permeability.

The fine clastic component consists of very fine-

12

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Interpretationline

F.

Ambient conditions:explanation of flow

Stressed conditions: location and percent of borehole transmissivity of dominant permeable intervals

during injection

Injected water travels downhole along casing and upper part of open borehole with no loss

No measurable flow in casing, nor in upper part of open hole

Water exits at fractureConsistent upflow past confining unit

Water enters through intergranular pores and travels uphole

No measurable flow No measurable flow

No measurable flow

Partial loss of injected flow: exits through fractures—18% of transmissivity

Consistent downflow of remaining injected water

Loss of remaining injected water: exits through intergranular pores—82% of transmissivity

No measurable flow in casing, nor in upper part of open hole

Water enters abruptly at five thin intervals of secondary pores and travels downhole. Entrances are separated by confining units.

Consistent, relatively strong downflow past confining unit

Water exits gradually through intergranular pores

Consistent weak downflow past confining unitWater exits abruptly

Ambient conditions:explanation of flow

Casing bottom

Casing bottom

Interpretationlines

Figure 5. Flowmeter logs are a depiction of vertical water movement in a borehole: positive values on thelogs correspond to flow up a borehole, negative values correspond to flow down a borehole, and zero representsno measurable flow. Ambient borehole flow in a vertical direction is driven by vertical hydraulic gradient.Trolling flowmeter logs are a continuous record of flow measured by a slimline electromagnetic probe as it israised at 10 feet per minute up the borehole. Stationary logs show a series of flow measurements taken atvarious depths in the borehole with the probe stopped, or "stationary." These two kinds of flowmeter logs areused in conjunction with geophysical logs that measure physical rock properties (such as gamma, caliper, video,and BHTV logs) to interpret flow conditions in the borehole, shown graphically as an "interpretation line" onthe stationary logs.

A. Schematic hydrogeologic setting and corresponding stationary flowmeter log. Flowmeter logging recordsno vertical borehole flow because the open borehole exposes only a single aquifer with no vertical gradient.

Figure 5 explanation continued on page 14

Figure 5 explanation continued from page 13

14

grained sandstone, siltstone, and shale in thin to mediumbeds that are strongly to moderately cemented. Thiscomponent has very low to low relative permeability,several orders of magnitude less than that of the coarseclastic component described above. Plug tests indicatea vertical permeability that typically ranges from 10-6

to 10-2 md. Horizontal permeability is commonly about

two orders of magnitude greater than vertical.

The carbonate rock component consists of veryfine- to fine-grained dolostone and limestone withvariable amounts of silt, sand, and shale as interbedsor admixed in the carbonate matrix. Matrix porosityand vertical permeability values are typically less than15 percent and 10-1 md, respectively. Limited tests ofhorizontal permeability indicate that it is commonlyabout two orders of magnitude greater than verticalpermeability in laminated carbonate rocks. Horizontalpermeability is probably roughly equal to verticalpermeability in plug samples of structureless carbonaterocks such as those common in the Oneota Dolomite.

Secondary porosity: fractures anddissolution features

The hydrostratigraphic character of the threecomponents described above is affected by lateral andvertical variability in the abundance andinterconnectivity of fractures and dissolution features(Plates 1, 2). Calculated values for porosity andpermeability within each of these components can vary

B. Schematic hydrogeologic setting and corresponding stationary flowmeter log. Flowmeter logging recordsno vertical flow in the borehole, even though the hole fully penetrates a confining unit, because the aquifersabove and below the confining unit have similar heads.C. and D. Schematic hydrogeologic settings and corresponding stationary flowmeter logs. Flowmeter logsshow vertical flow that occurs in boreholes that intersect two (intergranular) aquifers with heads that differfrom one another.

E., F., and G. Flowmeter logs collected in southeastern Minnesota and used in this report. Changes in magnitudeof vertical flow along the interpretation line mark permeable intervals through which water enters (inflow) orexits (outflow) the borehole. Abrupt changes in magnitude of vertical flow correspond to relatively thin intervalsof hydraulically active secondary pores, most commonly bedding-plane fractures; gradual changes correspondto intervals where intergranular flow is dominant. The beds that separate these hydraulically active intervalsare of relatively low permeability and can be considered confining units at the scale of the immediate vicinityof the borehole. The confining units that directly separate an entrance from an exit maintain differential headsabove and below them, which drives ambient borehole flow. Confining units that separate successive entrancesor exits along a borehole may or may not separate heads that differ from one another. G. also provides anexample of flowmeter logging under stressed conditions. The borehole was injected with water at a rate of 9gallons per minute. The relative transmissivity of the two permeable intervals that accommodate the injectedwater is quantified to the right of the column following the procedure described by Paillet and others (2000).

substantially depending on the scale of the tested rocksample, and the degree of development of fractures anddissolution features. Permeability is very high wheresuch features are well developed and interconnected,and very low, even on a large scale, where minimallydeveloped (for example Liesch, 1973; Libra andHallberg, 1985; Graese and others, 1988; Gianniny andothers, 1996; Eaton and others, 2000).

Core logging, borehole videos, geophysical logs,dye-trace investigations, and field observations ofexposed bedrock in southeastern Minnesota presentedin this report, and studies of generally similarsedimentary bedrock in other parts of North America(for example Ferguson, 1967; Nichols, 1980; Wyrickand Borchers, 1981; Graese and others, 1988) suggestthat bedrock conditions can be separated into twogeneral categories based on the nature of secondaryporosity: "shallow" bedrock conditions, and "deep"bedrock conditions (Fig. 7; Plates 1, 2). Shallowbedrock conditions differ from deep conditions becausethey have a relatively high density of large, well-connected fractures and dissolution cavities.

Shallow bedrock conditions are characterized byrelatively strong development of three kinds ofsecondary pores (Fig. 8). Systematic fractures are flat-sided openings oriented perpendicular to bedding. Theyare also referred to as "joints," and are typically themost prominent fractures in large outcrops—commonly

15

D.

A. B. C.

Figure 6. Examples of the three principle matrixhydrostratigraphic components in core.A. Coarse clastic component from the Mt. SimonSandstone consisting of medium- to coarse-grained, friablesandstone.

B. Fine clastic component in the Eau Claire Formation.Consists of very fine-grained sandstone and siltstone withthin shale laminations.

C. Fine clastic component in the Franconia Formation.Consists largely of shale (dark beds) with interbedded veryfine-grained sandstone and siltstone.D. Carbonate rock in the Platteville Formation. The coreon the right has thin, irregular interbeds of shale. Coresare from Ramsey County (ATES Project, cores AC-1 andBC-1).

16

evident at distances of hundreds of feet as straight,vertical openings with a more or less consistent spacing.The walls of systematic fractures typically have strikeorientations that fall within one or two tightly clusteredsets (Olsen, 1988b; Ruhl, 1995; Runkel, 1996a).Nonsystematic fractures are more randomly distributedand more variable in their orientation and shape. Theyinclude openings that parallel bedding planes as wellas irregular, curved, or conchoidal fractures that intersectbedding obliquely. Both systematic and nonsystematicfractures are common in all three matrixhydrostratigraphic components where they occur inshallow bedrock conditions. Dissolution features area secondary pore developed through the dissolution ofcarbonate rock. Dissolution can enlarge the aperturesof nonsystematic and systematic fractures, and can alsocreate cavities that have no apparent relationship tofractures. In shallow bedrock conditions, thepermeability of the coarse clastic, fine clastic, andcarbonate rock components may be several orders ofmagnitude higher than that of deep conditions at scalesgreater than that of plugs because of the greaterdevelopment of these three kinds of secondary pores(for example Donahue and Associates, Inc., 1991;Gianniny and others, 1996; Wenck and Associates, Inc.,1997).

Our understanding of secondary porosity in deepbedrock conditions relies mostly on examination ofcores (Fig. 9, for example) and borehole video andcaliper logs collected from southeastern Minnesota(Donahue and Associates, Inc., 1991; Walton and others,1991; Delta Environmental Consultants, Inc., 1995;Runkel, 1999; Runkel and others 1999; this study) andon recent studies of analogous Paleozoic bedrocksettings in Iowa, Wisconsin, Illinois, and Michigan(Witzke and Bunker, 1984; Graese and others, 1988;Hurley and Swager, 1991; Gianniny and others, 1996;Eaton and others, 2000). Collectively, this informationsuggests that deep bedrock conditions differfundamentally from shallow conditions in that secondarypores are diminished in abundance, size, and degreeof interconnectivity, principally because dissolutionfeatures and nonsystematic fractures are less common.Our limited borehole data in southeastern Minnesotaindicate that open nonsystematic fractures andmacroscopic dissolution cavities are apparentlyuncommon to absent in the fine clastic and coarse clasticcomponents. Where present, discrete bedding-planefractures are separated by tens of feet of strata withno evident secondary pores, and some individual coresand boreholes have no recognizable bedding-planefractures across hundreds of feet of strata. Carbonaterock in deep settings varies in its development ofsecondary porosity (Fig. 9). Some carbonate intervals,

such as the lower two-thirds of the Oneota Dolomite,the Platteville Formation, and Galena Group, haverelatively few open fractures and macroscopic cavitiescompared to their character in shallow bedrockconditions (Graese and others, 1988; DeltaEnvironmental Consultants, Inc., 1995; Tipping andRunkel, 2001; this report). In contrast, core andborehole video logs analyzed in this report (and byTipping and Runkel, 2001) demonstrate that othercarbonate intervals in deep bedrock settings, such asmuch of the Shakopee Formation, parts of the St.Lawrence Formation, the uppermost Oneota Dolomite,and a thin, carbonate-rich interval in the lower part ofthe Franconia Formation, have a relatively high densityof dissolution features, including large cavities (greaterthan 4 inches), and dissolution-enlarged, mesoscopicfractures oriented in directions both perpendicular andparallel to bedding.

It is not known whether interconnected networksof open systematic fractures are common or rare in deepbedrock conditions of southeastern Minnesota, in partbecause subsurface information is almost entirelylimited to vertical boreholes that have a smallprobability of intersecting such features. Becausesystematic fractures are probably the result of regional-scale stresses (Price and Cosgrove, 1990), their presencein outcrop indicates that they are likely present inindividual layers of strata even at depths hundreds offeet below the bedrock surface. They are most likelyto occur locally in well-indurated layers such as thosedominated by carbonate rock and cemented siliciclastics(Price and Cosgrove, 1990; Helgeson and Aydin, 1991;Hurley and Swager, 1991; Narr and Suppe, 1991). Theyare theoretically less likely to occur in friable sandstonesand poorly indurated shales, although exceptions arewell-documented (for example Ryder, 1996). Boreholevideo logs of a few wells open to deep bedrockconditions in the Twin Cities Metropolitan area revealthe presence of vertical systematic fractures withapertures of several inches in coarse clastic strata ofthe Jordan Sandstone and the fine clastic strata of theEau Claire Formation (Minnesota Department of Healthborehole video library; for example unique wellnumbers 200519, 206169, and 205821). These waterwells were developed to increase productivity and itis possible that the apertures of these fractures werewidened when the borehole was blasted with dynamiteand bailed. Nevertheless, their presence demonstratesthat Paleozoic strata do contain systematic planes ofweakness in deep bedrock settings. The abundance,dimension, aperture size, and interconnectivity of thesefractures are entirely unknown, but are presumablydiminished compared to systematic fractures in shallowbedrock settings.

17

~100 feet

Coarse clastic component

Fine clastic component

Carbonate component

Non-systematic fractures(some dissolution enlarged)

EXPLANATION

Surficial deposits

Figure 7. Typical development of stress-relief fractures in layered Paleozoic bedrock. Note that nonsystematicstress-relief fractures decrease in abundance at greater distances from the bedrock surface.

A. Diagrammatic sketch based on studies of Paleozoic bedrock in eastern North America (Ferguson, 1967),modified with observations from southeastern Minnesota discussed in this report.

B. Quarry exposing carbonate rock of the Shakopee Formation and Oneota Dolomite near Mankato in BlueEarth County. Nonsystematic fractures are abundant in the upper part of the bedrock exposed in the quarry.Only widely spaced, systematic fractures are evident in the lower part of the quarry. The depth to whichnonsystematic and systematic fractures extend continuously beneath the bedrock surface will vary from placeto place in southeastern Minnesota.

B.

A.

18

C.

B.

A. Figure 8. Characteristic secondary poresin shallow bedrock conditions.

A. Systematic fracture in interbedded fineclastic and carbonate rock component ofthe St. Lawrence Formation at Barn Bluffin Red Wing, Goodhue County. Note thevertical systematic fracture with largeaperture (hammer for scale is circled) andflat surfaces of the outcrop characteristicof systematic fractures.

B. Systematic fractures in the coarseclastic component of the Jordan Sandstonenear Whitewater State Park, northeasternWinona County.C. Carbonate rock of the OneotaDolomite in Stillwater, WashingtonCounty. Nonsystematic fractures occurparallel to bedding and as irregular,subvertical fractures typically confined toindividual beds. Systematic fractures arerelatively straight, and have wideapertures that cut vertically across theentire outcrop. Many of the fractures havesome evidence of enlargement bydissolution. Staff is 5 feet tall.

D. Nonsystematic, stress-relief fracturesin interbedded fine clastic and carbonaterock of the St. Lawrence Formation atBarn Bluff in Red Wing, Goodhue County(hammer for scale).

E. Similar fractures in fine clastic rock(chiefly shale) of the Decorah Shale atLilydale Regional Park in Ramsey County.F. Large dissolution cavities (marked byarrows) developed in carbonate rock inthe upper part of the Oneota Dolomite ineastern Wabasha County. These cavitiestypically are preferentially developedalong discrete beds. The large cavity inthe center of the photograph is about 2feet in height.

G. Small dissolution cavities in acarbonate bed within the upper OneotaDolomite near Mankato, Blue EarthCounty.

19

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21

The greater development of secondary porosity inshallow bedrock conditions compared to deep conditionsis the result of several processes. Uplift, unloading ofyounger bedrock, and weathering in shallow conditionsopens the apertures of systematic planes of weaknessin addition to producing the ubiquitous nonsystematicbedding-plane and curvilinear fractures (Ferguson, 1967;Wyrick and Borchers, 1981; Price and Cosgrove, 1990)characteristic of all bedrock outcrops in southeasternMinnesota. These latter features are commonly referredto as "stress-relief fractures" in reference to theircommon origin during the removal of overlyingmaterial. Vertical and horizontal stresses thataccompany glacial advances and retreats acrossPaleozoic bedrock in southeastern Minnesota cancontribute to the production of these features (forexample Moerner, 1978; Liszkowski, 1993). In addition,dissolution of carbonate rock is typically morepronounced in relatively near-surface settings comparedto conditions of relatively deep burial. The depth towhich these processes collectively producehydrogeologically significant secondary porosity willvary from place to place depending on several factors.For example, a system of interconnected systematicfractures in layered bedrock typically terminatesdownward at or near the uppermost friable sandstone,relatively ductile shale, or along a discontinuity suchas a bedding-plane fracture (Price and Cosgrove, 1990;Helgeson and Aydin, 1991; Narr and Suppe, 1991).Dissolution of carbonate rock typically diminishes withdepth (Goldstrand and Shevenell, 1997; Shevenell andGoldstrand, 1997), especially below the uppermostimpermeable layer of siliciclastic bedrock that can serveas a confining bed.

There is no precise or consistent depth at whichthe boundary between "shallow" and "deep" bedrockconditions occurs. A study of the Prairie du Chien andJordan aquifers by Runkel and others (1999) placed thelower boundary of shallow bedrock conditions at 100feet below the bedrock surface everywhere in theMinneapolis–St. Paul metropolitan area becauseexamination of borehole videos, core, and outcropsindicated that open fractures and dissolution featuresare relatively uncommon below that depth. Runkel(1999, 2000) conducted a larger-scale investigation,which included most of the Paleozoic stratigraphy acrossa nine-county area of southeastern Minnesota, andproposed a 200-foot-deep boundary between deep andshallow bedrock conditions as a regional-scalegeneralization (Plates 1, 2). Such an interpretation isconsistent with studies outside of Minnesota thatsimilarly depict a relatively well-connected, high-densitysystem of secondary pores in the uppermost 100 to 200feet of layered sedimentary bedrock (Ferguson, 1967;

Wyrick and Borchers, 1981; Williams and others, 1984;Graese and others, 1988; Hatcher and others, 1992;Soloman and others, 1992; Sasowsky and White, 1994;Michalski and Britton, 1997; Morin and others, 1997).A synthesis of engineering data from five dam sites inNorth America by Snow (1968) suggested that fractureporosity decreases an order of magnitude from the landsurface to a depth of 200 feet regardless of the dominantlithology at the individual sites. Investigations of theCambrian Maynardville Limestone in Tennesseedemonstrated that secondary pores were more prevalentin the uppermost approximately 110 feet of bedrockand extremely rare below a depth of about 240 feet,except in specific facies with a high susceptibility todissolution, such as evaporites (Goldstrand andShevenell, 1997; Shevenell and Goldstrand, 1997).

The term "shallow bedrock conditions" in thisreport refers to the upper 200 feet of Paleozoic bedrockregardless of the thickness and composition of overlyingunconsolidated materials. This 200-foot boundary ischosen with the understanding that the change fromwhat we have characterized as shallow bedrockconditions to deep bedrock conditions is in realitytransitional, and will vary in depth from place to place.

Hydraulic characterThe geologic controls on hydraulic conductivity of

Paleozoic bedrock is evaluated in this report bycomparing values of conductivity calculated forindividual wells and scientific boreholes to thehydrostratigraphic setting of their open-hole interval(Fig. 10, for example). Additionally, for selected partsof southeastern Minnesota, plots showing the spatialdistribution of hydraulic conductivity for individualhydrostratigraphic units were compared to maps ofbedrock topography, bedrock geology, structure, andisopachs to assess the effects of features such as bedrockvalleys and the influence of faults and folds to aquiferperformance.

Our evaluation indicates that calculated hydraulicconductivity at an individual borehole largely reflectsthe hydrostratigraphic character of its open-hole interval.For example, borehole measurements of hydraulicconductivity within individual lithostratigraphic unitsin deep bedrock settings largely reflect the permeabilityand thickness of its matrix hydrostratigraphiccomponent(s), and the degree of development ofsecondary pores (Fig. 10, for example). Rocksdominated by a fine clastic or carbonate rock componentwith few secondary pores have relatively low hydraulicconductivity and serve as confining units in deepbedrock settings (for example Libra and Hallberg, 1985;Nicholas and others, 1987; Graese and others, 1988;Miller and Delin, 1993). Horizontal hydraulic

22

conductivity of the fine clastic component based ondiscrete interval packer tests commonly ranges fromas low as 10-7

foot per day for units composed almost

entirely of shale (Freeze and Cherry, 1979; Graese andothers, 1988; Eaton and others, 2000), to 10-2

to 10-1

foot per day for interbedded, very fine-grained sandstoneand shale (for example Miller and Delin, 1993). Verticalconductivity in the fine clastic component is estimatedto be about two orders of magnitude less than horizontal(MUGSP, 1980; Miller, 1984; Kanivetsky, 1989;Setterholm and others, 1991; Miller and Delin, 1993).Packer tests of discrete intervals of unfractured, densePaleozoic carbonate rock in Wisconsin, Illinois, andIowa have typically indicated horizontal conductivitiesof about 10-4 foot per day or less (Graese and others,1988; Gianniny and others, 1996). In contrast, thecoarse clastic component, and intervals of carbonaterock containing abundant dissolution features, haverelatively high hydraulic conductivity values in deepbedrock settings. Discrete interval packer tests, specificcapacity data, and standard aquifer tests of wells openonly to the coarse clastic component typically rangefrom a few feet per day to as much as 60 feet per day(for example Nicholas and others, 1987; Young, 1992;Miller and Delin, 1993; Runkel and others, 1999). Thecarbonate rock component can have conductivity valuescommonly as high as hundreds of feet per day, and dyetraces through locally deep bedrock settings demonstrateflow speeds as rapid as miles per day along discreteintervals where well-developed conduit systems arepresent (Libra and Hallberg, 1985; Wheeler, 1993;Alexander and Lively, 1995; Paillet and others, 2000;Tipping and Runkel, 2001).

The inverse relationship between the degree in thedevelopment of secondary porosity and depth of burialbeneath bedrock (Plates 1, 2) is reflected by hydraulicperformance. The enhanced development of secondarypores in shallow bedrock conditions corresponds to ameasurable increase in hydraulic conductivity for thePaleozoic bedrock of southeastern Minnesota. Thescatter and box plots (Fig. 11) of 8,626 conductivityvalues calculated from specific capacity tests comparedagainst the depth of the open-hole interval below thebedrock surface show increased conductivitycorresponding to decreased burial beneath youngerbedrock. Similar comparisons of hydraulic propertiesto depth of burial beneath the bedrock surface are madefor each individual Paleozoic lithostratigraphic unit insubsequent sections of this report, and they demonstratethat individual matrix hydrostratigraphic units insoutheastern Minnesota, including those dominated bycoarse clastic and fine clastic strata, have a much higheraverage conductivity, and a greater range in conductivitywhere they occur in shallow bedrock conditionscompared to where the same units occur in deep bedrock

conditions. Additionally, ongoing borehole flowmeterinvestigations (for example Paillet and others, 2000;Tipping and Runkel, 2001) indicate that ambient flowrates in boreholes exposed only to deep bedrockconditions are typically subdued compared to the muchhigher and more variable flow rates in boreholes opento shallow bedrock conditions. This relationship likelyreflects the higher permeability due to enhancedsecondary porosity, and the greater stresses of near-surface recharge and discharge in shallow bedrockconditions.

A number of studies of sedimentary bedrock inNorth America likewise show a positive correlationbetween hydraulic performance and proximity to thebedrock surface. Bedding-plane fractures that providepreferential flow paths in siliciclastic strata have beendemonstrated to decrease in frequency and in hydraulicconductivity with depth (Michalski and Britton, 1997;Morin and others, 1997). Packer tests of Ordovicianand Silurian carbonate rock and shale in northern Illinoisdemonstrated that the highest conductivities wereconsistently within the uppermost 100 feet of thebedrock surface (Kempton and others, 1987). The samestudy indicated that bedrock strata within the uppermost40 feet of the bedrock surface are on average 100 timesmore permeable than the rocks below (Kempton andothers, 1987; Curry and others, 1988). A regionallyextensive "confining unit" composed of Paleozoic shaleand limestone in Illinois and adjacent states has ahydraulic conductivity of 10-7 to 10-5 foot per day wheredeeply buried by younger bedrock, and 10-3 to 12 feetper day closer to the bedrock surface (Eberts andGeorge, 2000). A study of the karstic MaynardvilleLimestone in Tennessee demonstrated that "quick flow"conduits of the highest conductivity were rare below110 feet and absent 240 feet below the bedrock surface,where only slow flow through matrix pores was recorded(Goldstrand and Shevenell, 1997; Shevenell andGoldstrand, 1997). The Galena Group in northeasternIowa has similar attributes: where relatively deeplyburied by younger bedrock that includes a shalyconfining unit, the Galena Group is characterized bydiffuse flow along relatively unmodified, narrowfractures. In conditions of shallow burial, particularlywhere it occurs as the uppermost bedrock, it ischaracterized by cavernous pores and more rapid conduitflow (Rowden and Libra, 1990; Keeler, 1997). Thegreater abundance, interconnectivity, and aperture offractures, and increased susceptibility of dissolutionaccounts for the relatively high magnitude andvariability in conductivity in shallow bedrock conditionsat each of the sites described above.

The results of our study revealed our limitedunderstanding of the relative hydraulic importance of

23

Increasing counts

Eau

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Figure 10. Hydrostratigraphic and hydraulic attributes for the Eau Claire through St. Lawrence Formationsin a deep bedrock setting at the ATES Project site in Ramsey County. Plug tests of porosity and permeabilitycharacterize the small-scale matrix hydrostratigraphic attributes. Visual examination of core provides informationon the distribution and character of macroscopic secondary pores. Discrete interval packer tests measure thehydraulic performance of the hydrostratigraphic components. Note that plug-scale permeability values positivelycorrelate with hydraulic conductivity where secondary pores are absent or rare. Fine clastic and carbonaterock components that test at relatively low permeability have a correspondingly low conductivity except in thelower part of the St. Lawrence Formation where dissolution cavities are present. Based on data in Walton andothers (1991), Miller and Delin (1993), and core logging as part of this investigation.

24

secondary pores at scales larger than that measured byan individual borehole pump test, especially in deepbedrock conditions of southeastern Minnesota. Anumber of studies summarized in this report havedemonstrated that stratigraphically controlled networksof dissolution cavities can be important hydraulicconduits along discrete intervals of carbonate rock indeep bedrock settings. The abundance and hydraulicimportance in deep bedrock settings of secondary poressuch as networks of systematic and bedding-planefractures, and other enhanced permeability features inclastic rocks, is less defined. Borehole flowmeter testsof clastic strata in deep bedrock conditions (describedlater in this report) document the dominance ofintergranular flow in some individual boreholes and,conversely, of flow along bedding-plane fractures inother boreholes. Our limited subsurface datademonstrate that hydraulically active secondary porestherefore clearly exist in deep bedrock conditions, andsuch features may be significant in the transport of waterthrough the aquifer at larger horizontal scales.

Indirect evidence such as water chemistry andpotentiometric levels indicates that any such fracturenetworks are poorly connected vertically across unitsdominated by fine clastic and carbonate strata. Thoselow-permeability components have been demonstratedto provide vertical hydraulic separation in deep bedrocksettings. Furthermore, discrete interval packer tests ofindividual boreholes (Fig. 10, for example) open tosiliciclastic strata in deep bedrock conditions generallyshow a positive correlation between hydraulicconductivity and intergranular porosity and permeability.The results of less rigorous, "standard" aquifer testsand of thousands of specific capacity tests insoutheastern Minnesota largely show the same positivecorrelation between conductivity and intergranularporosity and permeability. Such results are compatiblewith either single or multiple porosity interpretations.

On the other hand, the pump test results from asmall percentage of boreholes open to deep bedrockconditions do not reflect intergranular permeabilityalone, which suggests that a larger-scale conductivityaccommodated by secondary pores exists. For example,the specific capacity database for some units dominatedby the coarse clastic component such as the Mt. Simon,Ironton–Galesville, and Jordan Sandstones includes asmall percentage of statistically outlying conductivityvalues that are higher than expected if intergranularpermeability was the only control on well yield. Aquifertests of large diameter wells, particularly those that havebeen developed by blasting, include the greatestpercentage of relatively high conductivity values. Thesevalues of hydraulic conductivity possibly correspondto the small percentage of wells in deep bedrock

conditions that intersect networks of hydraulicallysignificant secondary pores such as bedding-plane andsystematic fractures. The majority of wells do notintersect such fracture networks, and pump tests of thosewells do not clearly reflect the enhanced permeabilityprovided by this larger-scale pore system. Theseobservations are not compatible with simple, singleporosity, intergranular flow interpretations but arecompatible with multi-porosity interpretations.

Hydrogeologic frameworkOur hydrogeologic framework (Plates 1, 2) is based

on hydraulic data interpreted within the context of thehydrostratigraphic attributes of the Paleozoicstratigraphic section, described in detail in thesubsequent sections of this report. The framework wepresent is more complex than those depicted in previouspublications because many of the individuallithostratigraphic units that were formerly consideredsingle hydrogeologic units across their entire extent arein this report subdivided at a regional scale into twoor more aquifers and confining units.

In deep bedrock conditions, the Paleozoic strataof southeastern Minnesota include at least elevenaquifers, chiefly hydrostratigraphic units dominated bycoarse clastic rock or intervals of carbonate rock withrelatively abundant secondary pores. These aquifersare separated by ten regional confining units composedof fine clastic strata or carbonate rock with fewinterconnected secondary pores. In shallow bedrockconditions, each of the individual hydrostratigraphicallydefined units, including those that provide confinementin deep bedrock settings, are of much greater bulkconductivity, and could be considered aquifers becauseeach has been demonstrated to yield economic quantitiesof water. The ability of the confining units to providehydraulic separation in such conditions is diminishedand greatly variable on a local scale because of therelatively enhanced development of secondary porosity.

Our hydrogeologic framework also delineates threemajor "karst systems" (Plates 1, 2), based largely onthe work of Alexander and Lively (1995), Alexanderand others (1996), and Green and others (1997). A karstsystem is an integrated mass-transfer system in solublerocks with a permeability structure dominated byconduits dissolved from the rock and organized tofacilitate the circulation of fluid (Klimchouk and Ford,2000). Southeastern Minnesota karst systems composedof carbonate-dominated strata where they lie in shallowbedrock conditions in ascending stratigraphic orderinclude the Prairie du Chien, Galena–Spillville, andCedar Valley karst systems. Each karst system ischaracterized by relatively abundant secondary poresincluding large cavities, and dissolution enlarged

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Figure 11. Hydraulic conductivity data for 8,626 wells open only to Paleozoic bedrock in southeastern Minnesota.Calculated from specific capacity tests in the County Well Index database. Box plots (C and D) provide aneasily observable manner to view the distribution of hydraulic conductivity values. Each box in this and subsequentconductivity figures encloses 50 percent of the values with the median value of the variable displayed as aline. The top and bottom of the box mark the limits of 25 percent of the variable population. The lines extendingfrom the top and bottom of each box mark the minimum and maximum values that fall within an acceptablerange. Any value outside of this range, called an outlier, is displayed as an individual point. Note, outliersare used in the calculations for the box plot, as well as in calculations of mean values in subsequent figures.

A. and B. Scatter plots showing the relationship between the depth of the open-hole interval below the bedrocksurface and hydraulic conductivity. Shallower wells tend to have higher conductivity. The two plots show thesame set of data at different scales.

C. Box plot of hydraulic conductivity values for deep bedrock conditions. Plot does not show 43 outlyingvalues greater than 100 feet per day.D. Box plot of hydraulic conductivity values for shallow bedrock conditions. Plot does not show 682 outlyingvalues greater than 100 feet per day.

C. D.

B.A.

26

systematic and nonsystematic fractures, and rapid, directconnections between the surface and ground water (Fig.12). These features may be expressed at the land surfaceby caves, numerous springs, and many sinkholes in areaswith only a thin cover of unconsolidated material, butthe same aquifer properties often exist in areas withfew if any obvious surface karst features. In thesubsurface the hydraulic properties of karst systems arevery heterogeneous, with large conduits that allow waterto travel as rapidly as miles per day draining matrixblocks of very low conductivity that store water movingmuch slower. The karst aquifers, also called "tripleporosity aquifers" (Worthington, 1999), are importantto ground-water management because the ground-watermovement through conduits can be extremely rapid anddifficult to predict. Catastrophic introduction and rapidtravel of contaminants are well-documented insoutheastern Minnesota (Alexander and Book, 1984;Alexander and others, 1993; Wheeler, 1993).

HYDROGEOLOGIC ATTRIBUTES OFINDIVIDUAL LITHOSTRATIGRAPHIC

UNITS

The hydrostratigraphic and hydraulic attributes ofeach of the major lithostratigraphic units of southeasternMinnesota are described in detail in the remainder ofthis report. We present our results organized in amanner in accordance with the standardlithostratigraphic nomenclature of southeasternMinnesota because these lithostratigraphic units areentrenched in our literature, databases, maps, andvocabulary. In addition, only lithostratigraphic unitsare delineated for individual water-well sites in theCounty Well Index database. Presented in this manner,our characterization can be used in combination withthe County Well Index database and bedrock geologicmaps as a guide to predict internal hydrostratigraphicand hydraulic variability within individuallithostratigraphic units at regional as well as site-specificscales.

The characterization of the hydrogeologic attributesof individual lithostratigraphic units begins with adescription of the character and distribution of the threeprincipal matrix hydrostratigraphic components and adiscussion of the development of secondary porosityin deep and shallow bedrock conditions, features thatare shown on Plates 1 and 2. This is followed by asynopsis of hydraulic properties, compiled on Figures13 and 14. Lastly, a discussion combines thehydrostratigraphic and hydraulic information in anintegrated synthesis of the hydrogeologic character ofeach lithostratigraphic unit, including delineation ofindividual aquifers and confining units.

Hydraulic conductivity and flow speeds measuredfor individual lithostratigraphic units are divided intosubsets of values where measurable differences inhydraulic conductivity and productivity correspond tointernal variations in hydrostratigraphic character. Forexample, hydraulic properties of each lithostratigraphicunit are described separately for deep and shallowbedrock conditions. Conductivity values for shallowbedrock conditions are based on pump and flowmetertests of wells that are open at least in part to within200 feet of the bedrock surface, and attributes of deepbedrock conditions are based on tests for wells wherethe uppermost 200 feet of bedrock is cased off.

The degree to which individual units arecharacterized varies greatly. The Prairie du ChienGroup, Jordan Sandstone, and Franconia Formation havebeen the subject of a large number of detailedinvestigations in a wide range of geologic settings insoutheastern Minnesota, and as a result we summarizesignificant recent advances in our understanding of thehydrogeologic attributes of those units. Other intervals,such as the Mt. Simon Sandstone and Galena Group,are known to vary internally in hydraulic properties andtherefore to consist of multiple hydrostratigraphic units,but they have not been subjected to rigoroushydrostratigraphic and hydraulic study in Minnesota.These units are characterized in a more cursory fashion,and we draw heavily upon investigations of these strataconducted outside of Minnesota.

MT. SIMON SANDSTONE

Hydrostratigraphic attributes

Matrix porosityThe Mt. Simon Sandstone is broadly divisible into

two parts based on intergranular attributes (Figs. 15,16). The lower Mt. Simon Sandstone across most ofsoutheastern Minnesota consists chiefly of a coarseclastic component that is moderately to poorlycemented. Plug test permeability is commonly greaterthan 1,000 md in both vertical and horizontal directions(MUGSP, 1980). Fine clastic interbeds are a subordinatecomponent in the lower Mt. Simon Sandstone; they aremost abundant in Fillmore, Houston, and WinonaCounties in southeastern Minnesota

The upper part of the Mt. Simon Sandstone acrossmost of southeastern Minnesota is hydrostratigraphicallymore complex (Figs. 15, 16). It consists ofapproximately equal parts coarse clastic and fine clasticstrata intercalated in beds from a few feet to as muchas 30 feet thick. Plug tests from individual boreholesdemonstrate that vertical permeability ranges over tenorders of magnitude: coarse clastic intervals are

27

commonly over 1,000 md, whereas fine clastic intervalscommonly have values from 10-6 to 10-3 md. Someindividual plug samples are strongly anisotropic, witha horizontal permeability 1,000 times greater thanvertical.

Secondary porosityDeep bedrock conditions—Our knowledge of

secondary porosity in the Mt. Simon Sandstone in deepbedrock settings is extremely limited. Conventionalwisdom maintains that as a friable, mostly high porosityunit covered by layers of younger bedrock of contrastingmaterial properties, interconnected networks of open

systematic fractures may not be developed to anappreciable degree (Price and Cosgrove, 1990; Helgesonand Aydin, 1991; Narr and Suppe, 1991). An unprovedassumption by previous investigators is thatinterconnected networks of fractures and dissolutionfeatures are rare to absent in the Mt. Simon Sandstonein deep bedrock settings and therefore porosity andpermeability are determined chiefly by intergranularattributes.

Shallow bedrock conditions—The Mt. SimonSandstone occurs in a relatively shallow bedrock settingalong the Mississippi River and the lower reaches ofits tributaries in southeastern Minnesota (Mossler and

Streamsinks

Blind valley Sinkholes

Fractures

Caves

Sump

Spring

Surfacestream

Seepage



Carbonate component

Non-systematic fractures (some dissolution enlarged)

Systematic fractures (some dissolution enlarged)

Dissolution features—cavities and enlarged bedding-plane fractures

EXPLANATION

Surficial deposits Water

Figure 12. Typical attributes of a karst system in southeastern Minnesota. Such systems are developed mostcommonly in carbonate rock in shallow bedrock conditions. Any fracture or cavity may contain ground waterin this karst system.

28

Book, 1984; Mossler, 1990, 2001; Mossler andBloomgren, 1990; Runkel, 1996a, 1998). However, itis deeply buried by unconsolidated Quaternary sedimentin these areas and has not been studied using subsurfacetechniques that provide information on secondary pores.

The Mt. Simon Sandstone has open, systematic andnonsystematic fractures where it is well exposed in largeoutcrops in west-central Wisconsin, such as in the citiesof Eau Claire and Chippewa Falls. Although Minnesotalacks large outcrops that can be confidently assignedto the Mt. Simon Sandstone, the characteristics of theformation in Wisconsin suggest that open fractures arepresent in the Mt. Simon Sandstone where it occursin shallow bedrock conditions in southeasternMinnesota.

Hydraulic attributesDeep bedrock conditions—A large number of

discrete interval and standard aquifer tests of the Mt.Simon Sandstone where it occurs in deep bedrockconditions (Fig. 13) in Illinois, southern Wisconsin, andsoutheastern Minnesota have a range in hydraulicconductivity from 0.38 to 21 feet per day (Nicholasand others, 1987; Young, 1992; Carlson and Taylor,1999). The lowest values are calculated from tests ofwells where the Mt. Simon Sandstone is buried byseveral hundred to thousands of feet of younger bedrock.Studies in Illinois have demonstrated that in suchsettings the intergranular permeability in the sandstoneis reduced compared to shallower conditions of burialas a result of the enhanced development of pore-fillingcement and compaction (Hoholick and others, 1984).The highest values of hydraulic conductivity, thosegreater than 12 feet per day, are calculated from testsof wells within a few miles of the Mississippi River,where the Mt. Simon Sandstone occurs much closerto the bedrock surface.

The hydrostratigraphically complex upper Mt.Simon Sandstone has not been individually tested todetermine its conductivity in Minnesota. Nicholas andothers (1987) calculated a bulk horizontal hydraulicconductivity of 1.3 feet per day for the upper part ofthe Mt. Simon Sandstone in Illinois, where it containsalternating beds of coarse clastic and fine clastic strata(Fig. 15), similar to Minnesota. This bulk conductivityvalue most likely is a measure of the individual, highpermeability coarse clastic beds in the upper part ofthe formation. In contrast, individual beds of the fineclastic component in the upper Mt. Simon Sandstonewill have a horizontal hydraulic conductivity of about10-3 to 10-1 foot per day, and a vertical hydraulicconductivity between 10-5 and 10-3 foot per day basedon tests of strata with similar matrix porosity andpermeability in other Paleozoic formations in

southeastern Minnesota (for example Miller and Delin,1993).

Hydraulic conductivity values based on specificcapacity tests for 25 wells open to the Mt. SimonSandstone in deep conditions of burial are depicted inFigure 17. The values typically range from less thanone to as much as 50 feet per day, and averageconductivity is 39.5 feet per day. Exclusion of twooutlying values greater than 240 feet per day from thedatabase results in a calculated average conductivityof 21 feet per day, a value more consistent with thehigher-quality pump tests described above, and withconductivity values calculated for other deeply buriedPaleozoic aquifers dominated by intergranular flowthrough the coarse clastic component in southeasternMinnesota (described later in this report). The twooutlying values of greater than 240 feet per day werecalculated from tests of wells that were developed byblasting or decompression techniques, and if accuratethey may reflect a significant contribution of yield froma network of hydraulic fractures.

Shallow bedrock conditions—Hydraulicconductivity values calculated from specific capacitytests for 165 wells open to the upper and lower partsof the Mt. Simon Sandstone under shallow bedrockconditions have a range from less than 1 to 70 feet perday. Average hydraulic conductivity is 29.3 feet perday (Fig. 17). The greater range in conductivitycompared to deep bedrock conditions probably reflectsa higher percentage of wells in which significantcontribution occurs through networks of fractures.

Hydrogeologic synthesisThe Mt. Simon Sandstone is hydro-stratigraphically

complex because it contains two intercalatedcomponents, the coarse clastic and fine clasticcomponents, that differ markedly from one another inpermeability (Figs. 15, 16). Similar hydrostratigraphicproperties have been described for the Mt. SimonSandstone in Illinois and Wisconsin, wherehydrogeologic investigations have demonstrated that theintercalations of fine clastic material serve as a confiningunit(s) in deep bedrock settings (Nicholas and others,1987; Carlson and Taylor, 1999). Packer tests in theIllinois study demonstrated that even though the upperMt. Simon Sandstone contains coarse clastic intervalsthat provide moderately high bulk horizontalconductivity, fine clastic interbeds have low enoughvertical conductivity to function as a confining unit(s)that separates an upper from a lower Mt. Simon aquifer(Fig. 15). Potentiometric heads in the two aquifersdiffered from one another by over 50 feet, and waterin the lower aquifer is different from the upper in

29

fundamental water chemistry, particularly in its orderof magnitude higher concentration of chloride andsodium (Nicholas and others, 1987). Preliminary resultsof an ongoing investigation of the Mt. Simon Sandstonein southeastern Wisconsin using borehole, flowmeter,temperature, and water chemistry data have similarlyindicated the presence of a "middle" confining unit thatdivides the formation into two distinct aquifers (K.Bradbury, unpub. data, 2001).

Unpublished investigations of the Mt. SimonSandstone in the three-county Waseca–Waterville areaof southeastern Minnesota (Fig. 1) indicated that fineclastic interbeds in the middle to upper Mt. SimonSandstone may serve as confining units (MUGSP, 1980),as they do in Illinois and Wisconsin (Fig. 15). Thereare at least two fine clastic intervals of Mt. SimonSandstone in that area that are of sufficient thicknessand low permeability to function as hydraulic caps thatcould potentially confine natural gas stored in the coarseclastic material beneath them. These intervals,designated Cap Rock A and Cap Rock B as part of theMUGSP, suggest that the Mt. Simon Sandstone in theWaseca–Waterville area may be divisible into a lower,middle, and upper aquifer. The lower and middleaquifers are dominated by high-conductivity coarseclastic strata, whereas the upper Mt. Simon aquiferconsists of alternating thin to medium beds of coarseand fine clastic material. Ground-water chemistry fromtwo boreholes indicated that water from the lower Mt.Simon aquifer has measurably greater dissolved solids,including chloride concentration that is as much as 6times higher than that of water in the middle and upperMt. Simon aquifers (MUGSP, 1980).

The common practice of depicting the entire Mt.Simon Sandstone as a single aquifer in Minnesotashould be reevaluated because the formation may beinternally compartmentalized into two or morehydrogeologic units (Fig. 15, for example). Althoughcoarse clastic beds across its entire stratigraphic extentcan yield economic quantities of water, the fine clasticbeds, which are most common in its upper one-half,are commonly as thick and of similar matrixhydrostratigraphic properties as younger Paleozoiclayers demonstrated to act as confining beds insoutheastern Minnesota, such as those intercalated withcoarse clastic layers in the lower St. Peter Sandstonein the Twin Cities Metropolitan area (Kanivetsky andCleland, 1992). Studies of ground-water chemistry inthe Mt. Simon Sandstone should in particular beconducted with consideration of multiple aquifers,particularly because water contributed from thePrecambrian "basement" may be confined largely to thelowermost part of the formation.

EAU CLAIRE FORMATION


Matrix porosityThe Eau Claire Formation is composed chiefly of

the fine clastic component, with vertical permeabilitiesthat typically range from 10-5 to 10-3 md (Figs. 16, 18).It varies little in its matrix character across most ofsoutheastern Minnesota, containing only relatively thin(typically less than 10 feet) coarse clastic interbeds inits upper part in Wabasha, Winona, and HoustonCounties (Runkel, 1996a). One exception is in Faribaultand adjacent counties where the Eau Claire Formationreaches its greatest thickness, and includes ahydrostratigraphically distinct "greensand" faciesdominated by fine- to medium-grained glauconiticsandstone as thick as 100 feet (Fig. 18; Mossler, 1992).This coarse clastic component has plug-scalepermeabilities that are orders of magnitude higher thanthe bulk of the Eau Claire Formation elsewhere insoutheastern Minnesota.


secondary porosity in deep bedrock conditions islimited. Intergranular permeability is widely believedto be the chief control on hydraulic behavior in deepbedrock settings. Logged cores and a few boreholevideos corroborate that secondary pores are rare in theEau Claire Formation (Figs. 16, 18), occurring mostlyas small dissolution cavities in dolostone beds thatcompose a relatively minor part of the Eau ClaireFormation across southeastern Minnesota. A boreholevideo of Brooklyn Park municipal well 4 (MinnesotaDepartment of Health borehole video library, uniquenumber 203265) revealed the presence of a systematicfracture with an inch-scale aperture over 300 feet belowthe bedrock surface in the Eau Claire Formation. Thewell had been blasted and bailed to increaseproductivity—procedures that may have increased theaperture of the fracture.

Shallow bedrock conditions—In shallow bedrockconditions, the Eau Claire Formation has features typicalof fine clastic rock that has been subjected to stressrelief and weathering. Nonsystematic fractures arecommon in outcrops of the Eau Claire Formation inMinnesota and adjacent parts of Wisconsin, includingirregular, sub-vertical fractures a few inches in widthand bedding-plane fractures that can be traced tens offeet laterally. Large quarries of Eau Claire Formationrock in west-central Wisconsin commonly havesystematic fractures with apertures as wide as several

30

DE

EP

BE

DR

OC

K C

ON

DT

ION

SS

HA

LLO

W B

ED

RO

CK

CO

ND

ITIO

NS

Kh

<10

-2 ft

/day

(7)

Kh

10-3

to 1

0-2

ft/da

y w

ith th

in in

terv

als

to 1

0-1

ft/da

y (4

)

Kh

<10

-6 ft

/day

(9)

Opv

l

Odc

r

Max

imum

of 2

to 7

ft/d

ayso

me

subs

tant

ially

low

er(p

umpe

d dr

y) (

6)

Kh

<10

-2 ft

/day

(7)

Kh

mos

tly 1

0-3

to 1

0-2

ft/da

y ex

cept

dis

cret

e th

in in

terv

als

1.4

to 1

4 ft/

day

(4)

Oga

l

Odu

b

Om

aq

Dsp

l

Kh

5.3

ft/da

y (1

)K

h 0.

5 ft/

day

(1)

Kh

39 ft

/day

(3)

Kh

>17

4 ft/

day

(1)

Dcl

p

Kh

25 ft

/day

(1)

Kh

0.8

ft/da

y (1

)

Kh

>19

0 ft/

day

(1)

Kh

26 ft

/day

(1)

Dcl

c

Dcu

m

Dcu

u

Trav

el ti

mes

of u

p to

1 m

ile/d

ay (

11)

Opv

l

Odc

r

Kh

3 to

11

ft/da

y (6

)O

gal

Odu

b

Om

aq

Dsp

l

Dcl

p

Dcl

c

Dcu

m

Dcu

u

Kh

<10

-1 to

> h

undr

eds

of ft

/day

(10

)

Kh

max

imum

of 1

to 2

ft/d

ay (

6)

Kh

8.9

x 10

-4 t

o 3

x 10

-2 ft

/day

(6)

Kh

92 ft

/day

(1)

Kh

5.3

ft/da

y (1

)

Pum

ped

at 1

7 gp

m

with

no

draw

dow

n (1

)

Kh

13.4

ft/d

ay (

1)

Pum

ped

at 3

0 gp

mw

ith n

o dr

awdo

wn

(1)

Kh

67 ft

/day

(2)

Hor

izon

tal f

low

spe

eds

0.23

m

iles/

yr to

1.8

mile

s/da

y (6

)

Ver

tical

flow

spe

ed ~

260

ftin

less

than

7 m

onth

s (6

)

Kh

<10

-4 to

10-

3 ft/

day

(4)

Kh

10-3

to 2

8 ft/

day

(4, 5

)

Kh

64.6

ft/d

ay (

2)

Kh

10-3

to 3

00 ft

/day

(8)

Kh

6.5

ft/da

y (2

)

Kh

72 ft

/day

(2)

Kh

60.1

ft/d

ay (

2)

Kh

1.8

ft/da

y (7

)

Kv

10-6

ft/d

ay (

9)

Ost

p

Ogw

d

Kv

10-3

ft/d

ay (

12)

Ost

p

Ogw

d

Kh

8.4

ft/da

y (4

)K

h 15

.9 ft

/day

(2)

Kh

1.3

to 3

.7 ft

/day

(13

)

Kh

38.7

ft/d

ay (

2)K

h 20

to 3

0 ft/

day

(14)

Kh

24 to

30

ft/da

y (1

5)K

h ~

20 ft

/day

(16

)

31

Kh

10-2

ft/d

ay (

27)

Kv

10-4

ft/d

ay (

27)

Kh

10-3

to 1

0-2

ft/da

y (3

5)�

ecr

�ig

l

�fr

n

Kh

1.6

to 7

.9 ft

/day

(27

)K

v 0.

16 to

0.7

9 ft/

day

(27)

Kh

5 ft/

day

(27)

Kv

0.5

ft/da

y (2

7)K

h 10

.8 ft

/day

(2)

Kh

3 ft/

day

(18)

Kh

10 ft

/day

(18

)K

h 11

ft/d

ay (

33)

Kh

2.9

to 3

1 ft/

day

(34)

Kv

≤ 10

-4 ft

/day

(27

)K

h ≤

10-2

ft/d

ay (

27)

Kv

0.14

to 0

.75

ft/da

y (2

7)K

h 1.

4 to

7.5

ft/d

ay (

27)

Kh

14.0

ft/d

ay (

2)K

h 10

-2 to

6.7

ft/d

ay (

27)

Kv

10-4

ft/d

ay (

28)

�st

l

Kh

10-2

ft/d

ay (

25)

Kv

10-4

ft/d

ay (

25)

Kh

0.1

to 1

00 ft

/day

(22

)

Kh

8.0

to 2

4 ft/

day

(26)

�jd

n

Flo

w s

peed

6.5

mile

s/da

y (1

7)

Kh

9 ft/

day

(18)

Kh

33.5

ft/d

ay (

2)

(like

ly y

ield

ed b

y di

scre

te in

terv

als

of h

igh

cond

uctiv

ity

sepa

rate

d by

inte

rval

s w

ith lo

w c

ondu

ctiv

ity)

Opo

d

Ops

h

Kh

837

ft/da

y (2

0a)

Kh

17.4

ft/d

ay (

2)

Kh

9.3

and

20 ft

/day

(29

)

Kh

5.9

ft/da

y w

here

Maz

oman

ieM

embe

r is

thin

to a

bsen

t (2)

Kh

27.8

ft/d

ay w

here

Maz

oman

ieM

embe

r is

thic

k (2

)

Coa

rse

clas

tic K

h 1.

3 ft/

day

(7)

Fin

e cl

astic

Kh

10-2

ft/d

ay; k

v 10

- 4 ft

/day

(25

)

Kh

1.5

ft/da

y (7

)

Kh

39.5

ft/d

ay (

2)K

h 17

ft/d

ay (

36)

Kh

1.5

to 5

ft/d

ay (

37)

Kh

0.38

to 2

1 ft/

day

(38)

�m

ts�

mts

Kh

29.3

ft/d

ay (

2)

�ec

r

�ig

l

�fr

n

Kh

46 ft

/day

(2)

�st

l

Kh

43.2

ft/d

ay (

2)

Kh

appr

ox. 3

0 to

>50

0 ft/

day

(22)

�jd

n

Kh

45 ft

/day

(22

)

Opo

d

Ops

h

Kh

10-2

to 8

5 ft/

day

(31)

Kh

163

ft/da

y (2

3)K

v 1.

75 ft

/day

(23

)K

h 0.

1 ft/

day

(21)

Kh

60.8

ft/da

y (2

)

10 ft

inte

rval

s K

h ra

nge

from

1.6

to 6

5 ft/

day

(19)

Up

to 8

00 ft

/day

trav

el (

19)

Dis

cret

e <

3 ft

inte

rval

sK

h ra

nge

from

2.2

to1,

023

ft/da

y (2

0)

Bul

k K

h ra

nges

from

5.3

to 1

8 ft/

day

(20)

Kh

7.5

x 10

-3 ft

/day

(23

)K

v 1.

5 x

10-4

ft/d

ay (

23)

Kv

10-4

ft/d

ay (

24)

Kh

32.3

ft/d

ay w

here

Maz

oman

ieM

embe

r is

thin

to a

bsen

t (2)

Kh

31.7

ft/d

ay w

here

Maz

oman

ieM

embe

r is

thic

k (2

)

Kh

3 ft/

day

with

dis

cret

e in

terv

als

avg.

220

ft/d

ay (

30)

Kh

10-1

ft/d

ay (

32)

Kh

26.8

ft/d

ay (

2)K

h 10

to 1

00 ft

/day

(35

)K

h av

g. 2

0 ft/

day

(35)

Kh

36.7

ft/d

ay (

2)

Exp

lana

t ion

t o F

igur

e 13

is o

n pa

ges

32 a

nd 3

3.

Figu

re 1

3

32



Carbonate component

Non-systematic fractures(some dissolution enlarged)

Systematic fractures (some dissolution enlarged)

Dissolution features—cavities and enlarged bedding-plane fractures

EXPLANATION

Kh—Horizontal hydraulic conductivity

Kv—Vertical hydraulic conductivity

ft/day—Feet per day

gpm—Gallons per minute

Figure 13. Figure appears on pages 30 and 31. Generalized stratigraphic column of Paleozoic strata insoutheastern Minnesota showing matrix hydrostratigraphic components, typical development of secondary porosity,and hydraulic data compiled for this report. Figure is not to scale. Hydraulic data for these figures (numbersin parentheses) are from the following (see Fig. 1 for the location of listed counties):

1. Discrete interval packer testing in northern Iowa (Floyd and Mitchell Counties) by Libra and Hallberg(1985).2. Average value of conductivity calculated based on specific capacity tests in the County Well Indexdatabase.

3. Standard aquifer pump test of the LeRoy municipal well (unique number 127280) by J. Green of theMinnesota Department of Natural Resources, Mower County.

4. Discrete interval packer tests in Illinois by Kempton and others (1987), Curry and others (1988), andGraese and others (1988).5. Discrete interval packer tests in Wisconsin by Eaton and others (2000).

6. Discrete interval packer tests and dye-trace studies at the Spring Valley Amoco terminal by DeltaEnvironmental Consultants, Inc. (1995, 1998), Fillmore County.

7. Discrete interval packer tests in Illinois by Nicholas and others (1987).8. Discrete interval slug tests in Wisconsin by Stocks (1998).

9. Based on laboratory analysis of plug tests and typical results of pump tests of similar strata (Freezeand Cherry, 1979).

10. Standard aquifer pump tests at Reilly Tar and Chemical site in St. Louis Park, Hennepin County, reportedby ERT (1987) and ENSR International (1991); at General Mills Solvent Disposal site in northeast Minneapolis,Hennepin County, by Barr Engineering (1991); and at Minnehaha Park tunnel in Hennepin County by Liesch(1973). These and other site investigations with similar results are summarized in Hoffman and Alexander(1998).11. Dye-trace investigations in western Wisconsin (Hoffman and Alexander, 1998) and at Camp ColdwaterSpring, Hennepin County (Alexander and others, 2001).

12. Standard aquifer pump tests and modeling in the seven-county Twin Cities Metropolitan area bySchoenberg (1991).

13. Seven boreholes packer tested in southern Wisconsin (Young, 1992).14. Standard aquifer tests of several wells at the Reilly Tar and Chemical site in St. Louis Park, HennepinCounty. Includes tests of St. Louis Park municipal well number 3, and monitor well W410 (Barr Engineering,1976; and an anonymous report in the Minnesota Pollution Control Agency site files).

15. Standard aquifer tests of four wells in Ramsey and Hennepin Counties reported in Norvitch and Walton(1979). Conductivity was calculated using an aquifer thickness of 100 feet.

16. Standard aquifer test at the Nutting Site in Faribault, Rice County (Barr Engineering, 1986).17. Fillmore County dye-trace study by Wheeler (1993).

18. Standard aquifer pump test of Spring Grove municipal well #4 (unique number 433257), HoustonCounty (Eder and Associates, 1997).

19. Discrete interval tests and dye-trace studies at Oronoco Landfill, Olmsted County, by Donahue andAssociates, Inc. (1991) and RMT, Inc. (1992).20. Borehole flowmeter logging and pumping at wells in:

33

A. Faribault (unique number 625327), Rice County.B. Rochester (unique number 485610), Olmsted County.

21. Standard aquifer pump test of a well at Chatfield Fish and Game Club in Fillmore County (uniquenumber 227394).

22. Twenty-six standard aquifer pump tests conducted in southeastern Minnesota. Tests of 12 boreholeslocated in the seven-county Twin Cities Metropolitan area are reported by Runkel and others (1999). Testsof 14 boreholes outside of the metropolitan area are from unpublished data compiled by the U.S. GeologicalSurvey and include the following: Rochester Municipal wells 23 (unique number 220660), 27 (unique number224212), 28 (unique number 180567), 29 (unique number 161425), 30 (unique number 239761), 31 (uniquenumber 434041), 32 (unique number 506819), and 34 (unique number 463536), Rochester public schoolswells for Ridgeway (unique number 235583), Burr Oak (unique number 220615), and Golden Hill (uniquenumber 220679), all in Olmsted County, and Rice County wells at Carleton College (unique number 171005),Dundas (unique number 132294), and St. Olaf College (no number).23. Standard aquifer pump tests at the New Brighton and Arden Hills Twin Cities Army Ammunition Plantsite, Ramsey County, by Camp, Dresser and McKee (1991).

24. Standard aquifer pump test by the Minnesota Department of Natural Resources and Minnesota Departmentof Health at Plainview in Wabasha County.

25. Based on discrete interval packer tests of similar strata of other parts of the Paleozoic section in RamseyCounty by Miller and Delin (1993).26. Three boreholes packer tested in southwest Wisconsin (Young, 1992).

27. Discrete interval packer test in Ramsey County by Miller and Delin (1993).

28. Discrete interval packer tests and thermal profiling at the ATES Project site in Ramsey County byKanivetsky (1989).29. Two boreholes packer tested in southwest Wisconsin (Young, 1992).

30. Borehole flowmeter and discrete interval slug tests in southeastern Wisconsin (Swanson, 2001). Stratatested include interbeds of the Mazomanie Member.

31. Slug testing of monitor wells at a proposed ash disposal site at Red Wing in Goodhue County byWenck and Associates, Inc. (1997).32. Standard aquifer pump tests at Lakeland, Washington County by Braun Intertec (1992).

33. Standard aquifer pump test of Peterson Fish Hatchery (unique number 467232), Fillmore County.

34. Nine wells packer tested in Illinois and southern Wisconsin, reported by Young (1992).35. Discrete interval slug tests in southeastern Wisconsin (Bradbury, 2001).

36. Standard aquifer pump test of Goodview municipal well 3 (unique number 449410), Winona County.

37. Packer tests in Wisconsin by Carlson and Taylor (1999).38. Seventeen wells packer tested in southern Wisconsin and Illinois, reported by Young (1992).

Key to lithostratigraphic units:

�mts—Mt. Simon Sandstone �ecr—Eau Claire Formation �igl—Ironton–Galesville Sandstone

�frn—Franconia Formation �stl—St. Lawrence Formation �jdn—Jordan Sandstone

Opod—Prairie du Chien Group– Opsh—Prairie du Chien Group– Ostp—St. Peter Sandstone Oneota Dolomite Shakopee FormationOgwd—Glenwood Formation Opvl—Platteville Formation Odcr—Decorah Shale

Ogal—Galena Group Odub—Dubuque Formation Omaq—Maquoketa Formation

Dspl—Spillville Formation Dclp—Pinicon Ridge Formation and Dclc—Chickasaw Member of the Bassett Member of the Little Cedar Formation

Dcum—Coralville Formation and Little Cedar Formation Hinkle and Eagle Center Members of the Little Cedar Dcuu—Lithograph City Formation Formation

34

0

20

40

60

80

100


�mts �igl �frn1 �frn2 �stl �jdn Opdc Ostp Ogal Dspl

0

20

40

60

80

100

�mts �ecr �igl �frn1 �frn2 �stl �jdn Opdc Ostp Opvl Odcr Ogal Dcom


Ran

ge

Ran

ge

0

100

200

300

400

500

600


�mts �igl �frn1 �frn2 �stl �jdn Opdc Ostp Ogal Dspl0

100

200

300

400

500

600


Ran

ge

Ran

ge

�mts �ecr �igl �frn1 �frn2 �stl �jdn Opdc Ostp Opvl Odcr Ogal Dcom

Figure 14. Comparison of conductivity values (in feet per day) calculated from specific capacity data. Thedatasets for deep and shallow bedrock conditons are each plotted at two different scales. In deep bedrock conditions,the formations with the highest conductivity are those that contain coarse clastic strata, or are composed ofcarbonate rock with dissolution cavities. The former include the Mt. Simon, Ironton–Galesville, Jordan, andSt. Peter Sandstones, and the Franconia Formation only where the Mazomanie Member is present. The formationsthat are composed of carbonate rock with dissolution cavities include the St. Lawrence Formation and the Prairiedu Chien Group. In shallow bedrock conditions, conductivity is much more variable, and each of thelithostratigraphic units can have moderate to high conductivities, regardless of matrix characteristics. Thereforeall are used as an economic source of ground water.

The database from which these conductivity values have been calculated consists of tests of water wellsconstructed expressly for the purpose of extracting economic quantities of water. The values of conductivityare therefore chiefly representative of the most productive intervals of Paleozoic strata in a given geologic setting,and do not include a large sample of values representative of intervals of strata with relatively low conductivity.

Lithostratigraphic units:

�mts—Mt. Simon Sandstone �ecr—Eau Claire Formation �igl—Ironton–Galesville Sandstone�frn1—Franconia Formation where the Mazomanie Member is thin to absent

�frn2—Franconia Formation where the Mazomanie Member forms a substantial component of the formation

�stl—St. Lawrence Formation �jdn—Jordan Sandstone Opdc—Prairie du Chien GroupOstp—St. Peter Sandstone Opvl—Platteville Formation Odcr—Decorah Shale

Ogal—Galena Group Dspl—Spillville Formation Dcom—All strata above the Galena Group

35

Eau

Cla

ireF

orm

atio

n

Upp

erM

t. S

imon

aqui

fer

Mt.

Sim

onco

nfin

ing

unit

Low

erM

t. S

imon

aqui

fer

Incr

easi

ng c

ount

s

200 feet

Illin

ois

B.

500 feet

Incr

easi

ng c

ount

s

St.

Pau

lV

erm

illio

nLo

nsda

leW

asec

a –W

ater

ville

Ow

aton

naH

olla

ndal

eR

ushf

ord

2230

8246

7232

2190

2221

3644

and

213

646

2355

2622

6617

2001

90E

au C

laire

For

mat

ion

Upp

erM

t. S

imon

San

dsto

ne

Low

erM

t. S

imon

San

dsto

ne

Pre

cam

bria

n"b

asem

ent"

Upp

er (

tran

sitio

n)M

t.Sim

on a

quife

r

Cap

Roc

k A

Mid

dle

Mt.

Sim

onaq

uife

r

Cap

Roc

k B

Low

erM

t. S

imon

aqui

fer

Pre

cam

bria

n "b

asem

ent"

Eau

Cla

ireF

orm

atio

n

A

A'

A.

Coa

rse

clas

tic c

ompo

nent

Fin

e cl

astic

com

pone

nt

AA

'

Upp

erM

t. S

imon

San

dsto

ne

Low

erM

t. S

imon

San

dsto

ne

Figu

re 1

5. A

. H

ydro

stra

tigr

aphi

c at

trib

utes

of

the

Mt.

Sim

onS

ands

tone

and

rep

rese

ntat

ive

natu

ral g

amm

a lo

gs a

cros

s pa

rtof

sou

thea

ster

n M

inne

sota

. N

ote

that

alt

houg

h th

e M

t. S

imon

San

dsto

ne c

onsi

sts

chie

fly

of c

oars

e cl

asti

c st

rata

, it

als

oco

ntai

ns s

ubst

anti

all y

thi

ck i

nter

vals

dom

inat

ed b

y th

e fi

necl

asti

c co

mpo

nent

, pa

rtic

ular

ly i

n it

s up

per

part

. S

uch

inte

rbed

s w

ere

dete

rmin

ed to

be

of lo

w c

ondu

ctiv

ity

and

have

the

abil

ity

to p

rovi

de c

onfi

nem

ent

in t

he W

asec

a–W

ater

vill

ear

ea.

Uni

que

num

bers

are

lis

ted

abov

e th

e bo

reho

les.

B.

Na t

ura l

gam

ma

log

a nd

hydr

ogeo

logi

c un

its

defi

ned

for

the

hydr

ostr

a tig

raph

ica l

ly s

imil

a r M

t. S

imon

San

dsto

ne i

nIl

lino

is (

Nic

hola

s a n

d ot

hers

, 19

87).

36

750'

700'

650'

600'

550'

500'

450'

400'

350'

300'

250'

200'O

neot

a D

olom

iteJo

rdan

San

dsto

neS

t. La

wre

nce

For

mat

ion

Fra

ncon

ia F

orm

atio

nIr

onto

n–G

ales

ville

San

dsto

neE

au C

laire

For

mat

ion

Mt.

Sim

on S

ands

tone

0 100% 0 5 0 40% 10-6 100 mdLith

ostr

atig

raph

icun

it

Mat

rixhy

dros

trat

igra

phic

com

pone

nt

Dep

th b

elow

bedr

ock

surf

ace

Vis

ual

poro

sity

Fra

ctur

espe

r fo

ot

Por

osity

Ver

tical

perm

eabi

lity

Gamma log

Increasing counts

Mt. SimonCap Rock A

Pre

hn #

3P

ratt

#3P

rehn

#3

Lloy

d W

illia

ms

#4an

d S

tein

haus

#1


Carbonate component


Interval of no core recovery

Figure 16. Distribution of porosity andpermeability of the Mt. Simon Sandstonethrough Oneota Dolomite in deep bedrockconditions in the Waseca–Waterville area.Matrix porosity and permeability is low exceptin the coarse clastic component. Secondaryporosity is minimally developed except fordissolution cavities developed in specifichorizons of carbonate rock in the St. LawrenceFormation and Oneota Dolomite, and incarbonate-rich intraclasts in the FranconiaFormation. The Oneota Dolomite cavities weredeveloped about 150 to 200 feet below thebedrock surface, in the transition betweenshallow and deep bedrock conditions. Plugsamples were collected at approximately one-foot intervals. Gray shading on the porosityand permeability logs are estimated valuescorresponding to intervals of no core recovery,chiefly where coarse clastic beds were present,and are based on plug tests of nearby cores andfrom Setterholm and others (1991). Cores Pratt3, Prehn 3, Lloyd Williams 4, and Steinhaus1.

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inches in the fine clastic component.

Hydraulic attributesDeep bedrock conditions—Discrete interval pump

tests of the fine clastic component in the Eau ClaireFormation in deep bedrock conditions measured ahorizontal hydraulic conductivity value of less than 10-

2 foot per day (Miller and Delin, 1993). Fine clasticstrata of the Eau Claire Formation in southeasternWisconsin yielded similar values of horizontalconductivity based on slug tests, about 10-3 to 10-2 footper day (Bradbury, 2001; Swanson, 2001). Verticalconductivity has been estimated at 10-4 foot per day(Fig. 10; Miller and Delin, 1993), which is consistentwith the low intergranular permeability and anisotropymeasured in plug samples. Coarse clastic beds in theupper part of the formation have not been individually

tested, but can be expected to have a hydraulicconductivity of as much as a few tens of feet per daybased on tests of similar coarse clastic strata in deepbedrock conditions.

Only seven wells in the County Well Index databasedraw water from the Eau Claire Formation in deepbedrock conditions. These wells are open to the upperpart of the formation in Winona, Houston, and WabashaCounties, where the formation is known to containcoarse clastic interbeds. Well records did not containthe information necessary to calculate hydraulicconductivity.

Shallow bedrock conditions—Scientifically rigoroushydraulic tests of the Eau Claire Formation undershallow bedrock conditions have not been conducted,but detailed, site-specific studies of

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Figure 17. Hydraulic conductivity data for the Mt.Simon Sandstone calculated from specific capacitytests. See Figure 11 for an explanation of box plots.A. Scatter plot showing the relationship betweenthe depth of the open-hole interval below the bedrocksurface and hydraulic conductivity. Shallower wellstend to have higher conductivity.

B. Box plot of hydraulic conductivity values fordeep bedrock conditions. Two outlying valuesgreater than 200 feet per day are not shown.

C. Box plot of hydraulic conductivity values forshallow bedrock conditions. One outlying value of990 feet per day is not shown.

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hydrostratigraphically similar parts of the FranconiaFormation (described later in this report) suggest thatthe Eau Claire Formation in shallow bedrock settingsprobably has a great range in hydraulic conductivity,from 10-2 foot to a few tens of feet per day.

Hydraulic conductivity calculated from specificcapacity tests of 249 wells open to the Eau ClaireFormation in shallow bedrock settings typically rangefrom less than one to as much 100 feet per day, andaverage 36.7 feet per day (Fig. 19). These values arewithin the typical range for coarse clastic units that havea much greater matrix permeability than the Eau ClaireFormation, such as the Jordan aquifer, and thereforesupport the hypothesis that fracture porosity in the EauClaire Formation can at least locally result in moderatelyhigh bulk hydraulic conductivity.

Hydrogeologic synthesisPrevious work based on potentiometric data plotted

at both local and regional scales has clearlydemonstrated that all or part of the Eau Claire Formationhas the ability to function as a confining unit in deepbedrock conditions (for example Delin and Woodward,1984; Miller, 1984), even though intercalations of thecoarse clastic component in its upper part may beutilized as local aquifers in saturated conditions (Runkel,1996a). The "greensand" facies in Faribault andadjacent counties could yield moderate quantities ofwater in deep bedrock conditions, based on relativelyhigh values of plug permeability, but it has not beenhydraulically tested as a discrete hydrogeologic unit.

The hydrogeologic character of the Eau ClaireFormation in shallow bedrock conditions may be muchmore complex than in deep conditions as a result offlow along fractures. Visible seeps occur along bedding-plane fractures in bluffside exposures of the Eau ClaireFormation in Wabasha County along U.S. Highway 61,2.5 miles south of its intersection with U.S. Highway60. In addition, the Eau Claire Formation is used asa source of water for over 400 wells in the County WellIndex database, demonstrating that it can yield moderateto even large quantities of water, and be properlyclassified as an aquifer, in shallow bedrock conditions.The majority of these wells (335) are located in thenorthern part of the Twin Cities Metropolitan area andalong the St. Croix and Mississippi Rivers from ChisagoCounty south to the Iowa border, where the Eau ClaireFormation is the uppermost bedrock. Given the lowintergranular permeability of most of the Eau ClaireFormation, water in these wells is probably drawnchiefly through fracture networks. The relativeeffectiveness and scale at which the Eau ClaireFormation can provide confinement of the underlying

Mt. Simon Sandstone in such a setting is in need offurther investigation.

IRONTON AND GALESVILLESANDSTONES


Matrix porosityThe Ironton and Galesville Sandstones have

historically been combined as a single map unit becauseboth are dominated by the coarse clastic component(Figs. 10, 16, 18). However, this unit can be divided,at least locally, into two subcomponents: a "clean"coarse clastic component and a shaly coarse clasticcomponent (Runkel, 1996a). The latter componentcontains an appreciable amount of silt and shale as thininterbeds, and as matrix between sand grains. Ittypically occurs in the upper one-half of the Ironton–Galesville Sandstone, an interval corresponding to theuppermost Galesville and lowermost Irontonlithostratigraphic units.

The clean and shaly coarse clastic components havenot been studied in the detail necessary to quantitativelycompare their permeability. Plug samples of the cleancoarse clastic component commonly have a permeabilityof 102 to 103 md in both a horizontal and verticaldirection (Figs. 10, 16, 18). The shaly component mayhave a much lower permeability, especially in a verticaldirection, because of the presence of clay and silt-sizedparticles filling pore spaces between sand grains andas thin laminations.


secondary porosity in deep bedrock conditions isextremely limited. Inasmuch as the Ironton–GalesvilleSandstone is a friable, high porosity layer of stratacovered by younger bedrock of contrasting materialproperties, networks of interconnected, open verticalfractures are believed to be poorly developed at best(Price and Cosgrove, 1990; Helgeson and Aydin, 1991;Narr and Suppe, 1991). Thin (a few inches or less)bedding-plane fractures can be seen on video logs ofsome boreholes (for example unique number 255768,Minnesota Department of Health Borehole VideoLibrary), but their abundance and lateral extent is notknown. An unproven assumption by previousinvestigators is that fractures and dissolution featuresare uncommon in the Ironton–Galesville Sandstone indeep bedrock settings and therefore porosity andpermeability is determined chiefly by intergranularattributes.

Shallow bedrock conditions—The

39

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Figure 18. Distribution of porosity andpermeability of the Mt. Simon Sandstonethrough Oneota Dolomite in deep bedrockconditions at Bricelyn. Matrix porosityand permeability is low except in thecoarse clastic component. Secondaryporosity is minimally developed exceptfor dissolution cavities developed inspecific beds of carbonate rock in the St.Lawrence and lower FranconiaFormations, the Oneota Dolomite, and incarbonate-rich intraclasts in the upper partof the Franconia Formation. Plug sampleswere collected at approximately one footintervals. Gray shading on the porosityand permeability logs are estimated valuescorresponding to intervals of no corerecovery, chiefly where coarse clasticbeds were present, and are based on plugtests of nearby cores and from Setterholmand others (1991). Cores Schroeder 1 andSchroeder 5.

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hydrostratigraphic character of the Ironton–GalesvilleSandstone in shallow bedrock conditions has not beenstudied in detail. Systematic and nonsystematicfractures with apertures as wide as a few inches arecommon in the only laterally extensive exposures inMinnesota—a series of road cuts on the west side ofU.S. Highway 61 between the cities of Wabasha andLa Crescent.

Hydraulic attributes Deep bedrock conditions—Discrete interval and

standard aquifer pump tests in southeastern Minnesotaand nearby parts of southwestern Wisconsin yielded arelatively narrow range of hydraulic conductivity valuesfrom a few feet per day to as high as 15 feet per day(Young, 1992; Miller and Delin, 1993). Discreteinterval packer tests of nine boreholes in southeasternWisconsin and Illinois are consistent with those valueswith the exception of one value of 31 feet per day (Figs.10, 13). Vertical hydraulic conductivity values areestimated to be about one-tenth of horizontal values(Miller and Delin, 1993). Discrete interval packer testsby Miller and Delin (1993) at the Aquifer ThermalEnergy Storage Project (ATES) site in St. Paul (Fig.1) indicated that an interval dominated by the cleancoarse clastic component has a horizontal hydraulicconductivity value about 58 percent greater than aninterval dominated by the shaly coarse clastic

component (Fig. 10).Borehole flowmeter investigations of the Ironton–

Galesville Sandstone near Savage, Hastings, and PriorLake (Fig. 1) demonstrate that the coarse clastic stratahave a relatively high intergranular permeabilitycompared to adjacent fine clastic strata of the Eau Claireand Franconia Formations (Fig. 20; Paillet and others,2000; this study). At each of these sites, ambient andinduced borehole flow was not concentrated in thin,discrete intervals within the Ironton–GalesvilleSandstone, but instead was evenly distributed acrossthe coarse clastic component, a characteristic consistentwith flow chiefly through intergranular pore spaces.

Hydraulic conductivity values calculated fromspecific capacity tests of the Ironton–GalesvilleSandstone are shown in Figure 21. In deep bedrockconditions, values typically range from 1.5 to 28 feetper day, with an average of 10.8 feet per day. Thesecalculations are consistent with those determinedthrough higher-quality pump tests and typical inmagnitude for fine- to coarse-grained sandstoneaquifers.

Shallow bedrock conditions—Controlled pump testdata for the Ironton–Galesville Sandstone in shallowbedrock conditions of Minnesota are not available.Bradbury (2001) and Swanson (2001) reported theresults of discrete interval slug tests and borehole

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Figure 19. Hydraulic conductivity data for the Eau Claire Formation calculated from specific capacity tests.See Figure 11 for an explanation of box plots.A. Scatter plot showing the relationship between the depth of the open-hole interval below the bedrock surfaceand hydraulic conductivity. Shallower wells tend to have higher conductivity.

B. Box plot of hydraulic conductivity values for shallow bedrock conditions.

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flowmeter logs of the Ironton–Galesville Sandstone inshallow conditions of southeastern Wisconsin,demonstrating a range in conductivity from 10 to 100feet per day, with a geometric mean of 20 feet per day.They identified two discrete intervals of preferentialflow within the Ironton–Galesville Sandstone.

Hydraulic conductivity values calculated fromspecific capacity tests of wells constructed in shallowbedrock conditions of southeastern Minnesota rangefrom less than one to 60 feet per day, with an averagevalue of 26.8 feet per day (Fig. 21). The relatively highaverage conductivity and large number of outlyingvalues greater than 60 feet per day likely reflect somecomponent of fracture flow.

Hydrogeologic synthesisThe Ironton–Galesville Sandstone can be treated

as a single aquifer in which water moves chiefly throughlarge, well connected intergranular pore spaces in deepbedrock settings. The relative proportion of the shalycoarse clastic component to the clean coarse clasticcomponent may have a measurable control on aquiferproductivity, much as the relative abundance of distincthydrostratigraphic components within the JordanSandstone are known to control its productivity acrosssoutheastern Minnesota (Runkel, 1996b; Runkel andothers, 1999; this report). The relative proportion ofthese two components in the Ironton–GalesvilleSandstone can be estimated, with some difficulty,through gamma logs and cuttings at individual well sites.

The presence of abundant fractures no doubt playsan important role in the hydraulic behavior of theIronton–Galesville Sandstone in shallow bedrockconditions, although fracture flow in this unit has notbeen studied in detail in Minnesota. Preferential flowpaths along narrow intervals of the Ironton–GalesvilleSandstone in Wisconsin (Bradbury, 2001; Swanson,2001) may reflect the presence of hydraulicallysignificant bedding-plane fractures that can dominateflow systems of siliciclastic aquifers where they occurin shallow bedrock conditions (Michalski and Britton,1997; Morin and others, 1997).

FRANCONIA FORMATION


Matrix porosityThe intergranular attributes of the Franconia

Formation are better documented than those of any otherPaleozoic lithostratigraphic unit because porosity andpermeability have been calculated for hundreds of plugssampled from cores collected as far north as St. Pauland as far south as Faribault County (Figs. 10, 16, 18;MUGSP, 1980; Miller and Delin, 1993). Matrix

porosity in outcrops along the Mississippi River hasalso been studied (Runkel and Tipping, 1998).Collectively, this work demonstrates that the bulk ofthe Franconia Formation across southeastern Minnesotais composed of the fine clastic component, withsubordinate beds of carbonate strata. As such theFranconia Formation is similar in grain size,stratification, and degree of cementation to the dominantcomponent of the Eau Claire Formation. Plug samplesof fine clastic and carbonate rock components in theFranconia Formation have a vertical permeability thattypically ranges from 10-6 to 10-2 md, similar to valuescalculated for the Eau Claire Formation.

Coarse clastic beds of relatively high permeabilityare common in the upper part of the FranconiaFormation, and are of sufficient thickness in the northernand eastern Twin Cities Metropolitan area to be definedas a mappable member, the Mazomanie Member (Fig.22). Stratigraphic cross-sections and isopachs of theMazomanie Member indicate that it forms a substantialpart of the Franconia Formation (greater than 20 percentof the formation) in parts of Anoka, Chisago, Hennepin,Isanti, Ramsey, Sherburne, Washington, and WrightCounties. Outside of this eight-county area, thin tomedium coarse clastic beds are a relatively minorcomponent of the Franconia Formation, most commonlypresent in its uppermost part, and in strata that aretransitional with the underlying Ironton Sandstone.Such interbeds tested as high as 340 md in a verticaldirection (Runkel, unpub. data, 1998).

Secondary porosityDeep bedrock conditions—Most of what is known

about secondary porosity in fine clastic strata insoutheastern Minnesota is based on comprehensive, sitespecific investigations of the Franconia Formationconducted over the past ten years (Delta EnvironmentalConsultants, Inc., 1992; Miller and Delin, 1993; Wenckand Associates, Inc., 1997; core logging for this report).In deep bedrock conditions, secondary porosity isbelieved to be relatively uncommon based onexamination of core and borehole video and geophysicallogs (Figs. 10, 16, 18). Bedding-plane fractures in theupper and lower parts of the formation are documented,however (Fig. 20). Additionally, subvertical mesoscopicfractures occur in some intervals but are rare, andtypically are open less than one millimeter or are closed.Dissolution cavities are common in carbonate intraclastsand in a relatively thin, carbonate-dominated intervalin the lower part of the formation.

Shallow bedrock conditions—The FranconiaFormation in shallow bedrock conditions is welldocumented because of an ash disposal site evaluationnear Red Wing (Wenck and Associates, Inc., 1997), and

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a similar but less comprehensive study near Lakeland(Delta Environmental Consultants, Inc., 1992). Wenckand Associates, Inc. (1997) used borehole video logs,cores, and surface exposures at the proposed ashdisposal site near Red Wing to demonstrate that theFranconia Formation is characterized by a densenetwork of nonsystematic vertical and horizontalfractures in shallow bedrock conditions. In one monitorwell at this site (unique number 575374), the FranconiaFormation had 10 horizontal and 2 vertical fracturesthat were of sufficient width to be recognized on aborehole video log along a 131-foot section of openhole. A test pit at the bedrock surface revealed 11subvertical fractures, ranging from 36 to 110 inchesin length, over a surface area of only 162 square feet.Outcrops along the Mississippi River and its tributariesfrom Red Wing south to the Iowa border have verticaland horizontal fractures developed to a comparable orgreater degree than that documented at the proposedash disposal site. Borehole caliper logs collected inAnoka and Washington Counties similarly show the

presence of fractures in the lower Franconia Formation(Fig. 23). These features are a classic manifestationof stress-relief unloading and weathering in thinlybedded, moderately cemented, sedimentary rocks andshould be expected in abundance anywhere theFranconia Formation is within tens of feet of thebedrock surface, particularly along valley walls.

Hydraulic attributesDeep bedrock conditions—Values of horizontal

hydraulic conductivity in the fine clastic componentof the Franconia Formation in deep bedrock conditionsrange from 10-3 to 10-2 foot per day (Fig. 10) based ondiscrete interval packer tests conducted by Miller andDelin (1993). Vertical hydraulic conductivity is roughlytwo orders of magnitude less than horizontal hydraulicconductivity. Several 20-foot intervals of FranconiaFormation tested by Miller and Delin (1993) yieldedno discharge and must have significantly lower hydraulicconductivity values. Coarse clastic beds of the

Figure 20. Borehole geophysical logs of the Eau Claire through St. Lawrence Formations from sites in Savage,Prior Lake, and Hastings. All three logs demonstrate that the Ironton and Galesville Sandstones together havethe properties of an aquifer, whereas the overlying lower to middle Franconia Formation strata serve as a confiningunit. Bedding-plane fractures in the upper Franconia and the St. Lawrence Formations are hydraulically active,even though strata between the fractures may serve as confining units. See Figure 1 for locations of thesesites, and Figure 5 for an explanation of flowmeter logs.A. Flowmeter data were collected while trolling up at 10 feet per minute during injection, and also with thetool stationary at various depths under ambient and 9 gallons per minute injection conditions. The percent ofborehole outflow through the open-hole interval of the well during injection is noted to the right of the flowmeterlogs. Note that the bulk of the outflow is through the coarse clastic component of the Ironton–GalesvilleSandstone. The flowmeter log has a relatively even signature across that interval, consistent with outflow throughintergranular pore spaces. The fine clastic component accounts for only 18 percent of the outflow, from asingle discrete horizon in the lower part of the Franconia Formation. Savage, unique number 593579; modifiedfrom Paillet and others (2000).

B. Trolling (at 10 feet per minute) and stationary flowmeter logs indicate that bedding-plane fractures in thelowermost St. Lawrence and upper part of the Franconia Formations yield water that travels down the boreholeat a mimimum rate of about 7 gallons per minute under ambient conditions. This downflow exits the holegradually, in intergranular fashion, in the Ironton–Galesville Sandstone. The intervening middle to lower FranconiaFormation serves as a confining unit. Prior Lake, unique number 672729.

C. On page 44. Trolling (at 10 feet per minute) and stationary flowmeter logs indicate that the Ironton–GalesvilleSandstone yields water that travels up the borehole at a minimum rate of about 1 gallon per minute underambient conditions. Additional borehole upflow is added through bedding-plane fractures at the St Lawrence–Franconia Formation contact and in the lower part of the St. Lawrence Formation. Water exits the boreholethrough bedding-plane fractures in the upper St. Lawrence Formation, and through a hole in the casing at 84feet (not shown on this figure). The lower to middle Franconia Formation, and the middle part of the St. LawrenceFormation serve as confining units at this site. Hastings, unique number 255768.

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Figure 20 continued on page 44.

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Mazomanie Member in the upper part of the FranconiaFormation, which are about 40 feet thick at the site,have a moderately high horizontal conductivity of about1.4 to 7.5 feet per day, and a bulk conductivity of about2 feet per day.

Hydraulic conductivity calculated from specificcapacity data for the Franconia Formation (Fig. 24)under deep bedrock conditions reflects its intergranularhydrostratigraphic variability. In the northern andeastern Twin Cities Metropolitan area, where theMazomanie Member forms a substantial part of theFranconia Formation (Fig. 22), conductivity typicallyranges from less than one to about 65 feet per day, andaverages 27.8 feet per day. These values are analogousto those calculated for the Jordan Sandstone, which issimilarly dominated by the coarse clastic component.In contrast, outside of the northern and easternmetropolitan areas, where the Mazomanie Member isthin or absent, the Franconia Formation typically rangesin conductivity from less than one to 10 feet per day,with an average value of 5.9 feet per day.

The comprehensive borehole geophysical study ofan observation well near Savage (Paillet and others,2000) yielded information about the relationshipbetween hydrostratigraphic character and ground-waterhydraulics in the lower part of the Franconia Formationand subjacent Ironton–Galesville Sandstone (Fig. 20).Flowmeter logs across 150 feet of open hole thatexposes the upper Eau Claire through lower FranconiaFormations indicated that all the borehole outflowduring injection occurred through intergranular porespaces in the Ironton–Galesville Sandstone and to amuch lesser degree through a four-foot interval withinthe lowermost part of the Franconia Formation. Incontrast, the open-hole intervals that intersect fine clasticstrata, which are the dominant component of both theEau Claire and Franconia Formations, were ofinsufficient conductivity to accommodate measurableoutflow of injected water.

Shallow bedrock conditions—Hydraulicconductivity values under shallow bedrock conditions

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are variable because of the effect of flow along discretefractures. At the ash disposal site near Red Wing,hydraulic conductivity values of the fine clasticcomponent ranged over four orders of magnitude,from 10-2 to 85 feet per day (Wenck and Associates,Inc., 1997). Such a large range in values reflectsthe variable size and degree of connectivity offractures across the site. The hydraulic conductivityof the lower one-half of the Franconia Formationwhere it occurs as the uppermost bedrock inLakeland in southern Washington County (Fig. 1)was measured at between 0.14 and 0.21 foot per day(Delta Environmental Consultants, Inc., 1992).Flowmeter logging of an injected well in Anoka County(Fig. 24) indicated that a single fracture had aconductivity of 90 feet per day, but the overlying 35feet of fine clastic Franconia Formation were ofinsufficient conductivity to accommodate measurable

outflow of the injected water.The Franconia Formation has recently been studied

in southeastern Wisconsin where it shows a similar largerange in hydraulic conductivity in shallow bedrockconditions (Swanson, 2001). It is composed ofinterbedded fine and coarse clastic strata in that area,and the bulk of the formation has an averageconductivity of about 3 feet per day. Ground waterflows preferentially along thin (less than 5 feet) bedding-plane parallel intervals that average 220 feet per dayin conductivity. These intervals appear to be laterallycontinuous across an entire watershed that exceeds 10square miles (Swanson, 2001).

Hydraulic conductivity of the Franconia Formationcalculated from specific capacity tests in southeasternMinnesota also reflects the importance of fractureporosity in shallow bedrock conditions (Fig. 24),especially in areas where the Mazomanie Member is

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Figure 21. Hydraulic conductivity data for theIronton–Galesville Sandstone calculated fromspecific capacity tests. See Figure 11 for anexplanation of box plots.A. Scatter plot showing the relationshipbetween the depth of the open-hole intervalbelow the bedrock surface and hydraulicconductivity. Shallower wells tend to havehigher conductivity.

B. Box plot of hydraulic conductivity valuesfor deep bedrock conditions.

C. Box plot of hydraulic conductivity valuesfor shallow bedrock conditions.

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thin or absent and wells are open chiefly to fine clasticstrata. Outside of the northern and eastern Twin CitiesMetropolitan areas, the Franconia Formation typicallyranges in conductivity from less than one to 40 feetper day, with an average value of 32.3 feet per day,roughly five times greater than its conductivity inconditions where it is deeply buried by younger bedrock.Within the counties where the Mazomanie Memberforms a substantial part of the formation, conductivitytypically ranges from less than one to 75 feet per day,with an average value of 31.7 feet per day.

Hydrogeologic synthesisThe Franconia Formation has previously been

characterized as a single, more or less homogeneoushydrogeologic unit across all of southeastern Minnesota:either as a moderately productive aquifer in goodhydraulic connection with the underlying Ironton–Galesville aquifer (for example Kanivetsky, 1978), oras a confining unit combined with the overlying St.Lawrence Formation (for example Delin and Woodward,1984). Our characterization of the Franconia Formation,which interprets hydraulic data within the context ofits hydrostratigraphic properties, demonstrates that itis very heterogeneous in its hydrogeologic properties.It therefore is best divided into two distincthydrogeologic units: the upper part of the Franconiaaquifer is a widely-used aquifer that yields waterthrough high permeability coarse clastic strata and alongbedding-plane fractures across much of the Twin CitiesMetropolitan area. In contrast the middle to lowerFranconia Formation in the metropolitan area, and theentire formation to the south, is analogous to other lowpermeability fine clastic units described in this report,serving as a regionally extensive confining unit abovethe Ironton–Galesville aquifer.

In deep bedrock conditions, the hydrostratigraphicand hydraulic character of the bulk of the FranconiaFormation is similar to that of the Eau Claire Formation.The plug analyses (Figs. 10, 16, 18), packer tests, andborehole flowmeter logs described above collectivelydemonstrate that from a regional perspective, the bulkof both the Eau Claire and Franconia Formations arecomposed of low conductivity, fine clastic rock that hasthe ability to serve as a confining unit (MUGSP, 1980;Miller, 1984; Miller and Delin, 1993; Paillet and others,2000; core logging for this report). In contrast, theMazomanie Member, which from a regional perspectiveforms a subordinate component of the FranconiaFormation (Fig. 22), is a high-conductivityhydrostratigraphic body that is best considered adiscrete, local aquifer within the regional Franconiaconfining unit. It is largely composed of the coarseclastic component, with hydraulic properties roughly

analogous to those of the Jordan, St. Peter, and Ironton–Galesville Sandstones.

Flowmeter logging of the Franconia Formation andadjacent units in deep bedrock conditions at Prior Lake(Fig. 20B) measured dynamic, strong, ambient flow thatsubstantiates the ability of its middle to lower part tofunction as a confining unit, and also demonstrated thatbedding-plane fractures in its upper few tens of feetcan be hydraulically active. Bedding-plane fracturesin the Franconia Formation (and possibly the lowermostSt. Lawrence Formation) collectively yield a minimumof seven gallons per minute of water to the boreholeunder ambient conditions. This water travels down theborehole, exiting gradually across the Ironton–GalesvilleSandstone. The relatively strong ambient flow thatbypasses the middle and lower Franconia Formation,without significant contribution or subtraction of flow,is evidence that this part of the formation is ofsufficiently low vertical conductivity to serve as aconfining unit. Borehole tests of a number of wellsopen to the Franconia Formation in the western TwinCities metropolitan area (conducted as this report wascompleted) in both shallow and deep bedrock conditionshave yielded similar results.

In shallow bedrock conditions, the FranconiaFormation is much more hydrogeologically complex.At the proposed ash disposal site near Red Wing, groundwater moves in intergranular fashion as well as throughdiscrete hydraulic fractures (Wenck and Associates, Inc.,1997). Borehole videos at the site reveal water thatseeps through intergranular spaces in thin beds of fine-to medium-grained sandstone, particularly in the upperFranconia Formation. Water can also be observedflowing at a much higher rate along fractures developedin fine clastic strata. For example, water cascades froma horizontal fracture 100 feet below the ground surfacein one borehole. The importance of fracture flow isalso reflected by the wide-ranging and locally highvalues of hydraulic conductivity. Three of the fiveintervals of Franconia Formation rock tested at the ashdisposal site had hydraulic conductivities at least twoorders of magnitude higher than the same fine clasticcomponent tested in a borehole with no open fractures(Miller and Delin, 1993). Recently collected flowmeterlogs of injected boreholes open to the fine clastic strataof the Franconia Formation in Anoka and WashingtonCounties show similar conditions whereby theintergranular permeability is of insufficient conductivityto accommodate measurable outflow of injected water.Narrow fractures of a few inches, or coarse clasticinterbeds of the underlying Ironton–GalesvilleSandstone dominate the open-hole hydraulics (Fig. 23).

Potentiometric data indicate that the fine clastic

47

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49

Figure 23. Borehole geophysical logs of the Franconia Formation in shallow bedrock conditions. Flowmeterdata were collected while trolling up at 10 feet per minute during injection, and also with the tool stationaryat various depths under ambient and injection conditions. The logs of both holes demonstrated that the open-hole hydraulics (measurable outflow in these examples) was dominated by thin (less than 1 foot) intervals ofpreferential flow in the Franconia Formation. The remaining parts of the Franconia Formation were of insufficientconductivity to accommodate measurable outflow. See Figure 5 for an explanation of flowmeter logs.

A. Water injected at a rate of about 2.5 gallons per minute exits the borehole through a thin bedding-planefracture that occurs within fine clastic strata of the middle part of the Franconia Formation. Hydraulic conductivityof the fracture is about 90 feet per day based on flow rate and static water level increase during injection.Anoka County unique number 165573.

B. Water injected at a rate of about 3.5 gallons per minute exits the borehole through narrow bedding-planefractures or thin coarse clastic interbeds in the lower Franconia Formation and upper Ironton–Galesville Sandstone.Washington County unique number 227031.

strata of the Franconia Formation have the ability toserve as a confining unit regardless of burial depthbeneath younger bedrock. Differential static waterlevels of 10 to 15 feet between the Ironton–GalesvilleSandstone and upper part of the Franconia Formation(Mazomanie Member) in nested well screens measuredas part of the ATES project investigation demonstratesthat the fine clastic component of the FranconiaFormation serves as a confining unit in deep bedrockconditions (Walton and others, 1991). The lower tomiddle, fine clastic-dominated part of the FranconiaFormation in shallow bedrock conditions near Red Wingis a confining unit that hydraulically separates afractured upper Franconia Formation aquifer from alowermost Franconia/Ironton–Galesville aquifer (Wenckand Associates, Inc., 1997). Potentiometric levels inthe upper part of the formation mimic the land surfaceand show a pattern of local recharge and discharge, traitscharacteristic of water-table aquifers (Figs. 25, 26). Thelowermost Franconia Formation and Ironton–GalesvilleSandstone, perhaps connected by fractures at this site,have potentiometric levels as much as 60 feet lowerin elevation than the water-table aquifer, and consistentwith levels for the larger-scale, regional ground-waterflow system. A site remediation study of a generallysimilar geologic setting near Lakeland in southernWashington County by Braun Intertec (1992) and DeltaEnvironmental Consultants, Inc. (1992) yielded resultsanalogous to those at Red Wing: the middle to lowerpart of the Franconia Formation hydraulically separatesan upper water-table aquifer of fractured bedrock froman Ironton–Galesville aquifer, creating differential headsbetween the two aquifers of more than 40 feet. Pumpingof a remediation well open only to the Ironton–Galesville aquifer caused only a 0.010-foot drawdownin the water-table aquifer at this site.

Field observations of outcrops and springs insoutheastern Minnesota are compatible with subsurfaceresults that suggest ground-water flow in shallowbedrock conditions occurs chiefly along a few discreteintervals with well developed secondary pores (Fig. 26,for example). Secondary pore networks parallel tobedding planes are particularly common in thecarbonate-rich lowermost part of the FranconiaFormation in Minnesota, and are principal water sourcesfor bluffside springs along the Mississippi River, manyof which emit ground water that has a "mixed" tritiumchemistry (Runkel, 1996a; Runkel and Tipping, 1998;E.C. Alexander, Jr., unpub. data). Based on thecharacterization of the Franconia Formation made byWenck and Associates, Inc. (1997) at Red Wing, suchsprings may be supplied by two distinct ground-watersources, which produce a mixed-tritium chemistry whenmixed (Fig. 26): one source is vintage confined waterfrom the regional Ironton–Galesville system that hasupwelled through fractures in the lowermost FranconiaFormation. A second source is recent water locallysupplied through near-surface fractures along the sidesof bluffs. Field observations suggest that the coarseclastic strata of the Mazomanie Member may also bedominated by fracture flow in shallow bedrockconditions. Bluffside springs along the St. Croix Riverare emitted from bedding-plane fractures in theMazomanie Member, best exemplified by the"Boomsite" spring in the north end of Stillwater.

The hydrostratigraphic variability of the FranconiaFormation is reflected by the variable locations andgeologic settings in which it is used as a domestic sourceof water. Its hydraulic conductivity is great enoughto be utilized as an economic source of water chieflywhere it occurs in shallow bedrock settings and wherethe Mazomanie Member is of substantial thickness in

50

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51

both shallow and deep bedrock conditions. Only 4.6percent of the 5,126 wells open to the FranconiaFormation are constructed in deep bedrock settingswhere the Mazomanie Member is thin or absent (CountyWell Index database). Examination of well constructionrecords in nine counties outside of the Twin CitiesMetropolitan area by Runkel (2000) indicated that thinto medium, coarse clastic beds in the uppermost partof the Franconia Formation and carbonate rock havingsmall dissolution cavities and mesoscopic fractures inits lower part apparently are the most conductive partsof the formation in a deep bedrock setting, and supplyrelatively small to moderate yields of water for domesticuse. The remaining 95 percent of Franconia Formationwells in southeastern Minnesota are constructed wherethe formation occurs in shallow bedrock conditions orwhere the Mazomanie Member reaches thicknessesgreater than 20 feet.

The extreme hydrostratigraphic and hydraulicconductivity variability within the Franconia Formationand its ability to serve as a confining unit, even in areaswhere the formation can yield economic quantities ofwater, should be a consideration in ground-watermanagement practices. The thickness and aerial extentof the fine clastic component of the Franconia Formationthat provides confinement is equal to or greater thanthat of most other historically recognized confining unitsin southeastern Minnesota. Additionally, theincreasingly recognized importance of preferential flowalong bedding-plane fractures (this study) has significantimplications for predicting ground-water paths andtravel times.

ST. LAWRENCE FORMATION


Matrix porosityThe St. Lawrence Formation consists of interbeds

of the fine clastic and carbonate rock components.Across most of southeastern Minnesota, it is dominatedby the carbonate rock component in its lower part, andby fine clastic strata in its upper part (Figs. 10, 16, 18).An exception is in the St. Croix River Valley, wherethe St. Lawrence Formation consists almost entirely offine clastic strata. Individual shale beds in the St.Lawrence Formation as thick as several inches arecommon. Vertical permeability measured in plugsamples of the carbonate and fine clastic componentsis very low to low, commonly ranging from 10-6 to 10-

2 md.

Secondary porosityDeep bedrock conditions—Mesoscopic fractures and

dissolution cavities are common in the St. LawrenceFormation regardless of depth of burial. Examinationof seven cores of the St. Lawrence Formation collectedfrom deep bedrock conditions in Faribault, Fillmore,Freeborn, Ramsey, and Rice Counties, and video andcaliper logs of open boreholes in the Twin CitiesMetropolitan area revealed that secondary pores arecommon along discrete intervals (Figs. 3, 9, 16, 18,20B). Dissolution cavities are as large as two inches,and typically elongate in a direction parallel to bedding.The pores exhibit features typical of hydraulic pores,including oxidation and deposits of minerals such aspyrite and calcite.

Shallow bedrock conditions—Outcrops andborehole video log investigations demonstrate that inshallow bedrock conditions the St. Lawrence Formation

Figure 24. Hydraulic conductivity data for the Franconia Formation calculated from specific capacity tests.See Figure 11 for an explanation of box plots.

A. Scatter plot showing the relationship between the depth of the open-hole interval below the bedrock surfaceand hydraulic conductivity. Shallower wells tend to have higher conductivity.

B. Box plot of hydraulic conductivity values for deep bedrock conditions where the coarse clastic-dominatedMazomanie Member is greater than 25 feet thick (Anoka, Chisago, Hennepin, Isanti, Ramsey, Sherburne,Washington, and Wright Counties).C. Box plot of hydraulic conductivity values for shallow bedrock conditions where the Mazomanie Memberis greater than 25 feet thick. Nineteen outlying values greater than 200 feet per day are not shown.

D. Box plot of hydraulic conductivity values for deep bedrock conditions where the Mazomanie Member isless than 25 feet thick.

E. Box plot of conductivity values for shallow bedrock conditions where the Mazomanie Member is less than25 feet thick. One outlying value greater than 400 feet per day not shown (note the vertical scale change inthis plot).

52

contains dissolution features as well as systematic andnonsystematic fractures (Fig. 8). At the proposed ashdisposal site near Red Wing, the St. Lawrence Formationis broken by a dense network of fractures where itoccurs as the uppermost bedrock (Wenck andAssociates, Inc., 1997). A 200-square-foot surfaceexposure contained 28 subvertical fractures that rangedin length from 18 to 60 inches. A single borehole with60 feet of open hole at the site contained two bedding-plane parallel fractures large enough to be visible ona video log.

Outcrops of the St. Lawrence Formation along theMississippi River and its tributaries from Red Wingsouth to the Iowa border have nonsystematic fracturesdeveloped to a comparable or greater degree than thosedocumented at the proposed ash disposal site (Fig. 26).Accessible sites where classic stress-relief fractures anddissolution features typical of the St. LawrenceFormation in shallow bedrock conditions can beexamined are along Winona County Road 15 nearHomer (T. 106 N., R. 6 W., sec. 16, NE, NW, NE), andin a road cut and abandoned quarry along U.S. Highway16 in Houston County near Mound Prairie (T. 104 N.,R. 5 W., sec. 34, NE, SW). Large, laterally extensiveoutcrops of the St. Lawrence Formation are uncommon,but one such exposure on the northwest side of BarnBluff in Red Wing (T. 113 N., R. 14 W., sec. 20) showsthat the formation contains vertical flat-sided fractureswith large apertures (Fig. 8) that are similar to thoseknown to be part of systematic regional-scale joint setsin younger strata, such as in the Platteville Formationand Galena Group.

Hydraulic attributesDeep bedrock conditions—The bulk vertical

hydraulic conductivity of the St. Lawrence Formationwas calculated at 10-5 to 10-4 foot per day in deepbedrock conditions based on pump tests and thermalprofile data as part of the ATES project study(Kanivetsky, 1989). Individual shale beds may havea vertical hydraulic conductivity as low as 10-7 foot perday (Freeze and Cherry, 1979). Bulk horizontal

conductivity was not calculated as part of the ATESproject, but a single packer test of a discrete intervalin the lower part of the St. Lawrence Formation thatcontains dissolution cavities indicated a conductivityfor that part of the formation of 6.7 feet per day (Fig.10), a moderately high value analogous to theconductivity measured in the coarse clastic componentin deep bedrock settings. Packer tests of two boreholesopen to the St. Lawrence Formation in southwesternWisconsin, where the formation is similar inhydrostratigraphic attributes to southeastern Minnesota,yielded values of 9.3 and 20 feet per day (Young, 1992).

Hydraulic conductivity calculated from specificcapacity data for wells that draw water from only theSt. Lawrence Formation in deep bedrock conditionstypically ranges from less than one to 50 feet per day,and averages 14 feet per day (Fig. 27). Yield to thesewells must occur chiefly through bedding-planedissolution cavities, considering the very low to lowintergranular permeability of the fine clastic andcarbonate rock components that compose the St.Lawrence Formation.

Shallow bedrock conditions—Discrete interval andstandard aquifer test data for the St. Lawrence Formationin shallow bedrock conditions are not available at thistime. A limited amount of borehole data and fieldmeasurements indicate that its hydraulic properties inshallow bedrock conditions are markedly variable, andcharacteristic of an aquifer with low intergranularpermeability in which water travels chiefly throughsecondary pores. Preliminary information from anongoing site-remediation project near Blaine includespump tests that indicate a range in specific capacityof over four orders of magnitude, with values as highas 294 gallons per minute per foot (Blaine MunicipalWell Field State Superfund Site; H. Neve, unpub. data,2001). Springs are emitted from the St. LawrenceFormation through enlarged fractures in several placesin southeastern Minnesota, such as the Old HouseSpring in Wabasha County (T. 110 N., R. 11 W., sec.20, NE, SE, SW), where rates of over 100 gallons perminute have been measured (Tipping and others, 2001).

Figure 25. Contour map of the potentiometric surface of the Ironton–Galesville and lower Franconia aquifers(boxed values), and of the local water-table aquifer in the fractured upper Franconia Formation strata at theproposed ash disposal site near Red Wing, Goodhue County. Note that the potentiometric surface for the lowermostfew feet of the Franconia Formation and Ironton–Galesville aquifer, which are part of a regional flow system,is as much as 60 feet lower than the water table. Additionally, ground-water flow in the regional aquifer occursin a direction that varies from that in the water-table aquifer. This indicates that the fine clastic strata in themiddle to lower Franconia at this site provide confinement. See Figure 1 for location. Modified from Wenckand Associates, Inc. (1997).

53

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55

Hydraulic conductivity calculated from specific capacitydata for wells that draw water from only the St.Lawrence Formation in shallow bedrock conditionstypically ranges from less than one to 75 feet per day,and averages 46 feet per day, about three times greaterthan the conductivity in deep bedrock conditions (Fig.27).

Hydrogeologic synthesisIn deep bedrock settings, the St. Lawrence

Formation is best characterized as a unit that has a lowbulk hydraulic conductivity in a vertical direction, andcan therefore serve as a confining unit. Publishedpotentiometric maps (for example Delin and Woodward,1984) cannot be used to test the effectiveness of theSt. Lawrence Formation alone to function as a confiningunit in deep bedrock conditions because water levelsin the upper part of the Franconia Formation are notplotted separately from those in the Ironton–Galesvilleaquifer. However, the relatively low vertical bulkconductivity values measured as part of the ATESproject study, and differential static heads between theSt. Lawrence and upper Franconia Formations at thesame site suggest that the St. Lawrence Formation canprovide vertical confinement. In contrast, bulkhorizontal conductivity of the St. Lawrence Formationis apparently as much as four or five orders of magnitudegreater than vertical conductivity, based on themoderately high values measured with discrete intervalpacker and specific capacity tests (Young, 1992; Millerand Delin, 1993; County Well Index database; thisstudy). Therefore, even though the St. LawrenceFormation can serve as a confining unit in deep bedrocksettings, discrete intervals with interconnected secondarypores can yield moderate to large quantities of water(Howard and Nolen-Hoeksema, 1990).

The results of borehole flowmeter investigationsof the St. Lawrence Formation under ambient ground-water conditions in deep bedrock settings (Figs. 20C,28) reflect the formation's variable hydrogeologicattributes (Paillet and others, 2000; this study).Hydraulically active bedding-plane fractures canaccommodate relatively strong ambient flow, yet partsof the St. Lawrence Formation have the ability toproduce confinement in a vertical direction. At Winona,a substantial amount of measurable inflow to the welloccurs along bedding-plane fractures or dissolutioncavities in the upper and middle part of the St. LawrenceFormation (Fig. 28). Outflow of this water occurs alongSt. Lawrence–Franconia Formation contact strata. Thelowest part of the St. Lawrence Formation at this siteapparently serves as an aquitard, separatinghydraulically active conduits in the upper and middleparts of the formation from the underlying Franconia

Formation. At Hastings, bedding-plane fractures in thelower part of the St. Lawrence Formation contributeda minimum of about a half-gallon per minute of flowthat traveled up the borehole to exit along fractures inthe upper part of the formation (Fig. 20C). A boreholeinvestigation by Tipping and Runkel (unpub. data) nearHugo yielded evidence that similarly demonstrates thepresence of hydraulically significant secondary poresin the St. Lawrence Formation. Substantial downholeflow emitted from the Jordan aquifer exited near thebottom of the borehole chiefly through a dissolution-enlarged bedding-plane fracture in the upper St.Lawrence Formation (Minnesota Department of Healthborehole video library, unique number 645394).

The St. Lawrence Formation in shallow bedrockconditions exhibits several features characteristic ofyounger carbonate rock layers generally considered tobe karstic aquifers in southeastern Minnesota: it containsdissolution-enlarged nonsystematic fractures, cavities,and systematic fractures that may be part of a regionalnetwork. Borehole flowmeter logs collected frommunicipal test wells at Chaska (unique number 665713),Watertown (unique number 658174), and Greenfield(unique number 658157) as part of an ongoing projectconducted as this report was completed demonstratethat bedding-plane fractures are hydraulically active inthe St. Lawrence Formation in ambient conditions, andare among the principal contributors to well yield instressed conditions of pumping or injection. Highcapacity springs originate from these secondary poresalong the sides of bluffs. The St. Lawrence Formationdiffers from the three major karst systems describedlater in this report mostly because it occurs as theuppermost bedrock over a much smaller area ofsoutheastern Minnesota, and it lacks a classic land-surface expression of a karstic terrain. In part this mayreflect the relatively limited subcrop extent of the St.Lawrence Formation where it occurs close to the landsurface, compared to the thicker karst systems that occuracross broad plateaus that have only a thin cover ofunconsolidated material.

The St. Lawrence Formation clearly has all theattributes of a moderate to high yielding aquifer whereit occurs in shallow bedrock conditions, and wasrecently classified as such in Rice County where itoccurs as the uppermost bedrock across a large partof the county (Campion, 1997). Over 400 wells in theCounty Well Index database draw water from the St.Lawrence Formation in shallow bedrock conditions insoutheastern Minnesota, commonly constructed in areaswhere the underlying Franconia Formation is dominatedby the fine clastic component and therefore has pooryields.

56

The relative effectiveness of the St. LawrenceFormation to provide confinement in shallow bedrockconditions needs to be evaluated. It has never beendemonstrated to act as a confining unit across asignificant geographic extent where it occurs as theuppermost bedrock and is known to have high bulkhydraulic conductivity. An unpublished investigationconducted by the Minnesota Department of Health inBlue Earth, LeSueur, and Nicollet Counties measureddifferences in water chemistry above and below the St.Lawrence Formation locally where it occurs in shallowbedrock conditions, suggesting that some part(s) mayprovide confinement even where it is commonly usedas an aquifer. The St. Lawrence Formation maytherefore be analogous in hydrogeologic character tothe fine-clastic dominated Franconia Formation (Fig.23), and younger Paleozoic strata composed chiefly ofthe carbonate rock component. Discrete intervals withminimal development of secondary pores can at leastlocally provide confinement if they are laterallyextensive, whereas other intervals with a greaterabundance of interconnected fractures and dissolutionfeatures are of high enough hydraulic conductivity tosupply moderate to large quantities of water in saturatedconditions.

JORDAN SANDSTONE


Matrix porosity The Jordan Sandstone is composed of coarse clastic

and fine clastic components. Plug samples of the coarseclastic component commonly have permeabilities ofgreater than 1,000 md (Figs. 16, 18; MUGSP, 1980;Setterholm and others, 1991). The fine clasticcomponent is moderately to tightly cemented, very fine-grained sandstone with minor siltstone and shale. Plugsamples have a horizontal permeability that typicallyranges from 10-5 to 10-1 md and vertical permeabilityof 10-5 to 10-3 md.

Cross-sections based on correlated natural gammalogs and cuttings samples (Fig. 29) show that the JordanSandstone internally consists of varying proportions ofcoarse clastic to fine clastic material (Runkel, 1996b;Runkel and others, 1999). The lower 5 to 50 feet ofJordan Sandstone is typically composed of the fineclastic component, and the upper 50 to 80 feet typicallyconsists of the coarse clastic component. Additionally,the two components are intercalated within the JordanSandstone in a stratigraphically complex manner.Tongues of fine clastic component up to 40 feet thickrise diagonally up-section from the base of the JordanSandstone. Individual tongues are as much as several

miles wide and have a crescent shape in cross-sectionsoriented perpendicular to strike.

The open-hole interval interpreted as the "Jordanaquifer" on many water-well drilling records in theCounty Well Index database includes in its uppermostpart the Coon Valley Member of the Oneota Dolomite,a unit composed of an interbedded mixture of the fineclastic, coarse clastic, and carbonate rock components.The bulk of the Coon Valley Member is inferred to havea relatively low to moderate permeability based on asmall number of plug tests and water chemistry datathat suggest it serves as an aquitard (Setterholm andothers, 1991; Runkel, 1996b). There is a large variationfrom place to place in the relative proportions of theCoon Valley Member, fine clastic component, and coarseclastic component across the Jordan aquifer insoutheastern Minnesota. For example, the Jordanaquifer is 80 to 100 feet thick and is internally composedof 50 to 70 feet of the coarse clastic component, and20 to 40 feet of the fine clastic component in some partsof the city of Rochester (Fig. 30; Runkel, 1996b). TheCoon Valley Member is 20 to 30 feet thick in theseareas. Elsewhere in Rochester, the Jordan aquiferconsists entirely of the fine clastic component and CoonValley Member.

Secondary porosityDeep bedrock conditions—An unproven assumption

by previous investigators is that porosity andpermeability in the Jordan Sandstone is determinedchiefly by intergranular attributes. However, limitedcore, video, and caliper log data suggest that secondarypores may be more common than widely believed.Cores and borehole video logs of the Jordan Sandstonein deep bedrock conditions indicate that cavities arerarely present in the fine clastic component of the JordanSandstone, developed locally where carbonate-richintraclasts and fossils have been dissolved (Fig. 18).Mesoscopic fractures occur in the well-cemented fineclastic beds, but are open less than 1 millimeter. Caliperand video logs have also revealed bedding-planefractures in the Jordan Sandstone where it occurs about270 feet below the bedrock surface (MinnesotaDepartment of Health borehole video library, uniquenumber 658966).

Borehole video logs from three sites in the TwinCities Metropolitan area also have revealed the presencein deep bedrock conditions of vertical, systematicfractures with apertures of several inches in the upperpart of the Jordan Sandstone (Minnesota Departmentof Health borehole video library, unique numbers200519, 206169, 205821). Fracture apertures at thesesites were apparently widened when the borehole was

57

blasted with dynamite and bailed in an attempt toincrease productivity. It appears the Jordan Sandstoneat these sites has planes of weakness that were enlargedby these well development procedures. A tentativeinterpretation based on these observations is thatsystematic fractures are present in the Jordan Sandstone(and perhaps other units as well) in deep bedrocksettings, but individual fractures may have relativelynarrow apertures or are closed, and perhaps poorlyconnected at a large scale, compared to their characterin shallow bedrock settings.

Shallow bedrock conditions—Fractures are commonin the Jordan Sandstone in outcrop, particularly in theuppermost 20 feet of the formation where it is locallywell-cemented by calcite. Fractures range from small,irregular, closed mesoscopic fractures, to vertical flat

joints with inch-scale apertures that extend verticallyacross outcrops several tens of feet in height (Fig. 8).Fractures in the Jordan Sandstone have not beenrigorously studied, but cursory observations ofindividual outcrops indicate that vertical fractures aretypically more widely spaced than those in carbonatestrata above and below the Jordan Sandstone.

Hydraulic attributesDeep bedrock conditions—High quality, discrete-

interval tests have not been conducted on the JordanSandstone in southeastern Minnesota. Tests of threeboreholes in southwestern Wisconsin yielded hydraulicconductivity values ranging from 8.0 to 24 feet per day.Standard aquifer tests of 26 wells across southeasternMinnesota indicated that hydraulic conductivity in theJordan Sandstone ranges from about 0.1 to 100 feet

0

100

200

300

400

500

0 100 200 300 400 500 600

Con

duct

ivity

in fe

et p

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ay


0

100

200

300

400

500

Ran

ge

0

100

200

300

400

500

Ran

ge


25 samplesAverage 14 feet per day


Figure 27. Hydraulic conductivity data for the St.Lawrence Formation calculated from specificcapacity tests. See Figure 11 for an explanationof box plots.

A. Scatter plot showing the relationship betweenthe depth of the open-hole interval below thebedrock surface and hydraulic conductivity.Shallower wells tend to have higher conductivity.B. Box plot of hydraulic conductivity values fordeep bedrock conditions.

C. Box plot of hydraulic conductivity values forshallow bedrock conditions. One outlying valueof greater than 500 feet per day is not shown.

C.B.

A.

58

per day and averages 48.5 feet per day. TheseMinnesota values are based on aquifer tests conductedby many different scientists who presumably employeda number of different pumping procedures and methodsof analysis. The lowest values of less than 10 feet perday are based on tests of the Jordan Sandstone in the

Rochester area where the aquifer is known to have alenticular distribution. The fine clastic component ofthe Jordan Sandstone is inferred to have a horizontalhydraulic conductivity of about 10-3 to 10-1 foot per dayand a vertical hydraulic conductivity between 10-5 and10-3 foot per day based on tests of similar facies in the

200

200

250

300

300

350

400

400

450

500

500

550

600

Caliper(inches)

Stationary flow—ambient conditions(gallons per minute)D

epth

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Fine clasticcomponent

Carbonatecomponent

Figure 28. Results of a borehole geophysical investigation in deep bedrock conditions near Winona by Pailletand others (2000). The flowmeter data were obtained under ambient conditions. Note that ambient inflow tothe well is from interbedded carbonate and fine clastic rock in the upper part of the St. Lawrence Formation,and outflow occurs in the upper part of the Franconia Formation. These hydraulically active intervals are separatedby a confining unit that corresponds approximately to the St. Lawrence–Franconia Formation contact. Uniquenumber 235704. See Figure 1 for location, and Figure 5 for an explanation of flowmeter logs.

59

Franconia and Eau Claire Formations (Miller and Delin,1993).

Hydraulic conductivity values calculated fromspecific capacity tests of Jordan Sandstone wells aregraphically depicted in Figure 31. Under deep bedrockconditions, conductivity typically ranges from less thanone to as much as 35 feet per day, with an average valueof 17.4 feet per day. These values fall within the rangeobtained by standard aquifer pump tests, but are muchlower in average value, which may reflect the abilityof standard tests to more fully measure contributionof water from secondary pore networks.

The relatively large range in productivity andhydraulic conductivity values for the Jordan Sandstonein deep bedrock conditions (Fig. 31) in part reflectsconsiderable variability in the thickness of the coarseclastic component, even at the scale of an individualmunicipality (Runkel, 1996b; Runkel and others, 1999).The coarse clastic component has permeability valuesorders of magnitude higher than the fine clasticcomponent. Therefore, under deep bedrock conditions,variations from place to place in coarse clasticcomponent thickness have a measurable impact on theproductivity and transmissivity of wells that draw waterfrom the Jordan aquifer (Runkel, 1996b; Runkel andothers, 1999). Additionally, the large number of highoutlying values may be representative of wells thatintersect deep secondary pores such as bedding-planefractures or networks of systematic fractures.

Shallow bedrock conditions—Hydraulic propertiesof the Jordan Sandstone in shallow bedrock settingsare much more variable than in deep bedrock settings(Runkel and others, 1999). For example, two municipalwells open to the Jordan Sandstone in Bloomington havehydraulic conductivities of 56 and 533 feet per daybased on standard aquifer pump tests, an order ofmagnitude disparity that is characteristic of aquifersthat have some component of fracture flow (Runkel andothers, 1999). Springs with flow rates of hundreds ofgallons per minute are emitted from individual fracturesin the Jordan Sandstone (such as along U.S. Highway76 in Houston County, T. 103 N., R. 6 W., sec. 27).The presence of calcite cement along the walls ofsystematic fractures in many outcrops of JordanSandstone in southeastern Minnesota, and its absencein adjacent, nonfractured strata, indicates that fractureswere once common preferential pathways forpaleoground-water flow (McBride and others, 1994).

Hydraulic conductivity values calculated fromspecific capacity tests for Jordan Sandstone typicallyrange from less than one to as much as 95 feet per day,with an average value of 43.2 feet per day (Fig. 31).The greater range and overall higher average hydraulic

conductivity compared to the Jordan Sandstone in deepbedrock conditions reflects the greater importance offracture flow.

Hydrogeologic synthesisIn deep bedrock conditions, the lithostratigraphic

unit known as the Jordan Sandstone containscomponents best defined as an aquifer as well ascomponents best defined as a confining unit. The Jordanaquifer should refer to the coarse clastic componentthat commonly composes 20 to as much as 80 feet ofthe Jordan Sandstone. Defined in such a manner it ismore analogous in definition and properties to the highporosity intergranular Ironton–Galesville and St. Peteraquifers. The fine clastic component of the JordanSandstone is more similar in hydrostratigraphicproperties to the upper Eau Claire, middle Franconia,and parts of the St. Lawrence Formations. Where itoccurs in the lowermost part of the Jordan Sandstoneit is best considered a confining unit together with theunderlying St. Lawrence Formation. Where it occursas part of the Coon Valley Member on top of the JordanSandstone it is part of the overlying Oneota confiningunit.

Even though the Jordan Sandstone is oftendescribed as an intergranular aquifer, flow alongfractures may actually be volumetrically dominant incertain settings. Flow along fractures should beexpected in shallow bedrock conditions and may occurat least locally in deep bedrock conditions. The knownpresence of fractures in deep bedrock conditions, andthe locally high hydraulic conductivity of the Jordanaquifer compared to conductivity measured by discreteinterval packer tests of similar material in the Ironton–Galesville aquifer (Miller and Delin, 1993), suggest thatsignificant yield to some wells may occur throughfractures.

PRAIRIE DU CHIEN GROUP


Matrix porosityThe Prairie du Chien Group consists of two

formations: the Oneota Dolomite and the overlyingShakopee Formation (Fig. 2; Plates 1, 2). Bothformations consist largely of carbonate rock that hasa low matrix porosity of less than 10 percent, and verylow to low vertical permeability that ranges from 10-4

to 10-1 md (Figs. 16, 18; MUGSP, 1980; Setterholmand others, 1991). Horizontal permeability is typicallyas much as ten times greater than vertical permeability.Fine and coarse clastic interbeds are common in thelowest part of the Oneota Dolomite, the Coon Valley

60

Member, and as a subordinate component throughoutthe stratigraphic extent of the Shakopee Formation.Individual siliciclastic beds are typically less than 3feet thick except in the lower part of the ShakopeeFormation, where the coarse clastic component locallyis as thick as 40 feet and commonly referred to as the"Root Valley" or "New Richmond" Sandstone. The NewRichmond Sandstone has been mapped across much ofsoutheastern Minnesota outside of the Twin CitiesMetropolitan area (Plates 1, 2; Squillace, 1979) and isthe only regionally distributed, substantially thickinterval within the Prairie du Chien Group that has ahigh intergranular permeability.

Secondary porosityDeep bedrock conditions—The Prairie du Chien

Group is similar to the St. Lawrence Formation in thatmacroscopic secondary pores are common along specificintervals of strata even where buried by several hundredfeet of overlying bedrock in Minnesota, and bythousands of feet of bedrock in Iowa (Figs. 9, 32; DesMoines Water Works, 1995). The Shakopee Formationand uppermost Oneota Dolomite have especially welldeveloped secondary porosity, more than that knownfor any other Paleozoic unit in deep bedrock conditionsof southeastern Minnesota. Typical features includedissolution-enlarged horizontal and vertical fracturesup to a centimeter in width, decimeter-scale cavities,and oomoldic pores. Macroscopic secondary pores inthe Oneota Dolomite are much less abundant overall,and appear to be restricted to relatively few discretehorizons compared to their more ubiquitous distributionin the Shakopee Formation. A high density of largesecondary pores is common in a stromatolitic faciesthat is as much as 70 feet thick in the uppermost partof the Oneota Dolomite, and along individual beds asmuch as a few feet thick in the middle to lower partsof the formation (Fig. 32; Plates 1, 2). These highporosity intervals in the Oneota Dolomite are separatedfrom one another by strata as much as several tens offeet thick that have relatively few, small secondarypores.

Shallow bedrock conditions—In shallow bedrockconditions, the Prairie du Chien Group is ubiquitouslyfractured, and secondary pores are more extensivelydeveloped compared to its character in deep bedrockconditions (Fig. 32; Plates 1, 2). All outcrops of thePrairie du Chien Group contain nonsystematic, stress-relief fractures along bedding planes and at high anglesto bedding. Planar, vertical fractures with inch-scaleapertures are also known in the Prairie du Chien Groupand may be part of regional-scale orthogonal systems(Ruhl, 1995; Runkel, 1996a). Dissolution features inthe Prairie du Chien Group in shallow bedrock

conditions appear to be most pervasive in thestratigraphic intervals that contain the greatest densityof secondary pores in deep bedrock conditions; suchas within many of the carbonate beds in the ShakopeeFormation, in the uppermost part of the OneotaDolomite, and in the middle to lower Oneota Dolomite(Figs. 4, 8) along discrete horizons a few feet thick.

The Prairie du Chien Group hosts dozens of caves.Most of these "megascopic pores" are three-dimensionalphreatic maze caves and are developed through thelower Shakopee Formation, New Richmond Sandstone,paleokarstic Shakopee Formation–Oneota Dolomitecontact strata, and in the top portion of the OneotaDolomite. The largest Prairie du Chien Group cavein the region is Crystal Cave, a commercial cave locatedabout thirty miles east of the St. Croix River near SpringValley, Wisconsin. Crystal Cave extends verticallythrough the lower Shakopee Formation and NewRichmond Sandstone and is about one mile long. Atleast two other extensive Prairie du Chien Group cavesare known in Wabasha County, Minnesota (Tipping andothers, 2001). Several smaller Prairie du Chien Groupcaves occur as abandoned phreatic tubes near the top ofOneota Dolomite cliffs from Hastings south to HoustonCounty along the Mississippi River.

Hydraulic attributesHydraulic conductivity in the Prairie du Chien

Group reflects the relative development of secondarypores because the unit is composed chiefly of carbonaterock with very low intergranular permeability. As arelatively thick body of strata with variability in thesize, abundance, and interconnectivity of fractures anddissolution cavities, the Prairie du Chien Group can beexpected to have a range in conductivity of nine or moreorders of magnitude, in part depending on the scale atwhich it is measured (Liesch, 1973; Graese and others,1988; Gianniny and others, 1996; Hoffman andAlexander, 1998). The hydraulic data discussed beloware biased to some extent because most hydraulicconductivity values are calculated from tests ofboreholes constructed so that relatively high porositywater-producing intervals are exposed across the open-hole interval, or are based on field investigations thatmeasure rates of flow along well-developed conduitsystems. Conductivity values of greater than 1,000 feetper day and flow speeds measured in miles per day inthe Prairie du Chien Group are well documented throughthese kinds of investigations (Wheeler, 1993; Pailletand others, 2000). However, the hydraulic propertiesof relatively tight carbonate rock, which can be tensof feet thick in parts of the Prairie du Chien Group,are not well represented in our database because theyhave rarely been tested. Studies of similarly dense

61

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63

carbonate rock have demonstrated hydraulicconductivity values that typically range from 10-6 to10-4 foot per day (for example Liesch, 1973; Libra andHallberg, 1985; Graese and others, 1988; Gianniny andothers, 1996).

Deep bedrock conditions—Hydraulic propertiesobtained from pump tests (Fig. 13), dye-trace andborehole flowmeter investigations (Fig. 33) of the Prairiedu Chien Group in deep bedrock settings demonstratethat flow occurs along discrete intervals, many withvery high conductivity, that are preferentially locatedin the Shakopee Formation and uppermost OneotaDolomite strata. A dye-trace study in Fillmore Countydocumented flow speeds of greater than 6.5 miles perday, largely along Shakopee Formation–OneotaDolomite contact conduits that locally occurred in deepbedrock conditions along the trace. At Faribault, tworelatively narrow horizons with a high concentrationof cavities in Shakopee Formation–Oneota Dolomitecontact strata at a depth between 175 and 200 feet belowthe bedrock surface had conductivities measured at 93and 837 feet per day based on flowmeter logs analyzedin conjunction with pumping and drawdown data (Fig.33; Paillet and others, 2000). An ongoing investigationby Tipping and Runkel (unpub. data) also documentssimilar discrete intervals in the Shakopee Formationand uppermost Oneota Dolomite with high conductivityat a number of other sites in southeastern Minnesota.An approximately 150-foot-thick interval of upperPrairie du Chien Group rock under deep bedrockconditions at Spring Grove has a bulk hydraulicconductivity of 9 feet per day (Eder and Associates,1997).

Hydraulic conductivity of the middle and lowerparts of the Oneota Dolomite is much lower than thatof its upper part. Hydraulic models based on pumpand slug tests in the New Brighton and Arden Hills areasindicated a relatively low vertical conductivity of 1.5x 10-4 foot per day and horizontal conductivity of 7.5x 10-3 foot per day at this site, where lower to middleOneota Dolomite strata occur at a depth that istransitional between shallow and deep bedrockconditions (Camp, Dresser and McKee, 1991). Stratain the overlying Shakopee Formation are five ordersof magnitude higher in conductivity at the same site.A cooperative study by the Minnesota Department ofNatural Resources and Minnesota Department of Healthindicated a similarly low vertical conductivity of 10-4

foot per day at Plainview (Fig. 13).

Hydraulic conductivity values based on specificcapacity tests of wells that draw water from the Prairiedu Chien Group in deep bedrock conditions typicallyrange from less than one to as high as 50 feet per day,

and average 33.53 feet per day (Fig. 34). The largenumber of high outlying values and overall moderateto high average conductivity compared to other partsof the Paleozoic section is consistent with borehole andcore observations that large secondary pores arecommon along specific intervals in the Prairie du ChienGroup in deep bedrock conditions. Runkel (2000) usedspecific capacity values in nine southeastern Minnesotacounties to demonstrate that relative productivity ofwater wells is at least in part dependent upon thestratigraphic position of the open hole: wells that drawwater from the lowermost Shakopee Formation anduppermost Oneota Dolomite contact strata were threetimes more productive than those open only to the upperShakopee Formation or middle to lower part of theOneota Dolomite. The substantially higher averageproductivity of wells open across the contact betweenthese formations apparently reflects the relatively greatabundance of secondary pores and perhaps the localpresence of the New Richmond (Root Valley) Sandstone.

Shallow bedrock conditions—A wide range inhydraulic conductivity has been measured in the Prairiedu Chien Group where it occurs in shallow bedrockconditions, with most measurements collected as partof site-remediation projects (Fig. 13). Pump and slugtests of the Shakopee Formation in the Arden Hills andNew Brighton areas indicated a horizontal conductivityof 163 feet per day and vertical conductivity at 1.75feet per day. Bulk values of 18 and 5.3 feet per daywere calculated based on borehole flowmeter andpumping test data collected from two wells open to theuppermost Oneota Dolomite and Shakopee Formationnear Faribault and Rochester, respectively (Paillet andothers, 2000). The same investigation demonstratedthat contribution to these wells was accommodatedthrough a few discrete horizontal fractures anddissolution features having hydraulic conductivity valuesthat ranged from 2.2 to 1,023 feet per day (Figs. 33,35). Unfractured rock between major water-producinghorizons has much lower hydraulic conductivity, andwas demonstrated at both sites to provide hydraulicseparation of conduits. Variability in hydraulicconductivity was also recorded at the Oronoco Landfillsite near Rochester where discrete 10-foot intervals hadconductivities that ranged from 1.6 to 65 feet per day(RMT, Inc., 1992).

An ongoing investigation conducted by theMinnesota Geological Survey (Tipping and Runkel,2001) has yielded results consistent with those of RMT,Inc. (1992) and Paillet and others (2000). Flowmeterand packer tests of scientific boreholes at Northfieldand Cottage Grove (Figs. 36, 37) demonstrated thatwater in the Prairie du Chien Group travels chiefly along

64

0

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100

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Figure 31. Hydraulic conductivity data for the Jordan Sandstone calculated from specific capacity tests. SeeFigure 11 for an explanation of box plots.A. Scatter plot showing the relationship between the depth of the open-hole interval below the bedrock surfaceand hydraulic conductivity. Shallower wells tend to have higher conductivity.

B. Scatter plot showing positive correlation between the thickness of the coarse clastic component andtransmissivity for wells in the Jordan Sandstone in the Twin Cities Metropolitan area (modified from Runkeland others, 1999).

C. Box plot of hydraulic conductivity values for shallow bedrock conditions. Twenty-five outlying valuesgreater than 250 feet per day are not shown.D. Box plot of hydraulic conductivity values for deep bedrock conditions. Two outlying values greater than250 feet per day are not shown.

0

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65

a few discrete bedding-plane parallel conduit systemsseparated from one another by intervals of carbonaterock of sufficiently low vertical conductivity to serveas confining units. Static head variability within thePrairie du Chien Group is strong enough locally to driveambient flow in a borehole at rates greater than 12gallons per minute.

Dye-trace investigations have measured rapid flowspeeds in the Prairie du Chien Group where it occursin shallow bedrock conditions. Horizontal flow speedsof ground water at the Oronoco site were as rapid as800 feet per day (Donahue and Associates, Inc., 1991).The Fillmore County study by Wheeler (1993) measuredflow speeds as rapid as 6.5 miles per day within conduitsthat occurred largely in conditions of shallow burialbeneath younger bedrock. These and other dye tracesin shallow bedrock conditions of southeastern Minnesotaall yield relatively wide breakthrough curves that havea fine-scale structure. These breakthrough curves aredifferent from the narrow breakthrough curves seen inthe conduits of the Galena–Spillville karst system(described later in this report) and are consistent withmovement through complex anastomosing, turbulentflow systems.

Hydraulic conductivity values calculated fromspecific capacity tests for Prairie du Chien Group wellsin shallow bedrock conditions (Fig. 34) typically rangefrom less than one to 125 feet per day and have anaverage value of 60.8 feet per day, about twice theaverage value for deep bedrock conditions. Wellsconstructed to draw water from Shakopee Formation–Oneota Dolomite contact strata are substantially moreproductive than wells that are not (Runkel, 1999), arelationship similar to that found in deep bedrockconditions.

Hydrogeologic synthesisThe Prairie du Chien Group occurs as the

uppermost bedrock across a wide expanse ofsoutheastern Minnesota where it exhibits all of the usualattributes of karst, including sinkholes, springs, caves,stream sinks, and dry valleys. As such it is consideredthe stratigraphically lowermost of three major "karstsystems." Recharge occurs through fractures anddissolution cavities, and ground water can travel alongbedding-plane parallel conduits at rates that have beenmeasured in miles per day. The differential stratigraphicdistribution of secondary pores is apparently reflectedin some of the karstic characteristics of the Prairie duChien Group: relatively high densities of sinkholes andsprings, corresponding to major bedding-plane parallelconduit systems, occur where the Shakopee Formation–Oneota Dolomite and St. Peter Sandstone–ShakopeeFormation contact strata lie directly beneath the land

surface (Dalgleish and Alexander, 1984; Tipping andothers, 2001). Four catastrophic failures of threewastewater treatment lagoons (Alexander and Book,1984; Jannik and others, 1992; Alexander and others,1993) all occurred in lagoons built on top of ShakopeeFormation–Oneota Dolomite contact strata.

Discrete interval pump tests, dye-traceinvestigations, plug tests of permeability, and boreholeflowmeter studies collectively demonstrate that groundwater travels in the Prairie du Chien Group chieflythrough fractures and solution features having moderateto extremely high values of hydraulic conductivity.Significant flow may also occur along relativelypermeable coarse clastic interbeds, although this hasnot been documented. Intervals of rock that lack thesefeatures are orders of magnitude lower in conductivityand serve as confining units. The distribution of thesepreferential flow paths in deep and shallow bedrockconditions appears to be largely stratigraphicallycontrolled (Fig. 32) and therefore generally predictableon a regional scale. The principal intervals withparticularly high porosity and conductivity include: theShakopee Formation where individual carbonate bedswith macroscopic secondary pores and thin, coarseclastic interbeds are common; uppermost OneotaDolomite strata where large cavities are abundant; andwithin the coarse clastic New Richmond Sandstone. Incontrast, the middle to lower Oneota Dolomite has alower porosity, and hydraulic tests demonstrate acorresponding lower conductivity (Camp, Dresser andMcKee, 1991). On the basis of this distribution ofporosity and related conductivity, the Prairie du ChienGroup can be generally divided into two hydrogeologicunits: a Shakopee aquifer of relatively high hydraulicconductivity that roughly corresponds to the ShakopeeFormation and upper one-third (as much as 70 feet)of the Oneota Dolomite; and an underlying Oneotaconfining unit.

The most comprehensive site-specifichydrogeologic study of the Prairie du Chien Group,conducted at a landfill site near Oronoco in OlmstedCounty (Donahue and Associates, Inc., 1991; RMT, Inc.,1992), indicated that its hydrogeologic character isgenerally similar to that described for many other karsticcarbonate rock aquifers studied over the past 10 years(Fig. 38A; for example Gianniny and others, 1996;Zanini and others, 1998), and in particular reflects thehydrogeologic differences between its upper and lowerparts. Donahue and Associates, Inc. (1991) and RMT,Inc. (1992) used borehole videos, dye tracing, ground-water chemistry, discrete-interval packer tests, andpotentiometric maps to demonstrate that the Prairie duChien Group is divisible into two hydrogeologic unitsthat correspond to the Shakopee aquifer and Oneota

66

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67

confining unit (Fig. 38A). The Shakopee aquifercontains a well-connected network of fractures anddissolution cavities that provide recharge to horizontalconduits across which water flows as rapidly as 800feet per day. In contrast, the Oneota confining unitlacks a well-connected system of vertical fractures anddissolution features and serves as a hydraulic barrierthat separates karstic carbonate rock above, from thecoarse clastic Jordan aquifer below.

Considerable evidence supports the interpretationof a confined Jordan aquifer at the Oronoco landfillsite (Donahue and Associates, Inc., 1991; RMT, Inc.,1992). The Jordan and Shakopee aquifers differ fromone another in potentiometric head by as much as 9feet, and in hydraulic gradients and inferred flowdirections. Furthermore, pumping of the Jordan aquifercaused no measurable drawdown in water levels in theShakopee aquifer. Lastly, the two aquifers arehydrogeochemically isolated from one another: waterin the Shakopee aquifer was extensively impacted bysurface contamination and water in the Jordan aquiferwas not.

The site-specific hydrogeologic depiction in Figure38 can be extrapolated to a regional scale because thelandfill at Oronoco lies in a plateau setting that is typicalof an enormous area of southeastern Minnesota wherethe Prairie du Chien Group occurs as the uppermostbedrock. The results of many other site-specific, aswell as regional-scale studies are consistent with themodel for the Oronoco landfill. For example, Hall andothers (1911) recognized a twofold hydrogeologicdivision of the Prairie du Chien Group that roughlycorresponds to the Shakopee aquifer and Oneotaconfining unit as we define them in this report. Theirwork included documentation of the ability of theOneota Dolomite to confine artesian water in theunderlying Jordan Sandstone across a large expanse ofsoutheastern Minnesota, and similar conditions remaintoday in places such as Northfield, Cannon Falls, andPreston despite heavy water withdrawals from theJordan Sandstone in those areas. A dye-traceinvestigation by Wheeler (1993) indicated that thehydrogeologic model for the Oronoco landfill site isapplicable to a much larger scale in nearby FillmoreCounty (Fig. 38B). Several other investigationsconducted over the past ten years are also best explainedby the presence of a confining unit in the OneotaDolomite. This includes independent hydrologicevidence outside of that collected from the Oronocolandfill study: potentiometric data that indicatedifferential heads above and below the Oneota Dolomite(Kanivetsky, 1988; Tipping, 1992; Barr Engineering,1996), pumping tests and barometric data that document

hydraulic confinement of the Jordan Sandstone (Camp,Dresser and McKee, 1991; Barr Engineering, 1996;Runkel and others, 1999), and by ground-waterchemistry (for example Setterholm and others, 1991;Tipping, 1992; Wall and Regan, 1994). Even thoughthese studies were conducted on the Prairie du ChienGroup in different parts of Minnesota and in differentconditions of burial depth and topography, they are allcompatible with a hydrogeologic model in which theShakopee Formation and upper part of the OneotaDolomite are a karstic aquifer with moderate to highhydraulic conductivity, whereas the middle to lowerOneota Dolomite is of markedly lower conductivity andconfines the Jordan aquifer. The results of recentborehole geophysical studies by Tipping and Runkel(unpub. data) at a number of sites in southeasternMinnesota (Fig. 37, for example) also demonstrate thepresence of an Oneota confining unit.

An evaluation of potentiometric data in Minnesotaand surrounding areas does not support the commonassertion of good hydraulic connection between thePrairie du Chien Group and Jordan Sandstone (forexample Kanivetsky, 1978; Delin and Woodward, 1984).A study in northern Iowa by Horick (1989) providedevidence that the lower part of the Prairie du ChienGroup provides effective confinement at a regional scale(Fig. 39). He showed that the potentiometric surfaceof water in the St. Peter aquifer is 50 to 200 feet higherthan that of the Jordan Sandstone across much ofnortheastern Iowa along its border with Minnesota. Inthe absence of any known beds in the lower St. Peteraquifer that could create such confinement, Horickconcluded that some part of the Prairie du Chien Groupis a "confining interval." In Minnesota, differences instatic levels between the upper part of the Prairie duChien Group and Jordan aquifer have been noted inat least seven counties (Hall and others, 1911;Kanivetsky and Palen, 1982; Kanivetsky, 1984, 1988;Kanivetsky and Cleland, 1990, 1992; Donahue andAssociates, Inc., 1991; RMT, Inc., 1992; BarrEngineering, 1996; Zhang and Kanivetsky, 1996). Thesedifferences in potentiometric head levels have typicallybeen attributed to impermeable beds of "limited" extentin the Prairie du Chien Group (for example Kanivetskyand Palen, 1982; Kanivetsky, 1984, 1988; Kanivetskyand Cleland, 1990, 1992), but no stratigraphic evidenceis provided to support the interpretation that such bedsare genuinely only locally distributed. The results ofthis study support a different interpretation: that thedifferences in head levels noted in these many differentareas reflect the ability of an interval of rock that isof regional extent across southeastern Minnesota, theOneota Dolomite, to provide confinement.

The data summarized above for the Prairie du Chien

68

1023

140

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837279

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Ambientflow

Pumpedflow

Interpretation

St. PeterSandstone

Sha

kope

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orm

atio

n

100

150

200

50

100

150

200 OneotaDolomite

Casingbottom




Location and hydraulic conductivity of dominant

permeable intervals(feet per day)M

atrix

hydr

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raph

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nent

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30 6200 -10 -5 0

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th in

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bel

ow

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ock

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ace

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0 100

Fine clastic component Carbonate component

Figure 33. Results of a borehole geophysical investigation of the carbonate dominated strata of the ShakopeeFormation and uppermost Oneota Dolomite strata at Faribault, Rice County (Paillet and others, 2000). Flowmeterdata were collected under ambient and 2 gallons per minute pumping conditions. Note that water is emittedthrough discrete horizons, typically less than one foot thick, with hydraulic conductivity as high as 1,023 feetper day. Ambient inflow to the well occurs in the upper half of the Shakopee Formation and outflow occurs inthe lower Shakopee Formation and uppermost Oneota Dolomite, demonstrating hydraulic separation of conduitswithin the Shakopee Formation. Unique number 625327. See Figure 1 for location, and Figure 5 for an explanationof flowmeter logs.

69

Group indicate that it should no longer be treated as asingle aquifer combined with the underlying JordanSandstone. Instead it should be divided into a Shakopeeaquifer and Oneota confining unit. The Oneotaconfining unit can be traced across all of southeasternMinnesota area using outcrops, cores, borehole logs,and cuttings. Although discrete beds with a relativelygreat density of secondary pores in the Oneota confiningunit may be of high horizontal hydraulic conductivity,there is strong evidence that bulk vertical conductivityis low enough to provide confinement, analogous inthat respect to the hydrogeologic attributes of the St.Lawrence Formation. As with all other confining unitsin southeastern Minnesota, it provides confinementwhere it is not breached by interconnected networksof secondary pores, a situation that most commonly

occurs locally in shallow bedrock conditions.

ST. PETER SANDSTONE


Matrix porosityThe St. Peter Sandstone is composed chiefly of the

coarse clastic component across most of southeasternMinnesota, but it contains thick (up to 30 feet), laterallyextensive fine clastic beds in its lower one-half in theTwin Cities Metropolitan area, and along the westernsubcrop of Paleozoic rocks south of the metropolitanarea (Fig. 40). Fine clastic beds in the lowermost partof the St. Peter Sandstone are known to occur locallyin other counties across southeastern Minnesota, but

0

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0

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200

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600

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ange


2,195 samplesAverage 60.8 feet per day


Figure 34. Hydraulic conductivity data for thePrairie du Chien Group calculated from specificcapacity tests. See Figure 11 for an explanationof box plots.

A. Scatter plot showing the relationship betweenthe depth of the open-hole interval below thebedrock surface and hydraulic conductivity.Shallower wells tend to have higher conductivity.B. Box plot of hydraulic conductivity values fordeep bedrock conditions. Two outlying valuesgreater than 1,000 feet per day are not shown.

C. Box plot of hydraulic conductivity values forshallow bedrock conditions. Twelve outlying valuesgreater than 1,000 feet per day are not shown.

C.B.

A.

70

are typically only a few feet thick or less, and theirlateral continuity is poorly understood. The coarseclastic component in the St. Peter Sandstone isespecially well-sorted and friable. It is one of the mosttexturally homogeneous units of sandstone known.Laboratory analyses of plug samples indicated it has

a high porosity and permeability (Norvitch and others,1973).

Interbeds of the fine clastic component have notbeen tested for permeability, but the attributesobservable in outcrop and core samples suggest thatthe fine clastic rocks are similar to beds of older

-2 20

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Ambientflow

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ata

Location and hydraulic conductivity of dominant

permeable intervals(feet per day)

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Dep

th in

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bel

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the

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sur

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ock

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ace

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Coarse clastic component Fine clastic component Carbonate component

Figure 35. Results of a borehole geophysical investigation of the carbonate-dominated strata of the ShakopeeFormation at Rochester, Olmsted County (Paillet and others, 2000). Flowmeter data were collected under ambientand 2 gallons per minute pumping conditions. Note that water is emitted through discrete horizons, typicallyless than one foot thick, with hydraulic conductivity as high as 440 feet per day. The ambient and pumpedflowmeter logs indicate that conduits are hydraulically separated: there is a small head difference driving inflow up theborehole from the conduit that lies 50 feet beneath the bedrock surface. Relatively minor inflow in the two conduitsbelow is not markedly affected by pumping because their inflow is driven up the borehole by a head difference that islarger than the drawdown produced by the pumping. Unique number 485610. See Figure 1 for location, and Figure 5for an explanation of flowmeter logs.

71

Paleozoic strata that have a vertical permeability from10-7 to 10-5 md.

Secondary porosityDeep bedrock conditions—The conventional view

is that interconnected networks of open vertical fracturesare poorly developed in a friable, high porosity unitsuch as the St. Peter Sandstone where it is covered byyounger bedrock layers of contrasting materialproperties. An unproven assumption by previousinvestigators is that fractures and dissolution featuresare rare to absent in the St. Peter Sandstone in deepbedrock settings and therefore porosity and permeabilityis determined chiefly by intergranular attributes.

Shallow bedrock conditions—The St. PeterSandstone in shallow bedrock settings containssystematic fractures with apertures as wide as a fewinches. They are similar to fractures developed inyounger carbonate strata, except they are more widelyspaced, are not solution enlarged, and are commonlyfilled with friable sand. Easily accessible examplesof such fractures occur in roadcut exposures in theRochester area (such as a roadcut in the southwestcorner of T. 107 N., R. 14 W., sec. 32), and in bluffsideexposures along the Mississippi River in downtown St.Paul. Norvitch and Walton (1979) also noted thepresence of fractures in shallow excavations of the St.Peter Sandstone beneath the overlying Glenwood andPlatteville Formations.

Large voids routinely develop in the St. PeterSandstone. Caves up to hundreds of feet long are knownin the sandstone from the Twin Cities Metropolitan areasouth to the Iowa border. The voids are believed tobe the result of either mechanical erosion of thesandstone by turbulent water flow or of collapse intosolutional voids in the underlying Shakopee Formationcarbonates. These voids are sufficiently common inthe Rochester area to have affected the foundationengineering of several buildings.

Hydraulic attributesDeep bedrock conditions—Hydraulic conductivity

of the St. Peter Sandstone in deep bedrock in Illinoisand southeastern Wisconsin falls within a relativelynarrow range of values from 1.3 to 8.4 feet per day(Nicholas and others, 1987; Graese and others, 1988;Young, 1992). The St. Peter Sandstone in those areasis similar in texture to the formation in Minnesota, butmay have a greater degree of cementation (Hoholickand others, 1984). Hydraulic conductivity valuescalculated from specific capacity data (Fig. 41) collectedfrom wells in deep bedrock conditions of southeasternMinnesota average 15.9 feet per day, with a typicalrange of about 2 to 50 feet per day. Fine clastic beds

in the lower St. Peter Sandstone have not been testedfor hydraulic conductivity, but leakage rates calculatedfrom pump tests and mathematical models suggest anoverall low vertical conductivity of 10-3 foot per day(Schoenberg, 1990).

Shallow bedrock conditions—Standard aquifer testsof several boreholes in the Twin Cities Metropolitanarea and of a single well in Rice County yieldedconductivity values that ranged from 20 to 30 feet perday for the St. Peter Sandstone in shallow bedrockconditions (Barr Engineering, 1976, 1986; Madsen andNorvitch, 1979). Conductivity values based on specificcapacity tests of wells in shallow bedrock conditions(Fig. 41) typically range from less than 1 to 75 feetper day, with an average value of 38.7 feet per day.The wide range and overall higher average conductivitycompared to the St. Peter Sandstone in deep bedrockconditions reflects the greater significance ofcontribution from fracture flow.

Hydrogeologic synthesisThe common depiction of the upper part of the St.

Peter Sandstone as a relatively homogeneous,intergranular aquifer of moderate to high hydraulicconductivity is possibly an adequate model for deepbedrock conditions. County-scale potentiometric mapsdemonstrate that fine clastic strata in the lower part ofthe formation serve as confining units that hydraulicallyseparate the St. Peter aquifer from the underlyingShakopee aquifer (for example Kanivetsky and Cleland,1992) in the Twin Cities Metropolitan area. Thepresence of similar fine clastic beds in the lower St.Peter Sandstone outside of the metropolitan areaindicates that the Shakopee aquifer may be hydraulicallyseparated from the St. Peter aquifer on a larger scaleacross southeastern Minnesota, particularly along thewestern subcrop extent of Paleozoic strata (Fig. 40).To the east, fine clastic strata are much thinner, andtheir ability to serve as a confining unit is not known.Lindgren (2001) suggested that the lower St. PeterSandstone in the Rochester area locally providesconfinement where the fine clastic beds are at most onlya few feet thick. The St. Peter and Shakopee aquifershave historically been inferred to be hydraulically wellconnected where these fine clastic interbeds are absent,and local hydrogeologic studies demonstrate someconnectivity (Delin, 1991).

The hydrogeologic significance of fractures andlarge voids in shallow bedrock conditions has not beeninvestigated in systematic fashion. The large aperturesand abundance of fractures in outcrop, and the relativelyhigh hydraulic conductivity of the St. Peter aquifer inshallow bedrock conditions across southeasternMinnesota compared to deep conditions of burial

72

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200

Qua

ter-

nary

casingbottom

staticwaterlevel

Figure 36. Results of a borehole geophysical investigation of the carbonate-dominated strata of the Prairie du ChienGroup and underlying coarse clastic strata of the Jordan Sandstone at Hamlet Park in Cottage Grove, Washington County.Flowmeter data were collected under ambient ground-water conditions with the tool moving up-hole at a rate of 10 feetper minute ("trolling" log), as well as with the tool at stationary positions. Note that water enters the borehole chieflythrough large secondary pores in the upper part of the Shakopee Formation. That water travels down the hole at a ratemeasured at nearly 15 gallons per minute, and exits the borehole through large cavities at a discrete horizon in thelower part of the Shakopee Formation. Such a borehole flow pattern, in conjunction with information from the caliperlog, static water levels, temperature profile, and chloride concentrations, all indicate that flow within the Shakopee aquiferoccurs along discrete, bedding-plane parallel conduits separated from one another by confining units. Similar boreholetests in other parts of southeastern Minnesota (Tipping and Runkel, unpub. data) are consistent with such an interpretation.

Measurable downflow of lesser magnitude occurs between the lower Shakopee Formation and Jordan Sandstone.Such flow is driven by differential heads between the Shakopee and Jordan aquifers. Static water levels measured atdiscrete intervals indicate that the Oneota Dolomite serves as a confining unit separating the two aquifers. Unique number658965. See Figure 1 for location, and Figure 5 for an explanation of flowmeter logs.

73

12

0Pum

p ra

te

(gal

/min

)

Below pump limit

Flow measurementtaken

Packer status: topand bottom closed

920

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ater

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f)

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9

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(C

)an

d pH

05

1015

Chl

orid

e (m

g/l)

pH

Jordan SandstoneOneota Dolomite

Prairie du Chien GroupShakopee Formation

Qua

ter-

nary

50 100

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Major cavities(based on videoand caliper logs)

56

78

910

Hol

e di

amet

er(in

ches

)20

250

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10050

casi

ngbo

ttom

stat

icw

ater

leve

l

Trol

ling

flow

—am

bien

t(g

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ns p

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inut

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tatio

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flow

(gal

lons

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ute)

05

1015

05

1015

Am

bien

t

Pum

ping

34%

14%

51%

Location and percent oftransmissivity of dominantpermeable intervals

Depth in feet below the land surface andbedrock surface (italics)

Matrix hydrostratigraphic component

Gam

ma

log

(AP

I uni

ts)

010

0

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ker

test

s

casi

ngbo

ttom

stat

icw

ater

leve

l

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rse

clas

tic c

ompo

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bona

te c

ompo

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ge 7

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re 3

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74

suggest that fracture flow may be regionally significant.Fractures in the St. Peter Sandstone are known to atleast locally provide conduits through which groundwater can move much more rapidly than rates predictedunder the assumption of intergranular flow only.Preferential flow through fractures was noted in shallowexcavations of the St. Peter Sandstone in the Twin CitiesMetropolitan area (Norvitch and Walton, 1979), andsmall streams in Fillmore County and surrounding areasare known to completely empty into fractures in theSt. Peter Sandstone. A good example of such a featureis a stream sink near the headwaters of an intermittenttributary to Watson Creek about 0.25 mile south ofFillmore County Highway 8 (T. 103 N., R. 10 W., sec.18, BCCAC).

GLENWOOD FORMATION


Matrix porosityThe Glenwood Formation is composed chiefly of

the fine clastic component, mostly shale and siltstone.Plug samples from a core in Faribault County hadvertical permeability of 10-5 to 10-4 md (MUGSP, 1980).Friable coarse clastic sandstone beds that are a minorcomponent of the Glenwood Formation have not beentested, but similar sandstone in other parts of thePaleozoic section is of high porosity and permeability.

Secondary porosityDeep bedrock conditions—As a relatively ductile,

high porosity unit in a layered sequence of bedrockstrata, open fractures in deep bedrock settings areprobably uncommon in the Glenwood Formation. Nonewere visible in the single core examined as part of ourinvestigation (Fig. 42).

Shallow bedrock conditions—Open fractures arecommon in the Glenwood Formation in shallow bedrock

Figure 37. Figure appears on page 73. Results of a borehole geophysical investigation of the carbonate-dominatedstrata of the Prairie du Chien Group and underlying coarse clastic strata of the Jordan Sandstone at Carleton College inNorthfield, Rice County. Flowmeter data were collected under ambient ground-water conditions with the tool movingup-hole at a rate of 10 feet per minute ("trolling" log), as well as with the tool at stationary positions. Note that waterenters the borehole chiefly through the coarse clastic strata of the Jordan Sandstone near the bottom of the hole. Thatwater travels up the hole, past the entire Oneota Dolomite with negligible loss, at rates greater than 12 gallons per minute.This upflow exits the borehole through large dissolution cavities at a discrete horizon that approximates the ShakopeeFormation–Oneota Dolomite contact. Such a borehole flow pattern, in conjunction with information from the caliperlog, static water levels, temperature profile, and chloride concentrations, all strongly indicate that the lower part of theOneota Dolomite confines the Jordan Sandstone at this site. Similar borehole tests in other parts of southeastern Minnesota(Tipping and Runkel, unpub. data) are consistent with such an interpretation.

A borehole video log of this well reveals that the Jordan aquifer contains bedding-plane and narrow, subvertical fractures.The gradual change in rate of flow across the Jordan aquifer on the flowmeter logs apparently records inflow throughintergranular pore spaces, whereas the abrupt shifts in rate records contribution through fractures. Unique number 658966.See Figure 1 for location, and Figure 5 for an explanation of flowmeter logs.

Figure 38. Hydrogeologic character of the Prairie du Chien Group and Jordan Sandstone in shallow to deep bedrockconditions at a landfill near Oronoco (Olmsted County), and in eastern Fillmore County. See Figure 1 for location.

A. At Oronoco, a water-table aquifer lies in the Shakopee Formation and uppermost Oneota Dolomite, which have abundantfractures and dissolution features. The Jordan Sandstone at this site is an intergranular aquifer with a regional ground-water system. The two aquifers are hydraulically separated by Oneota Dolomite with relatively few secondary pores.This depiction is based on borehole videos, gamma and caliper logs, cuttings, dye tracing, water chemistry, and potentiometriclevels. Modified from Donahue and Associates, Inc. (1991).

B. Conduit flow in the Prairie du Chien Group in eastern Fillmore County based on dye-trace investigations. Note thesimilarity of hydrogeologic conditions to those at Oronoco. Modified from Alexander and Lively (1995).

75

Spr

ings

Gal

ena

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orm

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n

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orah

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illow

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Mem

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76

conditions, and some fractures extend vertically acrossthe entire formation (outcrop observations for thisstudy). Their abundance, interconnectivity, and presencein the subsurface have not been studied.

Hydraulic attributesThe Glenwood Formation has not been subjected

to discrete interval packer testing, but based on its highshale content it can be expected to have a verticalhydraulic conductivity of about 10-7 to 10-5 foot per day(Freeze and Cherry, 1979) where secondary pores areabsent, and variable conductivity where it contains openfractures such as those known to occur in outcrop.

Hydrogeologic synthesis The Glenwood Formation is a low hydraulic

conductivity unit that is known to function as aconfining bed in shallow as well as deep bedrockconditions. Perched water-table aquifers are commonon top of the Glenwood Formation in shallow bedrocksettings, and its contact with the overlying PlattevilleFormation is a common source of springs (for exampleHall and others, 1911; Brick, 1997). However, theGlenwood Formation is commonly thin enough thatminor vertical fractures will entirely breach it, locallyallowing hydraulic connection between the overlyingPlatteville Formation and the underlying St. PeterSandstone. It is believed that such fractures accountfor large volumes of water locally recharged into theSt. Peter Sandstone where it is capped by the GlenwoodFormation (Delin and Almendinger, 1993; Lindgren,2001).

PLATTEVILLE FORMATION


Matrix porosityThe Platteville Formation is composed chiefly of

the carbonate rock component. Thin shale laminae arecommon, including regionally traceable bentonites.Plug tests of the carbonate rock indicate a very low tolow permeability ranging from 10-7 to 10-4 md (MUGSP,1980).

Secondary porosityDeep bedrock conditions—Little is known about the

Platteville Formation in deep bedrock conditions insoutheastern Minnesota. A single core examined aspart of this study had no visible fractures or dissolutionfeatures (Fig. 42). Open joints and fractures areuncommon in cores of the Platteville Formationcollected from deep bedrock conditions in Illinois(Kempton and others, 1987; Curry and others, 1988).

Shallow bedrock conditions—Outcrop and

subsurface studies in Minnesota and Wisconsin havedemonstrated that in shallow bedrock conditions thePlatteville Formation contains discrete intervals withrelatively well developed secondary porosity, separatedby intervals with much lower porosity (for example BarrEngineering, 1994, 2000; Brick, 1997; Stocks, 1998).The Platteville Formation is well known for containingbedding-plane and vertical fractures typical of stress-relief conditions, and it also has vertical, flat fracturesthat are part of a large-scale, orthogonal system (forexample Runkel, 1996a; Barr, 2001). Individualfractures commonly cut across the entire formationvertically, and are open as much as a few inches.

Dissolution-enlarged fractures and cavities arelocally large enough to permit human exploration andto lead to the development of sinkholes (Spong, 1980;Hoffman and Alexander, 1998). A 2,000-foot maze cavehas developed in the Platteville Formation in FillmoreCounty (Spong, 1980). In areas where the PlattevilleFormation is the uppermost bedrock, an extensiveepikarst system is commonly developed that presentsa variety of challenges to construction activity.

Hydraulic attributesDeep bedrock conditions—Discrete interval packer

tests of the Platteville Formation in deep bedrockconditions of Minnesota are not available. Acomprehensive hydraulic investigation of the formationin Illinois, where it is generally similar inhydrostratigraphic properties to the Platteville Formationin Minnesota, indicated that its hydraulic conductivitytypically ranges from 10-3 to 10-2 foot per day. Twoboreholes intersected discrete horizons with small, openfractures and packer tests indicated conductivities ashigh as 10-1 foot per day (Kempton and others, 1987;Curry and others, 1988). In Illinois, pump testsconducted across an open-hole interval exposinghundreds of feet of carbonate rock in the Galena Groupand Platteville Formation caused a drawdown of over256 feet, and failed to yield 15 gallons per minute, theminimum pump capacity. This implies a very low bulkhydraulic conductivity of less than 10-2 foot per day.Similar hydraulic conductivity values were obtained inanalogous geologic settings of deep burial in Indianaand Wisconsin (Nicholas and others, 1987). A monitorwell installed by Rowden and Libra (1990) in thePlatteville Formation in northeast Iowa remained dry.

Shallow bedrock conditions—The hydraulicconductivities of the Platteville Formation where itoccurs in shallow bedrock conditions (Fig. 13) havebeen measured in detail at a number of sites in the TwinCities Metropolitan area, such as at Minnesota PollutionControl Agency Superfund remediation sites (forexample ERT, 1987; Barr Engineering, 1991; ENSR

77

International, 1991), and at a proposed tunnel excavation(Liesch, 1973). Hoffman and Alexander (1998) reportedthat the hydraulic conductivity of the PlattevilleFormation at these and other Twin Cities Metropolitanarea sites ranges over at least six orders of magnitude.It has extremely high hydraulic conductivity wheresecondary porosity is well developed, and extremelylow conductivity where such features are poorlydeveloped. At individual sites, large-scale permeabilityvalues based on pump tests have been calculated to beso high as to be considered infinite for specific intervalsof the Platteville Formation, whereas other intervalstested were below the measurement threshold andtherefore may be substantially lower than 10-1 foot perday (Fig. 13). Flow speeds of ground water in the

Platteville Formation in a shallow bedrock setting havebeen calculated to be faster than one mile per day inWisconsin (Hoffman and Alexander, 1998) and in theTwin Cities Metropolitan area (Alexander and others,2001). The packer testing conducted by Kempton andothers (1987) and Curry and others (1988) on thePlatteville Formation in Illinois demonstrated that itsconductivity is roughly two orders of magnitude higherin shallow bedrock conditions than in conditions of deepburial, particularly where the rock is within theuppermost 40 feet of the bedrock surface. In thosesettings hydraulic conductivity ranged from 10-2 to 3feet per day.

Conductivity calculated from specific capacity testsof water wells in southeastern Minnesota typically

Fillmore Co.Mower Co. Houston Co. MINNESOTAIOWA

Mississippi

River

900

900

800800

700

700

600

900

900

1000

1100

1000

0 20 mi

0 30 km

Jordan aquifer potentiometric surface

St. Peter aquifer potentiometric surface

Figure 39. Potentiometric surfaces of the St. Peter aquifer (dashed line) and Jordan aquifer (solid line) in northeasternIowa, demonstrating that part(s) of the intervening Prairie du Chien Group strata serve as a confining unit that createshydraulic separation of the two aquifers. Modified from Horick (1989).

78

A'

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Ram

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2360

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100 50 0 fe

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up

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79

ranges from 2 to 130 feet per day, with an average valueof 72 feet per day (Fig. 43).

Hydrogeologic synthesisThe hydrogeologic properties of the Platteville

Formation vary tremendously. In deep bedrockconditions, its low bulk hydraulic conductivity andrelatively minor development of secondary pores suggestthat it is most properly considered a confining unit.However, the discrete interval hydraulic tests conductedin Illinois (Kempton and others, 1987; Curry and others,1988) demonstrated that relatively thin and widelyspaced intervals of the formation can yield moderatequantities of water in such conditions of deep burial.

In shallow bedrock conditions, the PlattevilleFormation is clearly a classic karstic aquifer, similarto the properties of the Galena aquifer described laterin this report. It contains hydraulically significantfractures and dissolution features, as well as sinkholesand large caverns characteristic of classic karsticaquifers (Liesch, 1973; Barr Engineering, 1983, 1991,1994; Hoffman and Alexander, 1998). Secondary poresappear to be most densely concentrated along a fewspecific stratigraphic intervals of the PlattevilleFormation, forming bedding-plane parallel conduitnetworks that provide preferential flow paths. Brick(1997) used outcrop attributes and the stratigraphicposition of springs to demonstrate that bedding-planeparallel conduits occur at predictable stratigraphicpositions within the Platteville Formation in the TwinCities Metropolitan area. Discrete bedding-planeparallel conduit systems were also identified in thesubsurface at the East Hennepin Avenue–General MillsSolvent Disposal Superfund site in Minneapolis (BarrEngineering, 1983, 1994, 2000). Individual conduitsystems at the site differ from one another in static headand in inferred flow direction. They are separated fromone another by bentonite beds or intervals of unfracturedcarbonate rock that serve as local confining units.Stocks (1998) described similar subsurface attributesin an investigation in Wisconsin, suggesting that groundwater traveled chiefly along discrete intervals ofrelatively high secondary porosity, separated from oneanother by low conductivity aquitards. A recentlycompleted dye-trace study by Alexander and others(2001) at the Camp Coldwater Spring site in the TwinCities Metropolitan area demonstrated that ground watercan travel at rates faster than one mile per day alongsuch bedding-plane systems. Dye pulses were recordedfor weeks to months after initial injection, reflectingan anastomosing pore system with variable flow speedsamong the individual interconnected conduits.

Recent ground-water models developed for site-

specific settings in the Twin Cities Metropolitan areaand near Rochester have demonstrated that thePlatteville Formation in those areas does not by itselfeffectively serve as a confining unit in shallow bedrockconditions. In the St. Louis Park area, contaminantswere transported through the Platteville Formation ina well-developed network of secondary pores where theformation occurs as the uppermost bedrock (Lindgren,1995). Near Rochester, a comprehensive ground-watermodel (Lindgren, 2001) demonstrated that a substantialamount of recharge to the St. Peter Sandstone occursvia fractures that are interconnected vertically throughthe Platteville Formation and underlying GlenwoodFormation where those units occur near the sides ofbluffs (Fig. 44).

The common hydrogeologic depiction of thePlatteville Formation as a confining bed (for exampleKanivetsky, 1978; Delin and Woodward, 1984) isapparently accurate only for areas where it is deeplyburied by younger bedrock and has negligibledevelopment of secondary porosity. In such a settingthe term "Decorah–Platteville–Glenwood confining unit"is appropriately applied to that part of the stratigraphicsection. In shallow bedrock conditions, however, thePlatteville Formation is more hydrogeologicallycomplex. It is a karstic carbonate aquifer that servesas a source of water for over 500 wells in the CountyWell Index database. It has the potential to provideconfinement locally only where it is not breached byinterconnected vertical fracture networks (BarrEngineering, 1983; Lindgren, 2001). The PlattevilleFormation is generally similar in hydrogeologicproperties to other carbonate rock layers in southeasternMinnesota, such as the Prosser Limestone, which havehistorically been accepted as karstic aquifers. ThePlatteville Formation is not considered a "major" karstsystem in this report because it is relatively thin andhas a limited distribution as uppermost bedrockcompared to the major karst systems.

DECORAH SHALE


Matrix porosityThe Decorah Shale is composed chiefly of the fine

clastic component, with subordinate interbeds ofcarbonate rock. It is over 90 feet thick in the TwinCities Metropolitan area, and thins to less than 30 feetat the Iowa border. Most of the Decorah Shale isactually shale, with a vertical permeability that rangesfrom 10-5 to 10-4 md based on plug tests of similar shalein the Glenwood Formation. The carbonate interbedsin the Decorah Shale are similar to those in the

80

Platteville Formation, and therefore probably havesimilarly low matrix permeabilities.

Secondary porosityDeep bedrock conditions—Secondary porosity

characteristics in the Decorah Shale have not beenstudied in deep bedrock conditions. As a relativelyductile, high porosity unit in a layered sequence ofbedrock strata, open fractures are assumed to beuncommon to absent.

Shallow bedrock conditions—Open fractures in theDecorah Shale are known to occur in shallow bedrocksettings (Fig. 8; Hall and others, 1911). These includenonsystematic stress relief fractures and orthogonalfracture sets that may be part of a regional system.

Hydraulic attributesDeep bedrock conditions—The Decorah Shale has

not been subjected to discrete interval packer tests, butbased on the large shale content it can be expected tohave a vertical hydraulic conductivity of about 10-7 to10-5 foot per day (Freeze and Cherry, 1979).

Shallow bedrock conditions—Hydraulicconductivity values calculated from specific capacitydata for six wells in Ramsey and Steele Counties rangefrom about 2 to nearly 160 feet per day, and average60.1 feet per day (Fig. 45). This suggests that fractureporosity can result in the development of moderatehydraulic conductivity.

Hydrogeologic synthesis

0

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Con

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et p

er d

ay


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Ran

ge


Ran

ge

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Figure 41. Hydraulic conductivity data for the St.Peter Sandstone calculated from specific capacity tests.See Figure 11 for an explanation of box plots.A. Scatter plot showing the relationship between thedepth of the open-hole interval below the bedrocksurface and hydraulic conductivity. Shallower wellstend to have higher conductivity.

B. Box plot of hydraulic conductivity values for deepbedrock conditions.

C. Box plot of hydraulic conductivity values forshallow bedrock conditions. Nineteen outlying valuesgreater than 250 feet per day are not shown.

C.B.

A.

81

Many studies have demonstrated that the DecorahShale serves as an effective confining bed, even inshallow bedrock conditions, and we classify it as aconfining unit together with the interbedded shale andcarbonate strata of the overlying CummingsvilleFormation. Perched water-table aquifers are commonabove the Decorah Shale, and springs are preferentiallylocated along hillsides at elevations that correspond tothe top of the formation or to the lower part of theCummingsville Formation. These springs approximatethe position of an important hydrogeologic boundarythat results in a process called "focused recharge" (Fig.44). Focused recharge occurs along hillsides wherelarge volumes of water emitted from the Galena aquifertravel rapidly downward across the eroded edge of theDecorah Shale and Platteville and GlenwoodFormations, eventually entering the St. Peter aquiferin a relatively limited, or "focused" area of recharge(for example Lindgren, 2001). Water moving from theGalena aquifer to the St. Peter aquifer in these areastravels at the surface, through thin surficial deposits,and along secondary pore networks in the shallowbedrock.

The hydrogeologic significance of open fracturesthat are known to occur in shallow bedrock conditionshas not been evaluated, but 25 wells in southeasternMinnesota use the Decorah Shale as a water source,suggesting that such fractures can yield economicquantities of water.

GALENA THROUGH CEDAR VALLEYGROUPS

The Paleozoic strata from the base of the GalenaGroup through the preserved thickness of the CedarValley Group are composed mostly of carbonate rock,with subordinate beds of the fine clastic component,chiefly shale (Fig. 46). Although the entire stratigraphicsection has historically been treated as a single aquifer,the "upper carbonate aquifer," investigations inMinnesota and adjoining states over the past 15 yearshave demonstrated that it contains distincthydrostratigraphic components that differ substantiallyfrom one another in their hydraulic properties. Theseinvestigations are ongoing (for example Campion,2002), and until they are completed our understandingof the hydrogeologic attributes for this part of thePaleozoic section remains relatively limited.

The limited amount of hydrogeologic informationavailable for Galena through Cedar Valley Groups strataprevents a reasonably thorough description ofhydrostratigraphic and hydraulic attributes for each ofthe many individual lithostratigraphic units to be

presented in this report. Therefore these strata aredescribed together, highlighting the general stratigraphicdistribution of their two major matrix hydrostratigraphiccomponents, carbonate rock and shale, as well as asummary of the available information on secondaryporosity and hydraulic attributes. This is followed bya synthesis of these data in which we subdivide thestratigraphic section into discrete hydrogeologic unitsand karst systems.


Matrix porosityThe dominant component of the Galena through

Cedar Valley Groups is carbonate rock, mostly finelycrystalline to dense, microcrystalline dolostone orlimestone. It has not been tested with laboratorymethods to calculate porosity and permeability, but canbe expected to have very low to low porosity andpermeability based on field examination and plug testsof generally similar older carbonate strata in thePaleozoic section.

Shale is abundant in only a few intervals (Fig. 46;Olsen, 1988a; Mossler, 1995a, 1998). The ChickasawMember of the Little Cedar Formation, the lower partof the Pinicon Ridge Formation, and the upper DubuqueFormation are the thickest intervals of strata composedlargely of shale. The lower Maquoketa Formation inwestern Fillmore County and across Mower County iscomposed of interbedded shale and shaly dolostone.Additionally, beds of shale as thick as a few feet areintercalated with carbonate beds of similar thicknessin the Coralville Formation, the upper part of the LittleCedar Formation, and the lower part of the GalenaGroup (Cummingsville Formation). Porosity andpermeability have not been calculated for plug samplescollected in Minnesota, but core samples of shale fromthe Maquoketa Formation in Wisconsin yielded verylow values (Eaton and others, 2000).

Secondary porosity Deep bedrock conditions—The abundance of

macroscopic secondary pores is known to be variablydistributed and stratigraphically controlled in thecarbonate rock of the Galena through Cedar ValleyGroups strata (Fig. 46) in deep bedrock conditions. Sitespecific studies indicate that secondary pores in theGalena Group are apparently concentrated in discrete,relatively thin intervals separated by much thickerbodies of relatively tight carbonate rock (for exampleCurry and others, 1988; Delta EnvironmentalConsultants, Inc., 1995, 2002). A site-remediation studynear Spring Valley revealed that dissolution featuresare concentrated in the lower part of the Prosser

82

Limestone and uppermost part of the CummingsvilleFormation in deep bedrock conditions. A core fromFreeborn County was examined as part of thisinvestigation (Fig. 42), and the distribution of visiblepores was similar: in deep bedrock conditions cavitiesand mesoscopic vertical fractures were present only inrelatively thin horizons clustered near theCummingsville Formation–Prosser Limestone contact.

Witzke and Bunker (1984) described thestratigraphic distribution of secondary pores incarbonate strata above the Galena Group where itoccurs in deep bedrock conditions in northern Iowa.They noted particularly high densities of cavities inthe uppermost part of the Maquoketa Formation, themiddle part of the Pinicon Ridge Formation, thelower part of the Coralville Formation, andthroughout much of the Spillville Formation andBassett Member of the Little Cedar Formation.

Secondary porosity in beds dominated by shaleunder deep conditions of burial has not been described.Fractures known to occur in shallow bedrock conditions(for example Eaton and others, 2000) have not beendescribed under deeper conditions of burial inMinnesota, although they could be present locally(Ryder, 1996).

Shallow bedrock conditions—The carbonate-dominated strata of the Galena through Cedar ValleyGroups contain all of the porosity attributes typical ofclassic karsted rock (Alexander and Lively, 1995;Alexander and others, 1996; Witthuhn and Alexander,1996), including nonsystematic fractures along beddingplanes and at high angles to bedding, as well assystematic fractures that are part of a large-scale,orthogonal system. Individual vertical fractures with


Carbonate component


Sha

kope

e F

orm

atio

nP

ross

er L

imes

tone

StewartvilleFormation

Cum

min

gsvi

lle F

orm

atio

n

Gal

ena

Gro

upD

ecor

ah S

hale

PlattevilleFormation

GlenwoodFormation

St.

Pet

er S

ands

tone

(inco

mpl

ete)

Pra

irie

du C

hien

Gro

upLi

thos

trat

igra

phic

unit

Mat

rixhy

dros

trat

igra

phic

co

mpo

nent

Dep

th in

feet

bel

ow

bedr

ock

surf

ace

Vis

ual p

oros

ity

0 100% 0 5

Frac

ture

s pe

r fo

otof

cor

e

600

550

500

450

400

350

300

250

200

150

Figure 42. Visual porosity and mesoscopic fracture abundance in coreH-1, Freeborn County. Macroscopic secondary pores are abundant inthe Shakopee Formation in deep bedrock conditions, and were also notedalong Cummingsville Formation–Prosser Limestone contact strata andin the lower part of the Stewartville Formation in shallower conditions.See Figure 1 for location.

83

apertures of a few inches are known locally to spanentire outcrops that are tens of feet in height.Dissolution has widened fracture apertures and has alsoproduced vuggy pores of wide-ranging abundance andsize. The horizons with relatively high secondaryporosity in deep bedrock conditions (Witzke andBunker, 1984) appear also to be of relatively highporosity in shallow bedrock conditions based on outcropand borehole investigations (Delta EnvironmentalConsultants, Inc., 1995, 2002; Mossler, 1998; Pailletand others, 2000). In addition, the presence of largecavern systems in the lower Dubuque, Stewartville, andupper Cummingsville Formations and relatively highdensity of sinkholes in the Prosser Limestone,Lithograph City and Stewartville Formations suggestthat those formations may be especially susceptible tothe development of large-scale, interconnected networksof pores.

The intervals dominated by shale are known to befractured in shallow bedrock conditions. Eaton andothers (2000) described open fractures in the MaquoketaFormation shale in southeastern Wisconsin where theformation is buried beneath less than 200 feet ofoverlying bedrock. In Minnesota, nonsystematic stress-release fractures are common, and larger-scale,

systematic fractures are known to cut entirely acrossrelatively thick shaly intervals such as the upperDubuque and lower Maquoketa Formations based onobservations of caves and outcrops (for exampleAlexander and Lively, 1995).

Hydraulic attributesDeep bedrock conditions—The results of hydraulic

tests of the Galena through Cedar Valley Groups (Fig.13) reflect the differential development of secondaryporosity: intervals of strata where secondary pores arerelatively large and abundant are of moderate to highconductivity, whereas thick intervals of tight strata areorders of magnitude lower in conductivity. Nicholasand others (1987) tested a saturated open-hole intervalexposing about 200 feet of Galena Group strata inIllinois and were unable to achieve the minimum pumpcapacity of 15 gallons per minute, while creating adrawdown of over 256 feet. This indicated a hydraulicconductivity of less than 10-2 foot per day. At a differentsite in Illinois, Graese and others (1988) conducteddiscrete interval pump tests that indicated the GalenaGroup typically has a conductivity of 10-3 to 10-2 footper day or less, with the exception of three packedintervals with conductivities that ranged from 1.4 to

0

50

100

150

200

0 10 20 30 40 50 60

Con

duct

ivity

in fe

et p

er d

ay


0

100

200

300

400

500

Ran

ge



Figure 43. Hydraulic conductivity data for the Platteville Formation calculated from specific capacity tests. See Figure11 for an explanation of box plots.

A. Scatter plot showing the relationship between the depth of the open-hole interval below the bedrock surface and hydraulicconductivity.

B. Box plot of hydraulic conductivity values for shallow bedrock conditions. One outlying value greater than 500 feetper day is not shown.

B.A.

84

14 feet per day (Kempton and others, 1987).Hydraulic conductivity data for the Galena Group

in deep bedrock conditions within Minnesota are scarce.Three wells in our specific capacity database rangedin conductivity from 4 to 10 feet per day and averaged6.5 feet per day (Fig. 47). Some Galena Group variancewells permitted by the Minnesota Department of Healthin Mower County have failed to yield adequate suppliesfor domestic purposes in deep bedrock conditions.Packer tests at the Spring Valley site remediationinvestigation in western Fillmore County yieldedvariable results. Two wells at the site were pumpeddry in 10 minutes or less when pumping at 5 to 6 gallonsper minute, indicating a low hydraulic conductivity,while a third well had a moderate hydraulic conductivityof between 2 and 7 feet per day (Delta EnvironmentalConsultants, Inc., 1995, 1998).

Hydraulic conductivity of the carbonate strata thatoverlie the Galena Group in deep bedrock conditionsis known chiefly from packer tests conducted by Libraand Hallberg (1985) in northern Iowa, along with onetest conducted on the Spillville Formation by Greenand others (1997) near LeRoy, Minnesota. Theseauthors reported values that range over three orders ofmagnitude. Three tests of the Spillville Formation rangefrom 0.5 to 39 feet per day (Fig. 13). The BassettMember of the Little Cedar Formation tested as lowas 0.8 foot per day at one site, whereas another boreholehad a hydraulic conductivity of more than 190 feet perday, greater than any other individual formation testedin deep bedrock conditions. A single test of theCoralville and upper part of the Little Cedar Formationsyielded a bulk hydraulic conductivity value of 26 feetper day.

Intervals dominated by shale have not been testedin Minnesota, but packer tests in Illinois under deepbedrock conditions resulted in hydraulic conductivityvalues that typically were less than 10-3 foot per day,with several tests below the measurement limit of 10-

4 foot per day (Curry and others, 1988; Graese andothers, 1988). Green and others (1997) demonstratedthat the vertical conductivity of shaly strata in thelowermost part of the Pinicon Ridge Formation was solow as to provide effective hydraulic confinement atLeRoy, Minnesota, on the basis of a pump testconducted on the underlying Spillville Formation andobservations of monitor wells higher in the stratigraphicsection.

Shallow bedrock conditions—It is well known thatin shallow bedrock conditions the carbonate strata inthe Galena through Cedar Valley Groups contain large,interconnected conduits that can accommodateenormous volumes of water traveling at rates measured

in hundreds of feet per day to several miles per day(for example Alexander and Lively, 1995; Green andothers, 1997; Delta Environmental Consultants, Inc.,1998) and therefore at a relatively large scale commonlyhave a high bulk hydraulic conductivity. Specificcapacity tests of the Galena Group in Minnesota average64.6 feet per day. The statistically acceptable rangeof conductivity calculated from specific capacity testsof the strata above the Galena group has a maximumvalue of 170 feet per day, and the average conductivityis 67 feet per day. Several packed intervals tested byLibra and Hallberg (1985) in northern Iowa hadconductivities greater than 50 feet per day, and theproductivity of wells that draw water from the Coralvilleand Lithograph City Formations in two boreholes inIowa, and of the Spillville Formation at Austin,Minnesota was so great that measurable drawdown didnot occur while pumping (Libra and Hallberg, 1985;Paillet and others, 2000). At the Austin site, 93 percentof the water was contributed from a discrete 5-foothorizon with large secondary pores (Fig. 48).

At a smaller scale, packer tests reveal extremevariability in hydraulic conductivity, and the presencewithin the Galena through Cedar Valley Groups ofintervals of carbonate rock with much lowerconductivity than that calculated for boreholes withlonger open-hole intervals. Packer testing of 0.6-meterintervals of the Galena Group in Wisconsindemonstrated that individual, carbonate-dominated,hydrostratigraphic units have conductivities as low as10-3 foot per day (Stocks, 1998). Rigorous hydraulictests of this kind, which are necessary to recognizediscrete horizons of low conductivity that may serveas confining units, have not been conducted inMinnesota.

Intervals dominated by shale have been tested atonly one site in Minnesota, near Spring Valley, wherehydraulic conductivity and ground-water flow speedsvaried dramatically. The Dubuque Formation had ahorizontal hydraulic conductivity ranging from 8.9 x10-4 to 3 x 10-2 foot per day (Delta EnvironmentalConsultants, Inc., 1995). The lower MaquoketaFormation at the same site yielded variable test results.Some wells were dry after a few minutes of pumpingat a rate of 5 to 7 gallons per minute whereas otherintervals yielded hydraulic conductivity values as highas 1 to 2 feet per day. Dye tracers in fracturedMaquoketa and Dubuque Formation strata traveledlaterally at rates ranging from 0.23 mile per year to1.8 miles per day, and reached depths of 260 feet intothe underlying Galena Formation in less than 7 months(Delta Environmental Consultants, Inc., 1998). Therange in conductivity measured at Spring Valley is

85

Foc

used

rec

harg

eto

St.

Pet

er a

quife

r

2 m

i

100

ft 0

Wat

er d

isch

arge

s fr

om G

alen

a G

roup

bedr

ock

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as

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avel

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ard

thro

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ture

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pid

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trat

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d ca

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tone

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orah

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le

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EX

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Sur

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and

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Figu

re 4

4. G

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. Pet

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and

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ased

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odel

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(200

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a, a

nd s

uppl

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n ba

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on o

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(for

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aps

(for

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ount

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86

generally consistent with that reported for theMaquoketa Formation in southeastern Wisconsin andIllinois, where fractured shaly beds commonly haveconductivities of a few feet per day, and relativelyunfractured shaly intervals are commonly 10-3 foot perday or less (Curry and others, 1988; Graese and others,1988; Eaton and others, 2000).

Hydrogeologic synthesisThe Galena through Cedar Valley Groups are

composed of discrete intervals having moderate toextremely high hydraulic conductivity separated byconfining units composed of unfractured carbonate rockor shale that are several orders of magnitude lower inconductivity (Fig. 46). The stratigraphic section isdivided into hydrogeologic units on the basis of matrixhydrostratigraphic properties: the shaly strata of theupper Dubuque/lower Maquoketa Formations, lowerPinicon Ridge Formation, and Chickasaw Memberseparate four intervals dominated by carbonate rock.In deep bedrock conditions, hydraulic tests andpotentiometric levels in nested well screens havedemonstrated that each of the shaly intervals have theability to serve as confining units (Libra and Hallberg,1985; Rowden and Libra, 1990; Green and others,1997), an interpretation additionally supported bydocumentation of ground-water ages that are stratified

in a manner consistent with hydraulic separation (Greenand others, 1997; Tipping, 1997; Campion, 2002). Theindividual bodies of carbonate rock separated by shalyconfining units can be considered aquifers as bulk unitsbecause they all yield economic quantities of water towells in Minnesota, even in deep bedrock conditions.These are the Galena aquifer (including the ProsserLimestone, Stewartville and lower DubuqueFormations), the upper Maquoketa–Spillville aquifer,and the lower and upper Cedar Valley aquifers (Greenand others, 1997). Water is produced from these bodiesof rock chiefly along discrete intervals of relatively highporosity. It is noteworthy that individual aquifers maycontain thick intervals of carbonate rock much lowerin conductivity and that such intervals have been provento serve as confining units in some areas (Buchmillerand others, 1985; Nicholas and others, 1987; Graeseand others 1988; Delta Environmental Consultants, Inc.,2002). For example, at Spring Valley, where the GalenaGroup occurs in a transitional setting between shallowand deep bedrock, such intervals provide confinementsufficient to separate discrete bedding-plane conduitsthat differ from one another in potentiometric level andinferred flow direction (Delta EnvironmentalConsultants, Inc., 2002). In Illinois and Iowa, wherethe Galena Group is much more deeply buried by

0

20

40

60

80

100

120

140

160

0 5 10 15 20 25

Con

duct

ivity

in fe

et p

er d

ay


0

20

40

60

80

100

120

140

160

Ran

ge



Figure 45. Hydraulic conductivity data for the Decorah Shale calculated from specific capacity tests. SeeFigure 11 for an explanation of box plots.A. Scatter plot showing the relationship between the depth of the open-hole interval below the bedrock surfaceand hydraulic conductivity.B. Box plot of hydraulic conductivity values for shallow bedrock conditions.

B.A.

87

younger bedrock, it is dominated by thick intervals ofcarbonate rock with low conductivity and therefore theentire group is typically classified as a confining unit(for example Buchmiller and others, 1985; Nicholasand others, 1987).

The hydrogeologic system in shallow bedrockconditions is more complex, with enormous variationsin hydraulic conductivity and flow features that arecharacteristic of "triple porosity" karstic aquifers (Figs.46, 49). The much greater degree to which secondaryporosity is developed, especially in the size, spacing,and interconnectivity of fractures, is reflected in anoverall greater range in conductivity, and at least locallycan compromise the ability of low permeability unitsto provide confinement. For example, outcrop and caveinvestigations have documented the presence of largevertical fractures that pass continuously through theMaquoketa and Dubuque Formations, and dye-tracestudies have demonstrated that ground water travelsacross them at rapid rates (Alexander and Lively, 1995;Delta Environmental Consultants, Inc., 1995, 1998).Tipping (1997) and Campion (2002) noted thatstratification in ground-water ages in Mower Countyis less pronounced in shallow bedrock conditions,particularly in areas with a relatively thin cover ofsurficial deposits and near bedrock faults. Nevertheless,even in such a setting, Campion (2002) delineatedseparate potentiometric surfaces for the upper CedarValley, lower Cedar Valley, and Spillville–Maquoketaaquifers, indicating that the confining units betweenthem provide some degree of hydraulic separation.

In shallow bedrock conditions, the strata betweenthe base of the Galena Group and the top of theLithograph City Formation contain two of southeasternMinnesota's major karst systems: the Galena–Spillvillekarst and the Cedar Valley karst (Figs. 46, 49). Thesekarst systems are separated by the shaly strata of thePinicon Ridge Formation. However, each karst systemcontains intervals of shaly strata that have beendemonstrated to act as confining units in relatively deepbedrock conditions, and therefore they may be furthersubdivided with continued study. Investigations thusfar have indicated that in shallow bedrock conditions,sinkholes, caves, and dye traces pass through shalyintervals within each of the individual karst systems.

Together the Galena–Spillville karst system andCedar Valley karst system occur as the uppermostbedrock across a large part of southeastern Minnesota,although the latter has been recognized only in MowerCounty at the time of this report. Karstic features suchas sinkholes, springs, stream sinks, and dry valleys arebest expressed and most abundant where overlyingunconsolidated glacial drift is relatively thin (less than50 feet). The differential stratigraphic distribution of

secondary pores is apparently reflected in some of thekarstic characteristics: relatively high densities ofsinkholes occur in the Prosser Limestone, Stewartvilleand Cedar Valley Formations, and springs mostcommonly lie at the approximate contacts betweencarbonate rock and shale, such as in the lowerCummingsville Formation. Lateral movement of groundwater can be expected to occur preferentially along afew discrete bedding-plane parallel intervals with well-developed secondary pore systems, such as thosedescribed for the Galena karst system in northeasternIowa (Keeler, 1997).

Recharge to karstic aquifers such as the Galena–Spillville occurs relatively rapidly through fractures anddissolution cavities, and ground water can travel laterallyat rates commonly measured in miles per day(Alexander and Lively, 1995; Alexander and others,1996). Flow paths in this aquifer system have beendemonstrated to commonly cross surface watersheddivides. Furthermore, the direction of ground-watermovement along conduit systems does not consistentlycorrespond to ground-water flow directions inferredfrom regional-scale potentiometric maps.

The Dubuque–Galena part of the Galena–Spillvillekarst system contains numerous caves where it occursas the uppermost bedrock. There are two fundamentallydifferent types of caves present. The caves that developin the Dubuque and Stewartville Formations tend to befracture-controlled, high-gradient, water-inlet mazecaves (for example Mystery Cave, Spring ValleyCaverns, Goliath Cave). These caves serve hydraulicallyto conduct surface water into the aquifer via sinkholes.The second type of caves typically develops in the midto lower Cummingsville Formation, and their attributesin part may reflect the presence of laterally continuousshale beds in the Cummingsville Formation. Such caves(Coldwater Cave in Iowa, Pine Cave, Tyson's SpringCave, and Deep Lake Cave) are large, low-gradientdendritic conduits that collect and drain water fromstratigraphically higher levels of the Galena Group. Inplanview these caves resemble surface drainages. Theyare typically flat and terminate at springs, draining largeconduits. Vertical hydraulic gradients are locally largerthan the horizontal gradient in this part of the karstsystem.

DISCUSSION: CLASSIFICATION OFAQUIFERS AND CONFINING UNITS

Our classification of aquifers and confining units(Plates 1, 2) recognizes eleven aquifers and tenconfining units at a regional scale. This newhydrogeologic classification is based on hydraulic datainterpreted within the context of a hydrostratigraphicframework that depicts the distribution of porosity and

88

Gal

ena

aqu

ifer

Maq

uo

keta

–S

pill

ville

aqu

ifer

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wer

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ar V

alle

yaq

uif

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m

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se s

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its

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tho

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host

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d ge

olog

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ruct

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are

mod

ifie

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om M

ossl

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1995

a, 1

998)

. H

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tigr

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m L

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s (1

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, an

d M

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).

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89

permeability in the Paleozoic bedrock. It thereforeprovides a more realistic depiction of aquifers andconfining units in a variety of geologic settings acrosssoutheastern Minnesota than previous classifications,and more accurately characterizes the hydraulicproperties of each of these hydrogeologic units atvarying conditions of burial.

Our classification follows standard conventions inthe use of the terms "aquifer" and "confining unit" (forexample Fetter, 1988; Subsurface-Water GlossaryWorking Group, 1989). An aquifer is a body of rockthat is sufficiently permeable to yield economicquantities of water to wells and springs. A confiningunit is a body of rock of relatively low permeabilitythat is stratigraphically adjacent to one or more aquifers.The standard definitions of "aquifer" and "confiningunit" are not entirely mutually exclusive—for examplebodies of rock that can yield economic quantities ofwater through bedding-plane parallel fractures can beof sufficiently low vertical conductivity to hydraulicallyconfine an underlying aquifer. The first step in ourclassification of hydrogeologic units in southeasternMinnesota therefore was to delineate major confiningunits in deep bedrock conditions, where secondary poresare relatively diminished. The "major" confining unitswe recognize are regionally extensive, relatively thickintervals of fine clastic and carbonate rock that havebeen demonstrated to be of sufficiently low bulk verticalconductivity to provide confinement under particularconditions of hydraulic stress, and where they are notbreached by vertical fractures. They meet all thestandard criteria characteristic of confining units (forexample Fetter, 1988; Subsurface-Water GlossaryWorking Group, 1989) such as having a verticalhydraulic conductivity of less than 10-2 foot per day.Furthermore, each of our regional confining unitsexcept the "middle Mt. Simon Sandstone" has beendemonstrated in this report to hydraulically separateaquifers with differential static heads in southeasternMinnesota or extreme northern Iowa. The middleMt. Simon Sandstone confining unit has beendemonstrated to do so in southeastern Wisconsin andIllinois.

The aquifers we define are the bodies of rockdominated by coarse clastic strata or relatively thickintervals of carbonate rock with abundant secondarypores that are known to yield moderate to large volumesof water in deep bedrock settings. The coarse clasticaquifers typically have a horizontal hydraulicconductivity between 5 and 60 feet per day in deepbedrock conditions. The carbonate rock aquifers aremuch more variable in conductivity, and typicallyconsist internally of relatively narrow intervals of high

to very high conductivity (tens to thousands of feet perday) separated by thick intervals of tight carbonate rockthat is orders of magnitude lower in conductivity.

The regional scale at which we have defined ouraquifers and confining units results in a generalizedclassification in which some individual hydrogeologicunits are internally variable in hydrostratigraphic andhydraulic properties. For example, the St. Lawrenceand lower St. Peter confining units internally containdiscrete, bedding-plane parallel intervals of secondarypores or coarse clastic interbeds that have moderate tohigh conductivity. Conversely, some aquifers we havedefined at a regional scale are known to contain internalconfining units, such as fine clastic beds in the upperMt. Simon aquifer, and carbonate rock with fewsecondary pores in the Galena aquifer. Our limitedunderstanding of the lateral distribution of theseheterogeneities prevents us from depicting them at aregional scale.

Regional-scale classification of aquifers andconfining units in shallow bedrock conditions is difficultbecause of the ubiquitous presence of fractures,relatively great abundance of dissolution features, andthe limited number of comprehensive ground-waterstudies conducted in such settings in Minnesota. Allof the confining units we recognize in deep conditionsof burial have been demonstrated to locally havemoderate to high bulk conductivities in shallow bedrocksettings. More importantly, it has been clearly shownthat these confining units are in places hydraulicallybreached by fractures and dissolution features wherethey occur close to the bedrock surface (Alexander andLively, 1995; Lindgren, 2001). On the other hand, partsof some confining units, such as the Decorah Shale andfine clastic strata in the Franconia Formation, have beendemonstrated to provide confinement at some scale inshallow bedrock conditions even though elsewhere theymay be breached vertically by fractures. Withconsideration of these complexities, we have tentativelyapplied a single hydrogeologic classification of aquifersand confining units for both shallow and deep bedrockconditions. Although each of the confining units havethe potential to provide hydraulic separation at somescale, in shallow bedrock conditions, the relativeeffectiveness and scale at which they can do so ispractically untested in southeastern Minnesota.Therefore as a practical matter for environmentalinvestigations, the ability of these confining units toprovide hydraulic separation in shallow bedrockconditions has to be established at individual sites—it cannot be assumed.

SUMMARY

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This study demonstrates that individuallithostratigraphic units in the Paleozoic bedrock ofsoutheastern Minnesota have great variability in theirinternal hydrostratigraphic character (Plates 1, 2).Variations in matrix and secondary pores result inmeasurable and predictable variability in hydraulicproperties within individual lithostratigraphic units. Thehydrogeologic system is best understood when studiedwithin the context of the hydrostratigraphic attributesof these rocks, such as within the context of the three-dimensional distribution of porosity and permeability.

The Paleozoic bedrock of southeastern Minnesotacan be divided into three principal matrixhydrostratigraphic components: 1. Coarse clastic rockof high porosity and permeability; 2. Fine clastic rockof low porosity and permeability; and 3. Carbonate rock,also of low porosity and permeability. The ground-water system appears to be relatively simple andpredictable in conditions of deep burial by youngerbedrock. Under these conditions, coarse clastic strataare of relatively high hydraulic conductivity, typicallyranging from a few feet per day to a few tens of feetper day, presumably reflecting flow through large, well-connected intergranular pore spaces. In contrast, thematrix conductivity of the fine clastic and carbonaterock components is sufficiently low in a verticaldirection (10-7 to 10-3 foot per day) that intervalsdominated by these components can provide hydraulicconfinement.

The abundance and distribution of secondary poresoverprinted on matrix hydrostratigraphic attributessubstantially affects hydraulic properties. Wedemonstrate that in deep bedrock conditions carbonatestrata contain discrete, stratigraphically controlledhorizons with abundant secondary pores, separated fromone another by relatively tight carbonate rock. These

horizons have a wide range in conductivity, in partdepending on the scale at which they are tested, butcan be as great as hundreds of feet per day. Ourunderstanding of the hydraulic importance of fracturesin deep bedrock conditions is much more limited.Bedding-plane parallel fractures and systematic fracturesare known to exist in all rock types at least locally, buttheir abundance and connectivity is not documented.Standard aquifer and specific capacity test datatentatively indicate that fracture networks may at leastlocally be hydraulically significant. Regional-scaleconnectivity of such networks may provide an enhanced,large-scale conductivity to the aquifers and confiningbeds in southeastern Minnesota that has not beenmeasured by the standard hydraulic tests performed thusfar.

In shallow conditions of burial, secondary poresincluding abundant fractures are common in all threematrix hydrostratigraphic components. Individual layerscomposed of coarse clastic, fine clastic, or carbonatecomponents in relatively shallow conditions are verydifferent hydrogeologically from the same layers inrelatively deep bedrock conditions because secondaryporosity is vastly different. In the shallow setting theyhave a higher bulk hydraulic conductivity, a greaterrange in conductivity, and may transmit the greatestvolumes of ground water through conduit networks.Water in the conduit network is typically rechargedthrough vertical fractures and transported laterallythrough an interconnected system of bedding-planeparallel secondary pores with high hydraulicconductivity. These preferential intervals of flow areseparated from each other by blocks with substantiallylower conductivity, which reflects the matrixpermeability. Flow paths in such conditions are muchless predictable than in deep conditions of burial andflow speeds have been documented to be faster than

Figure 47. Hydraulic conductivity data for Galena Group through Cedar Valley Group strata calculated fromspecific capacity tests. See Figure 11 for an explanation of box plots.

A. Scatter plot showing the relationship between the depth of the open-hole interval below the bedrock surfaceand hydraulic conductivity for wells open only to the Galena Group. Shallower wells tend to have higherconductivity.

B. Box plot of Galena Group hydraulic conductivity values for deep bedrock conditions.C. Box plot of Galena Group hydraulic conductivity values for shallow bedrock conditions. Two outlyingvalues greater than 500 feet per day are not shown.

D. Scatter plot showing the relationship between the depth of the open-hole interval below the bedrock surfaceand hydraulic conductivity for wells open to strata above the Galena Group. Shallower wells tend to havehigher conductivity.

E. Box plot of hydraulic conductivity values for strata above the Galena Group in shallow bedrock conditions.

92

those calculated on the assumption of intergranular flowonly.

The effectiveness and scale at which bedrock layerswith low matrix permeability can provide confinementis diminished in shallow bedrock conditions. Such unitsare ubiquitously fractured, and have a relatively greatrange in hydraulic conductivity in shallow conditionsof burial. Individual hydrostratigraphic units that areproven to serve as confining units in deep bedrockconditions are in places hydraulically breached byfractures and dissolution features where they occur closeto the bedrock surface (Alexander and Lively, 1995;Lindgren, 2001).

RECOMMENDATIONS

Our synthesis of the hydrogeologic attributes ofPaleozoic bedrock in southeastern Minnesota hasrevealed a number of topics that warrant furtherinvestigation. For example, among the major aquiferswe have defined, the upper and lower Mt. Simonaquifers and all of the aquifers above the Decorah Shaleare relatively poorly characterized. In addition, virtuallyall of the confining units need to be rigorously testedin shallow bedrock settings to adequately determine towhat degree fractures and dissolution featurescompromise their inferred confining capability. Thepresence, abundance, and hydraulic significance ofsystematic fractures and bedding-plane fractures are

Figure 48. Results of a borehole geophysical investigation at Austin, Minnesota (Paillet and others, 2000).Flowmeter data were collected under ambient and 2 gallons per minute pumping conditions. Note that 93percent of the contribution to the borehole is from a discrete horizon within the Spillville Formation. Ambientdownflow from this horizon moves out of the borehole into secondary pores just a few feet below. Pumpingproduced no measurable drawdown. Unique number 613746. See Figure 1 for location, and Figure 5 for anexplanation of flowmeter logs.

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93

topics that are particularly poorly understood. Suchfeatures may affect basin-scale hydraulics, particularlyin layers that have a low matrix permeability.Researchers are encouraged to analyze both new andexisting data in the context of our new hydrogeologicframework to further its development. Recentlydeveloped borehole geophysical techniques such asflowmeter, video, caliper, and acoustic televiewer logshave proven to be particularly useful for recognitionof major ground-water conduits and low permeabilityconfining units in subsurface conditions at both localand regional scales.

Models of ground-water flow in southeasternMinnesota should take into consideration the fact thatflow in some aquifers and confining units occurs chieflyalong discrete intervals of high hydraulic conductivitysuch as bedding-plane fractures. Such flow isincreasingly recognized to be important in aquifers andconfining units that were formerly treated as more orless homogeneous bodies (for example Gianniny andothers, 1996; Michalski and Britton, 1997; Morin andothers, 1997; Swanson, 2001). Prior to recognition ofpreferential flow paths, conductivity for such units wascommonly calculated on the basis of standard aquifertests and with the assumption that the entire thicknessof a hydrogeologic unit contributes equally to aborehole. Travel times of ground water calculated undersuch an assumption are known to be orders of magnitudeslower than travel times measured by tracer experiments(for example Bradbury and others, 2000). Wellheadprotection plans and other environmental managementstrategies in which ground-water travel times are ofcritical importance should take this into consideration.

Potentiometric and water chemistry maps shouldbe constructed within the context of the hydrogeologicframework presented in this report. Existing regional-scale maps are of limited value because they wereconstructed at scales that allow small but potentiallyimportant head differences to go unrecognized, and weredeveloped under the incorrect premise thatlithostratigraphic units directly correspond tohydrogeologic units, and that apparent similarities instatic water levels in adjacent units demonstrateshydraulic connection. Furthermore, previouslypublished potentiometric maps are based in part onmeasurements of static water levels in boreholes thatwe now know expose multiple aquifers and confiningunits.

ACKNOWLEDGMENTS

The framework for this report, and its inspiration,are attributed to Bea Hoffman of the SoutheastMinnesota Water Resources Board. Her recognition

that a comprehensive synthesis of hydrogeologic datafor southeastern Minnesota was needed to produceimproved wellhead protection plans led earlydevelopment of this report. The Southeast MinnesotaWater Resources Board was funded to initiate such asynthesis by the Minnesota Board of Water Resourcesthrough two "Challenge Grants" from its local waterresources protection and management program. Muchof the information on the Prairie du Chien Group andJordan Sandstone described in this report was compiledby the Minnesota Geological Survey as part of twoprojects approved by the Minnesota Legislature forfunding as recommended by the Legislative Commissionon Minnesota Resources: the 1989 project entitled"Geologic Factors Affecting the Sensitivity of the Prairiedu Chien–Jordan Aquifer," and the 1999 projectsupported specifically through the MinnesotaEnvironment and Natural Resources Trust Fund entitled"Groundwater Flow in the Prairie du Chien Aquifer,Southeastern Minnesota." Additionally, those projectsprovided part of the funds used to purchase boreholegeophysical equipment that has been essential incharacterizing the hydrogeologic attributes of all partsof the Paleozoic stratigraphic section. An ongoinginvestigation of the hydrogeologic attributes of theFranconia Formation and Ironton and GalesvilleSandstones funded by the Metropolitan Council alsoprovided important borehole flowmeter data used in thisreport.

A number people associated with local and stategovernment agencies have been particularly helpful overthe past 10 years in providing funds and logisticalsupport necessary to collect much of the informationused in this report. They include: Terry Lee, OlmstedCounty Planning Department; Doug Rovang and BarbHuberty, Rochester Public Works; Donna Rasmussen,Fillmore County Soil and Water Conservation District;Bill Buckley, Mower County Environmental Health;Daryl Franklin, Mower County Planning and Zoning;Ross Dunsmore, Winona County EnvironmentalServices; Jim Lundy, Sandeep Burman, and LarryLandherr, Minnesota Pollution Control Agency; LaurelReeves, Minnesota Department of Natural Resources,Division of Waters; and Bruce Olsen and PatrickSarafolean, Minnesota Department of Health.

94

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98

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