GEOLOGIC ATLAS OF MOWER COUNTY, MINNESOTA · Caution: The information on this map is a generalized...
Transcript of GEOLOGIC ATLAS OF MOWER COUNTY, MINNESOTA · Caution: The information on this map is a generalized...
Caution: The information on this map is a generalized interpretation of the sensitivityof ground water to contamination. The map is intended to be used for resource protectionplanning and to help focus the gathering of information for site-specific investigations.
COUNTY ATLAS SERIESATLAS C–11, PART B, PLATE 9 OF 10
Sensitivity to Pollution of the Uppermost Bedrock Aquifers
STATE OF MINNESOTADEPARTMENT OF NATURAL RESOURCESDIVISION OF WATERS
GEOLOGIC ATLAS OF MOWER COUNTY, MINNESOTA
1 0 1 2 3 4 5 MILES
SCALE 1:100 000
1 0 1 2 3 4 5 6 7 8 KILOMETERS
FIGURE 4. Sensitivity matrix for sensitivityratings. The first column lists geologic features (Plates 2, 3, and 5, Part A; Plate 10, Part B)relevant to sensitivity in Mower County. Columnheadings indicate thickness of till cover (modifiedfrom Plates 3 and 5, Part A). Sensitivity ratingsshow the influence of till cover and sandy depositson geologic features.
SCALE 1:200 000
Digital base map composite:Roads and county boundaries—Minnesota Department of Transportation GIS Statewide Base
Map (source scale 1:24,000)Hydrologic features—U.S. Geological Survey Digital Line Graphs (source scale 1:100,000)Digital base map annotation—Minnesota Geological Survey.
Project data compiled from 1997 to 2001 at the scale of 1:100,000. Universal TransverseMercator projection, grid zone 15, 1983 North American datum. Vertical datum is mean sealevel.
GIS data and metadata available through the DNR Waters website:http://www.dnr.state.mn.us/waters
This information is available in an alternative format on request.
This map was compiled and generated using geographic information systems(GIS) technology. Digital data products are available from DNR Waters.
This map was prepared from publicly available information only. Every reasonableeffort has been made to ensure the accuracy of the factual data on which this mapinterpretation is based. However, the Department of Natural Resources does notwarrant the accuracy, completeness, or any implied uses of these data. Users maywish to verify critical information; sources include both the references here andinformation on file in the offices of the Minnesota Geological Survey and the MinnesotaDepartment of Natural Resources. Every effort has been made to ensure theinterpretation shown conforms to sound geologic and cartographic principles. Thismap should not be used to establish legal title, boundaries, or locations ofimprovements.
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©2002 State of Minnesota,Department of Natural Resources, and theRegents of the University of Minnesota TM
SENSITIVITY TO POLLUTION OF THEUPPERMOST BEDROCK AQUIFERS
By
Moira Campion
2002
GROUND-WATER TRAVEL TIME, IN LOG10 HOURS
0 1 2 3 4 5 6 7
Hour Day Week Month Year Decade Century
Time range for tritium studies
SE
NS
ITIV
ITY
RA
TIN
G
Time range for dye trace studies
Time range for carbon-14
studies
Tritium age of ground water
Carbon-14 age of ground water
DNRWaters
FIGURE 1. Geologic sensitivity rating as defined by ground-watertravel time. Ratings are based on the time range required for water at ornear the surface to travel vertically into the uppermost bedrock aquifers(target zone) (see Figure 2). Longer travel times imply a lower sensitivityto pollution. The sensitivity ratings overlap because of uncertainty inestimating travel times, which can vary significantly because of localvariations in geologic and hydrologic conditions. Dye trace, tritium, andcarbon-14 studies indicate the relative ages of ground water.
Very Low
Low
Moderate
High
Very High
FIGURE 2. Schematic illustration of typical subsurface conditions in the study area. The illustration shows the vertical distributionof tritium and the factors that may influence ground-water travel time and contribute to lateral flow. Illustration not drawn to scale.
Estimated vertical travel time for water-borne surfacecontaminants to enter the uppermost bedrock aquifers(target zone)
Sensitivity Ratings
Very High—Hours to monthsVH
High—Weeks to yearsH
Low—Decades to a century
M Moderate—Years to decades
L
Map Symbols
Mapped features influencing sensitivity
MAP EXPLANATION
Well Symbols
Shape indicates aquifer type (Larger symbol , well in upper-most bedrock aquifer; smaller symbol , all other wells sampled)
Spillville-Maquoketa
Upper Cedar Valley
Lower Cedar Valley
St. Peter-Prairie du Chien-Jordan
Galena
Sinkhole
Quaternary
Vintage—Water with tritium concentration of lessthan 0.8 TU entered the ground water before 1953.
Recent—Water with tritium concentration of 10 or moretritium units (TU) entered the ground water since 1953.
Mixed—Water with 0.8 to less than 10 TU is amixture of recent and vintage.
Fault
If shown, ground-water age in years,estimated by carbon-14.
4000
6
5
7
3
90
90
218
5
6
56
2
1
20
7
63
4
90
20
6
218
2
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11
14
1
8
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5
7
7
1
105
56
14
16
3
3
4
56 7
105
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16
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T 102 N
T 104 N
R 18 W R 17 W R 16 W R 15 W R 14 W
T 101 N
T 103 N
R 14 WR 15 W
R 16 WR 17 WR 18 W
T 104 N
T 103 N
T 102 N
T 101 N
92°45'93°00'
43°45'
43°30'
43°45'
43°30'92°30'
92°45'93°00'
92°30'
Racine
MeadowGrand
Le Roy
Taopi
Adams
CreekRose
Austin
Maple View
Brownsdale
Waltham
Dexter
Elkton
Lyle
Lansing
Varco
Sargeant
UDOLPHO
WALTHAM
SARGEANT
PLEASANTVALLEY
RACINE
FRANKFORD
GRANDMEADOW
DEXTER
AUSTIN
WINDOM
LE ROY
LODINEVADALYLE
CLAYTON
RED ROCK
LANSING
MARSHALL BENNINGTON
Cedar
R
iver
Mill PondRamsey
Wolf Creek
Dobbins
Cre
ek
MurphyCreek
Orc hard
Cr eek
Woodbury Creek
Ceda
r
River
Ros e Creek
Otter Creek
Little
Cedar
River
Upper
Iowa
R iver
L ittle
Iow
a
River
Nor
thB
ranc
hUp
p
er I
ow
aR
iver
South
Branch
Roo
t
River
Deer Creek
Deer Creek
South Fork
BearCreek
North
Creek
BearFork
Creek
Robinso
n
Creek
North
Bra
nch
Root
Riv
er
Ro berts Creek
C
ar
ey
16
31 36
1
36
616
31 36 31
1661
3631
1
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31 36 31
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31 36 31 36
1616
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2
INTRODUCTION
Prevention of ground-water contamination is an important part ofwater resource management. The first step to preventing contaminationis to recognize where ground water is particularly sensitive to pollution.The 1989 Minnesota Groundwater Protection Act requires the MinnesotaDepartment of Natural Resources (DNR) to map geographic areas definedby natural features where there is a significant risk of ground-waterdegradation from activities conducted at or near the land surface (MS §103H.005). The natural features are defined as the geologic conditionsin the area. The sensitivity of ground water to pollution is related to theability of rocks and sediments to restrict the downward migration ofcontaminants that move with water. The sensitivity of ground water topollution based on geologic conditions is called geologic sensitivity.
This plate presents the geologic sensitivity to pollution map for theuppermost bedrock aquifers throughout Mower County. The sensitivitymap depicts the potential for ground-water contamination by usingcategories of travel time for water from the land surface to reach theuppermost aquifers (Figure 1). The map shows that areas with thin or notill cover and visible karst features such as sinkholes are rated as veryhigh sensitivity. Areas with till cover more than 75 feet thick are ratedas low sensitivity and mostly protected from downward migration ofcontaminants. Although lateral flow and focused recharge have thepotential to transport contaminants and adversely affect the ground-watersystem, they were not included in the geologic sensitivity interpretation.The conceptual illustration (Figure 2), the recharge map (Figure 3), and“Additional Sensitivity Information” are presented, however, to showthat lateral flow can affect nearby and distant ground-water resources.
GEOLOGIC SENSITIVITY APPROACH
The purpose of geologic sensitivity mapping by the Minnesota DNRis to assess the ability of the geologic materials to transmit water fromthe land surface to the uppermost bedrock aquifers (sensitivity target) inMower County and estimate the time required for water to travel (traveltime) to that resource (Geologic Sensitivity Workgroup, 1991). Figure1 shows the categories of travel time for assessing sensitivity for an area.The time of travel is the basis for the sensitivity rating. A thoroughpresentation of the lateral flow characteristics of ground water throughoutthe county was beyond the scope of this project. Therefore, this geologicsensitivity interpretation considers only a theoretical vertical pathwaytaken by water from the land surface to the aquifer. This approachsimplifies the range of sensitivity characteristics considered. Contaminanttransport is not evaluated in this geologic sensitivity assessment becausecontaminants behave differently according to their density, viscosity, andsolubility. Evaluation of contaminant behavior under various geologicsettings presented too many factors for the scope of this study.
The dominant factors influencing travel time and controlling theability of geologic material to transmit water are the flow characteristicsto the aquifer and the thickness and texture of the material between theland surface and the aquifer. Bedrock aquifers in southeastern Minnesotaare typically fractured and very porous. Water is transmitted into theaquifer directly if there is no cover material at the land surface. Materialsdeposited by glaciers, lakes, and streams cover most of the bedrock inMinnesota in varying thicknesses. Tills and lake sediments contain fine-grained clay minerals and do not transmit water quickly unless they arefractured (Figure 2, condition 10). Sandy materials, whether depositedon or near a glacier or deposited by stream processes, transmit waterreadily.
The target for this sensitivity study is the uppermost bedrock aquifers.As shown on Plates 7 and 8, four bedrock aquifers were mapped in MowerCounty. These uppermost bedrock aquifers were chosen as the geologicsensitivity target because it is the most relevant to the county residents.Most wells are drilled to the first reliable ground-water supply allowedunder the well code. If there is adequate low-permeability cover, the wellwill be completed in the first bedrock aquifer encountered. Althoughseveral municipalities and industrial facilities use one of several deeperaquifers, most wells in the county are completed in the uppermost bedrockaquifers.
GEOLOGIC SENSITIVITY MAP
The sensitivity ratings in Mower County range from very high tolow. The information on the other plates of this publication provides theframework for the geologic sensitivity interpretation. Plate 8 describesthe effect of faulting near Austin on ground-water movement and theground-water behavior of the multiple-porosity aquifers of Mower County.Karst features, karst hydrogeomorphic units, and the ground-water systemcreated within karst aquifers in the sensitivity target zone are explainedon Plate 10. Plates 2, 3, and 5 in Part A show the distribution of bedrockunits, surficial units, and the depth to bedrock, respectively. Plate 7illustrates the depth and heterogeneity of the glacial materials overlyingthe bedrock aquifers. The cross sections show the distribution of ground-water residence time. The sensitivity matrix (Figure 4) was developedusing a combination of factors from Plates 2, 3, and 5 in Part A and Plate10 in Part B.
Very High
The characteristics of areas rated as very high are bedrock at or nearthe land surface and shallow karst units with identified sinkholes. Theareas with bedrock at the land surface are shown on Plate 3, Part A. Theareas with cover less than 25 feet thick are shown on Plate 5, Part A.Both of these bedrock characteristics mostly occur within the shallowkarst hydrogeomorphic units described on Plate 10: Cedar River plain,Deer Creek plain, Le Roy plain, Ostrander plain, Spring Valley plain,and Le Roy upland. These karst units have sinkholes as shown on thesensitivity map on this plate. In these areas where bedrock aquifers areat or near the land surface, water is transported directly from the surfaceinto the fractured carbonate aquifers.
The faults shown on the sensitivity map indicate the areas of verticaldisplacement of the low-permeability bedrock units (see cross sectionson Plate 2, Part A, and Plate 7). Recent and mixed waters have beenfound very deep in the aquifers near the faults. These are the only faultsidentified, but there may be more unidentified faults in the area (see thewestern portions of cross-sections C–C’, D–D’, E–E’, and F–F’ onPlate 7).
High
Areas of high sensitivity include the Limestone plain karst unit (seePlate 10), except where cover is less than 25 feet thick, which is ratedas very high. The Limestone plain is a relatively large karst unit thatoccurs in eastern and western Mower County surrounding the shallowkarst units. Some sinkholes have been mapped in this karst unit in easternMower County, but these occur in the very high sensitivity area wherecover is less than 25 feet thick. Also rated as high sensitivity are areasof glacial sediments from 25 feet to 75 feet thick with overlying surficialsandy deposits. These sandy deposits can transmit water quickly.
Moderate
Areas of moderate sensitivity in Mower County occur in twosituations. A large deposit of sandy outwash in western Mower Countyallows water to travel from the land surface through the sandy materialinto the fractured carbonate aquifer more quickly than through till in thesurrounding area. Areas in the Covered karst unit with till less than 75feet thick are also rated as moderate. These areas are found in central andeastern Mower County.
Low
Areas mapped as till on Plate 3 with more than 75 feet to bedrock(Plate 5) are rated as low sensitivity. These low-sensitivity areas withthick till cover differ from the areas on Plate 5 with depth to bedrockmore than 75 feet and the Covered karst unit on Plate 10 because theydo not include thick and extensive sandy deposits on the land surface.
Some water samples were taken from the uppermost aquifers(sensitivity target) for carbon-14 age dating. The modeled ages rangedfrom 2500 years to 9000 years. Although their carbon-14 age dates arein the very low sensitivity range, the samples were taken from areaswhere the knowledge of the subsurface materials and ground-water flowconditions is insufficient to justify rating the target aquifer as very lowsensitivity.
Map Limitations
The sensitivity interpretation presented on the map was affected bylimitations of data and the sensitivity model. In some areas of the county,data were very sparse. Furthermore, the sensitivity interpretation is basedon a vertical pathway model, which does not include the influence oflateral ground-water flow. Some limitations of the sensitivity model werequalitatively resolved by the map (Figure 3) showing rechargecharacteristics, but this map is also limited by lack of information. Lateralflow is an important part of ground-water flow behavior but is difficultto characterize without quantitative aquifer parameters. Additionally, athorough understanding of aquifer characteristics on a countywide scalewas unrealistic for this project. Information on hydraulic conductivity,lateral and vertical flow gradients, and distribution of sediment texturesin the unconsolidated materials overlying the uppermost aquifers wouldbe needed to assess recharge and sensitivity more thoroughly.
ADDITIONAL SENSITIVITY INFORMATION
Lateral Flow
Lateral movement of ground water has relevance for protectingground-water resources from land use practices that could introducecontaminants. Ground water infiltrates the land surface under saturatedconditions and flows from higher areas to lower areas on the potentiometricsurface. Plate 7 shows the potentiometric surfaces of bedrock aquifersbased on available water-level information. Ground water often travelslaterally for two reasons. First, the discharge point is far from the pointwhere water is introduced into the ground-water system. Second, ground-water flow is influenced by heterogeneities in the aquifer material andsecondary porosity.
Figure 2 shows some examples of lateral ground-water flow inshallow, unconsolidated glacial sediments and in underlying bedrock.The enlargement in Figure 2 (condition 10) shows secondary porosity inshallow till resulting from fractures. Fractures become less abundantwith depth but are important to water movement in the shallow surfaceenvironment. In the glacial sediments, sandy deposits are more porousand permeable than the surrounding till. In well 7, recent ground wateris traveling in a local flow system downgradient to the right. The vintageground water in well 6 is less influenced by the local ground-watersystems near the surface. It is more likely to be traveling to the left withthe regional ground-water system dominated by the bedrock aquifer.
Ground water in the karst system of carbonate bedrock aquifers inMower County shows evidence of lateral flow, which is described indetail on Plate 8. The influence of lateral ground-water flow is shownschematically in wells 1 and 8 in Figure 2. Recent ground water hastraveled to the left through solution-enhanced fractures and conduits inthe carbonate bedrock aquifer under the less permeable materials at thesurface. Wells 2, 4, and 9 are completed in areas where lateral flow ispresent, but recent water has not yet reached the wells.
Recharge to Uppermost Bedrock Aquifers
Some recharge characteristics of aquifers are important to a sensitivityassessment. Recharge is defined here as the process that allows water toenter an aquifer. The meaning of the term recharge varies. For example,recharge can be described as water traveling from the land surface to thewater table, which is water-table recharge. Some recharge areas shownon Figure 3 were used in this sensitivity assessment.
Most ground water in Mower County is contained within a karstsystem of carbonate bedrock aquifers with solution-enhanced fracturesand conduits (see description of multiple-porosity aquifers, Plate 8).Generally, ground water flows from the topographically high areas in anorth-south trending ridge in the eastern part of the county in the Coveredkarst unit (Plate 10) to the surrounding, topographically low areas. Thehigh areas largely coincide with areas having the thickest tills, and thelow areas occur where there are major streams. Regional potentiometrichighs, however, should not be confused with the primary source ofrecharge.
Typically, ground-water recharge occurs where downward gradientsare present over the entire surface of an aquifer or ground-water resource,but it occurs at different rates. Figure 3 shows a map with six rechargecharacteristics. The bright pink areas have little or no cover material onthe surface of the carbonate bedrock aquifer. These areas are a combinationof all the shallow karst units (Cedar River plain, Deer Creek plain, LeRoy plain, Ostrander plain, Spring Valley plain, and Le Roy upland), andall have sinkholes. Water recharges directly into the aquifer and canquickly move great distances from the surface. The area in pale pinkcorresponds to the Limestone plain karst unit. There is limited cover here,and recharge is enhanced because karst features are close to the landsurface. The green areas occur in eastern Mower County between theLimestone plain unit and the area with till cover greater than 50 feetthick. These areas have slower recharge than the pink areas but still allowlarge quantities of water from the surface to enter the aquifer. The lightblue areas have till cover from 50 feet to 75 feet thick, and recharge alsois slower than in the pink areas. The extent of the dark blue recharge areais similar to the extent of the Covered karst unit. Ground water rechargesbedrock aquifers slowly here and can be thousands of years old.
The areas in pink have rather direct flow from the land surface tothe aquifer, whereas the areas in green and blue may have more lateralflow depending on the heterogeneities in the glacial deposits and the flowgradient. The tan area shows sandy deposits. Water can easily enter thesubsurface through this porous material and may also be transportedlaterally by a stream. A stream in the area with thick till could transporta contaminant downstream to an area with focused recharge. Becauselateral flow is not part of the sensitivity evaluation, these areas might beoverlooked but still have an impact on ground-water quality in the county.
REFERENCE CITED
Geologic Sensitivity Workgroup, 1991, Criteria and guidelines for assessinggeologic sensitivity of ground water resources in Minnesota: St.Paul, Minnesota Department of Natural Resources, Division ofWaters, 122 p.
ACKNOWLEDGEMENTS
Many people contributed to the success of this project. My co-workers provided input on many levels for the entire project. They allassisted with fieldwork. Julie Ekman also gave important suggestionsregarding data management and conceptual design. Jan Falteisek, programsupervisor, was instrumental in the direction of the project. Mike Tronrudand Randy McGregor created all maps, diagrams, and cross sections andcomposed clear visual elements using complex data. Nick Kroska providedcrucial editorial expertise that resulted in products that are well integratedand accessible. Jeffrey Green’s work on the karst features of MowerCounty was important to the sensitivity assessment. The DNR Bureauof Engineering survey team created an excellent network of elevationcontrol points at well sample locations used in my potentiometric analysis.E. Calvin Alexander, Jr. and Scott Alexander were instrumental in theconceptual design of the hydrogeology and the geochemistry and reviewedall plates. In addition to reviewing the plates, Geoffrey Delin contributedtechnical guidance especially regarding physical hydrogeology. MichaelTrojan, Anthony Runkel, Robert Tipping, Barbara Palen, Sandeep Burman,Julie Ekman, Jeffrey Green, Robert Libra, and Stephen Worthington allcarefully reviewed these documents and promptly returned relevantcomments.
143 5
6 8
Subsurface Conditions
FIGURE 3. Six areas with different rechargecharacteristics. Some correspond to units foundon the karst unit map (Plate 10). Generally, thepink areas have rapid recharge with relativelydirect flow from the surface to the aquifer. Thegreen and blue areas have slower recharge andmay have more lateral flow depending theheterogeneities in the glacial deposits and theflow gradient. The tan areas may transport largequantities of water laterally because of theirassociations with streams, as well as verticallybecause of their high porosity and permeability.
1
2
3
4
5
6
7
9
1250
1200
1150
Ele
vatio
n in
feet
6000
2500
7500
9000
6000
9000
3000
A
B
C
D
E
F
A’
B’
C’
D’
E’
F’
n.a., not applicable*Sandy deposits: outwash, stratified sediments, and alluvium found deposited over geologic feature
Greaterthan 75
n.a. n.a.
Geologic FeatureLess than
25 25–50 50–75
n.a.
Shallow karst n.a. n.a. n.a.
Bedrock at or near land surface
Limestone plain
Outwash deposit in westernMower County
Thickness of Till Cover (in feet)
Covered karst n.a. n.a.
n.a.
VH
VH
VH H M M
M L
MH*
HVH*VH
Tritium present—Water entered subsurface lessthan 50 years ago. Lighter shade indicates theunsaturated zone above the water table.
Arrow point and dashed line indicate water table
Till
Direction of ground-water flow
Bedrock surface
Well screen
Elevation of water in well
Sand and gravel
Shale
EXPLANATION
Carbonate bedrock aquifer
EXPLANATION
7
8
10
Stream
Wetland
Sinkhole
SinkholeSpring
10
9
Lateral flow from unprotected area to bedrock well under shale cover
Bedrock well, protected by shale cover
Vertical flow to bedrock well from area without till cover
Bedrock well, buried under till cover
Bedrock well, poorly constructed, conducting water along well casing
Well in sandy glacial deposit, deeply buried under till cover
Shallow well in sandy glacial deposit without till cover
Lateral flow to bedrock well from sinkhole
Bedrock well, protected by some till cover
Fractures in till that can reduce travel time for water
6
5
7
3
90
90
218
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6
56
2
1
20
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63
4
90
20
6
218
2
3
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11
14
1
8
8
7
5
7
7
1
105
56
14
16
3
3
4
56 7
105
5
56
16
16
16
16
56
T 102 N
T 104 N
R 18 W R 17 W R 16 W R 15 W R 14 W
T 101 N
T 103 N
R 14 WR 15 W
R 16 WR 17 WR 18 W
T 104 N
T 103 N
T 102 N
T 101 N
92°45'93°00'
43°45'
43°30'
43°45'
43°30'92°30'
92°45'93°00'
92°30'
Racine
MeadowGrand
Le Roy
Taopi
Adams
CreekRose
Austin
Maple View
Brownsdale
Waltham
Dexter
Elkton
Lyle
Lansing
Varco
Sargeant
UDOLPHO
WALTHAM
SARGEANT
PLEASANTVALLEY
RACINE
FRANKFORD
GRANDMEADOW
DEXTER
AUSTIN
WINDOM
LE ROY
LODINEVADALYLE
CLAYTON
RED ROCK
LANSING
MARSHALL BENNINGTON
Cedar
R
iver
Mill PondRamsey
Wolf Creek
Dobbins
Cre
ek
MurphyCreek
Orc hard
Cr eek
Woodbury Creek
Ceda
r
River
Ros e Creek
Otter Creek
Little
Cedar
River
Upper
Iowa
R iver
L ittle
Iow
a
River
Nor
thB
ranc
hUp
p
er I
ow
aR
iver
South
Branch
Roo
t
River
Deer Creek
Deer Creek
South Fork
BearCreek
North
Creek
BearFork
Creek
Robinso
n
Creek
North
Bra
nch
Root
Riv
er
Ro berts Creek
C
ar
ey
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LOCATION DIAGRAM
*Outwash, stratified sediments, and alluvium.**Karst units CRp; DCp; Op; and units LRp, SVp,and LRu with less than 25 feet of glacial cover.
Fast, direct recharge to bedrock
Slow, less direct recharge to bedrockFast vertical recharge,horizontal transport
Karst units with sinkholes**
Limestone plain karst unit
Greater than 25 feet butless than 50 feet of till
Greater than 50 feet butless than 75 feet of till
Greater than 75 feet of till
Surficial sandy deposits*
Direction of ground-watermovement