Nitrate-N Movement in Groundwater from the Land ... · Davis, J.H., Katz, B.G., and Griffin, D.W....
Transcript of Nitrate-N Movement in Groundwater from the Land ... · Davis, J.H., Katz, B.G., and Griffin, D.W....
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Davis, J.H., Katz, B.G., and Griffin, D.W.—
—USGS/SIR 2010-5099
U.S. Department of the InteriorU.S. Geological Survey
Scientific Investigations Report 2010-5099
Prepared in cooperation with the City of Tallahassee
Nitrate-N Movement in Groundwater from the Land Application of Treated Municipal Wastewater and Other Sources in the Wakulla Springs Springshed, Leon and Wakulla Counties, Florida, 1966-2018
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Nitrate-N Movement in Groundwater from the Land Application of Treated Municipal Wastewater and Other Sources in the Wakulla Springs Springshed, Leon and Wakulla Counties, Florida, 1966-2018
By J. Hal Davis, Brian G. Katz, and Dale W. Griffin
Prepared in cooperation with the City of Tallahassee
Scientific Investigations Report 2010-5099
U.S. Department of the InteriorU.S. Geological Survey
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U.S. Department of the InteriorKEN SALAZAR, Secretary
U.S. Geological SurveyMarcia K. McNutt, Director
U.S. Geological Survey, Reston, Virginia: 2010 Revised: 2011
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Suggested citation:Davis, J. H., Katz, B.G., and Griffin, D.W., 2010, Nitrate-N Movement in Groundwater from the Land Application of Treated Municipal Wastewater and Other Sources in the Wakulla Springs Springshed, Leon and Wakulla Counties, Florida, 1966-2018: U.S. Geological Survey Scientific Investigations Report 2010-5099, 90 p.
9781411328570
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Contents
Abstract ...........................................................................................................................................................1Introduction.....................................................................................................................................................2
Purpose and Scope ..............................................................................................................................4Previous Investigations........................................................................................................................4Description of the Study Area ............................................................................................................5Background and Approach .................................................................................................................5
Geohydrologic Setting of the Wakulla Springs Springshed ...................................................................6Geologic Setting ....................................................................................................................................6Hydrologic Setting ................................................................................................................................8Groundwater Flow ................................................................................................................................8
Data Collection and Field Methods ...........................................................................................................16Well and Core Samples ......................................................................................................................17Nitrate-N Loading and Concentrations at Land Surface from Various Sources ......................19
Southeast and Southwest Sprayfields ...................................................................................19Atmospheric Deposition ...........................................................................................................20Effluent Discharges from Onsite Sewage Disposal Systems .............................................20Disposal of Biosolids by Land Spreading ..............................................................................23Creeks Discharging into Sinks .................................................................................................23Fertilizer Application .................................................................................................................23Livestock Wastes .......................................................................................................................24
Nitrate-N and Chloride Concentrations in the Upper Floridan Aquifer and Wakulla Springs .....................................................................................................................25
Model Development ....................................................................................................................................30Groundwater Flow Model Description and Calibration ................................................................30
Subregional Model Geometry ..................................................................................................34Boundary Conditions ........................................................................................................34Simulated Hydraulic Conductivities ...............................................................................34Simulated Recharge to the Upper Floridan Aquifer ....................................................34
Subregional Model Calibration ................................................................................................35Simulated Effective Porosity ....................................................................................................44
Fate and Transport Model and Calibration .....................................................................................47Hydrodynamic Dispersion ........................................................................................................47Simulation of Nitrate-N and Chloride Concentrations from Various Sources .................47
Southeast and Southwest Farm Sprayfields ................................................................47Effluent Discharges from Onsite Sewage Disposal Systems, Fertilizer
Application, and Livestock Wastes ..................................................................51Inflow at Model Boundaries ...........................................................................................52Disposal of Biosolids by Land Spreading .....................................................................53Creeks Discharging into Sinks and Atmospheric Deposition ....................................53
Nitrate-N and Chloride Concentrations in Wakulla Springs ...............................................53Simulated Future Nitrate-N Concentrations in Wakulla Springs .......................................53Simulated Nitrate-N Concentration Distribution in the Upper Floridan Aquifer
at Selected Times .........................................................................................................55
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End of 1967 .........................................................................................................................55End of 1986 .........................................................................................................................55End of 2004 .........................................................................................................................55End of 2006 .........................................................................................................................55End of 2007 .........................................................................................................................64End of 2018 .........................................................................................................................64
Simulated Nitrate-N Loading to the Upper Floridan Aquifer ..............................................64Simulated Nitrate-N Loading to Wakulla Springs from All Sources .................................64
Southeast and Southwest Farm Sprayfields ................................................................64Atmospheric Deposition ..................................................................................................79Effluent Discharges from Onsite Sewage Disposal Systems ....................................79Inflow at Model Boundaries ...........................................................................................79Disposal of Biosolids by Land Spreading .....................................................................79Creeks Discharging into Sinks ........................................................................................79Fertilizer Application ........................................................................................................80Livestock Wastes ..............................................................................................................80
Model Sensitivity Analysis .......................................................................................................80Groundwater Flow Model Sensitivity Analysis ............................................................80Fate and Transport Model Sensitivity Analysis ............................................................81
Model Limitations................................................................................................................................83Summary........................................................................................................................................................84References ....................................................................................................................................................85
Figures 1. Map showing study area and potentiometric surface of the Upper Floridan aquifer
during late May through early June 2006 .................................................................................3 2. Graph showing nitrate-N concentration in Wakulla Springs from 1966 through 2007
compared to Leon and Wakulla County population ................................................................4 3. Map showing location of A, Southeast Farm sprayfield and B, Southwest Farm
sprayfield and airport biosolids disposal area .........................................................................5 4. Chart showing geologic units, hydrogeologic units, and model layers in north-central
Florida .............................................................................................................................................75–8. Maps Showing— 5. Top of the Upper Floridan aquifer used to set the top of model layer 1 ......................9 6. Base of the Upper Floridan aquifer used to set the bottom of model layer 2 ..........10 7. Thickness of the Upper Floridan aquifer ........................................................................11 8. Location of the Wakulla Springs cave system ..............................................................12 9. Diagram showing a generalized geologic cross section and model layers .....................13 10. Graph showing Wakulla Springs discharge from 1928 to 2008 ........................................15 11. Graph showing Wakulla Springs, Sopchoppy River, and Lost Creek discharges
from January 2005 to August 2008 ...........................................................................................15 12. Diagram showing hydrostatic balance between freshwater and saltwater
illustrated by a U-tube ................................................................................................................16
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13-15. Graphs Showing— 13. Nitrate-N loading to land surface and the Upper Floridan aquifer from 1966
through 2018 .......................................................................................................................18 14. Nitrate-N and chloride concentrations and recharge rates at the Southeast
Farm (SEF) and Southwest Farm (SWF) sprayfields ....................................................19 15. Actual and estimated number of onsite sewage disposal systems in Leon and
Wakulla Counties from 1966 to 2018 ...............................................................................20 16. Map showing locations of onsite sewage disposal systems in Leon and Wakulla
Counties in 2005 ..........................................................................................................................2117-22. Graphs Showing— 17. Land surface nitrate-N concentration and concentration recharging the Upper
Floridan aquifer from biosolids disposal for the Tallahassee airport, Southwest Farm (SWF) sprayfield, and Council, Petty, Strickland, and Young farm sites.........22
18. Nitrate-N load from domestic fertilizer application on Leon, and Wakulla Counties ...............................................................................................................24
19. Nitrate-N loading from animal wastes on A, Leon, and B, Wakulla Counties .........25 20. Measured and simulated nitrate-N and chloride concentrations in wells
SE-22, SE-53, SJ-1, and SJ-9 ............................................................................................26 21. Measured and simulated nitrate-N concentrations in wells LS-25 and SF-02 ........27 22. Measured and simulated nitrate-N and chloride concentrations in Wakulla
Springs, A-, B-, C- and D-tunnels ....................................................................................2923-28. Maps Showing— 23. Finite-difference grid (every tenth cell boundary shown) and general
locations for boundary conditions for the subregional groundwater flow and solute transport models ............................................................................................31
24. Bottom of model layer 1 ....................................................................................................36 25. Thickness of model layer 2 ...............................................................................................37 26. Simulated horizontal hydraulic conductivity for layer 1 ..............................................38 27. Simulated horizontal hydraulic conductivity for layer 2 ..............................................27 28. Simulated net recharge rates to the Upper Floridan aquifer .....................................40 29. Graph showing total onsite sewage disposal system discharges for Leon and
Wakulla Counties ........................................................................................................................41 30. Map and graph showing location of wells and comparison of measured and
simulated heads in 1991 .............................................................................................................42 31. Map and graph showing location of wells, and comparison of measured and
simulated heads for the late May to early June 2006 data set ...........................................4332-36. Maps Showing— 32. Particle pathlines showing groundwater flow directions with the simulated
Wakulla Springs stage at 5 feet and the Spring Creek Springs Group stage at 0 feet, and simulated Wakulla Springs stage at 5 feet and Spring Creek Springs Group stage at 6 feet ..........................................................................................44
33. Simulated porosity for layer 2 ..........................................................................................45 34. Particle tracking from monitoring wells SE-06, SE-11S, and SE-40 ...........................46 35. Simulated and measured nitrate-N concentrations at the Southeast Farm (SEF)
sprayfield in model layer 1 in 2006 ..................................................................................48 36. Simulated and measured nitrate-N concentrations at the Southeast Farm (SEF)
sprayfield in model layer 2 in 2006 ..................................................................................49
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37-42. Graphs Showing— 37. Measured and simulated A, chloride and B, nitrate-N concentrations at well
SE-22 ....................................................................................................................................50 38. Volume of recharge at the Southeast Farm (SEF) and Southwest Farm (SWF)
sprayfields ...........................................................................................................................50 39. Simulated nitrate-N concentrations reaching the Upper Floridan aquifer at each
onsite sewage disposal system (OSDS) location in Wakulla County .......................51 40. Simulated nitrate-N concentrations in recharge from atmospheric deposition,
fertilizer, livestock, and total sources in the Leon County part of the study area ...51 41. Nitrate-N and chloride in water-supply wells CW-5 and CW-17, located near
the model boundaries .......................................................................................................52 42. Simulated nitrate-N loads in Wakulla Springs from 1966 through 2018 ...................54 43-. Maps Showing— 43. Simulated nitrate-N concentrations in the Upper Floridan aquifer in model
layer 1 at the end of 1967 ..................................................................................................56 44. Simulated nitrate-N concentration in the Upper Floridan aquifer in model
layer 2 at the end of 1967 ..................................................................................................57 45. Simulated nitrate-N concentration in the Upper Floridan aquifer in model
layer 1 at the end of 1986 ..................................................................................................58 46. Simulated nitrate-N concentration in the Upper Floridan aquifer in model
layer 2 at the end of 1986 ..................................................................................................59 47. Simulated nitrate-N concentration in the Upper Floridan aquifer in model
layer 1 at the end of 2004 ..................................................................................................60 48. Simulated nitrate-N concentration in the Upper Floridan aquifer in model
layer 2 at the end of 2004 ..................................................................................................61 49. Simulated nitrate-N concentration in the Upper Floridan aquifer in model
layer 1 at the end of 2006, assuming that Wakulla Springs is fully capturing the flow in the A-, K-, and R-tunnels ...............................................................................62
50. Simulated nitrate-N concentration in the Upper Floridan aquifer in model layer 2 at the end of 2006, assuming that Wakulla Springs is fully capturing the flow in the A-, K-, and R-tunnels ...............................................................................63
51. Simulated nitrate-N concentration in the Upper Floridan aquifer for scenario 1 in model layer 1 at the end of 2007, assuming the flow in the R-tunnel is going to both Wakulla Springs and Spring Creek Springs Group ..........................65
52. Simulated nitrate-N concentration in the Upper Floridan aquifer for scenario 1 in model layer 2 at the end of 2007, assuming the flow in the R-tunnel is going to both Wakulla Springs and Spring Creek Springs Group .........................................66
53. Simulated nitrate-N concentration in the Upper Floridan aquifer for scenario 2 in model layer 1 at the end of 2007, assuming Wakulla Springs is fully capturing the flow in the A-, K-, and R-tunnels ...............................................................................67
54. Simulated nitrate-N concentration in the Upper Floridan aquifer for scenario 2 in model layer 2 at the end of 2007, assuming Wakulla Springs is fully capturing the flow in the A-, K-, and R-tunnels ...............................................................................68
55. Simulated nitrate-N concentration in the Upper Floridan aquifer for scenario 1 in model layer 1 at the end of 2018, assuming that the flow in the R-tunnel is going to both Wakulla Springs and the Spring Creek Springs Group .......................69
56. Simulated nitrate-N concentration in the Upper Floridan aquifer for scenario 1 in model layer 2 at the end of 2018, assuming that the flow in the R-tunnel is going to both Wakulla Springs and the Spring Creek Springs Group .......................70
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57. Simulated nitrate-N concentration in the Upper Floridan aquifer for scenario 2 in model layer 1 at the end of 2018, assuming that Wakulla Springs is fully capturing the flow in the A, K and R-tunnels ................................................................71
58. Simulated nitrate-N concentration in the Upper Floridan aquifer for scenario 2 in model layer 2 at the end of 2018, assuming that Wakulla Springs is fully capturing the flow in the A-, K-, and R-tunnels ............................................................72
59. Graphs showing simulated nitrate-N loads to the Upper Floridan aquifer and Wakulla Springs from all sources ............................................................................................73
60. Graphs showing results of the subregional fate and transport model sensitivity analysis .........................................................................................................................................82
Tables 1. Measured discharges for Wakulla and St. Marks Rivers and the Spring Creek
Springs Group ..............................................................................................................................14 2. Concentrations of nitrate-N and chloride in water samples from wells, springs, and
Tallahassee sprayfield effluent ................................................................................................17 3. Nitrate-N concentration in Wakulla Springs vent and tunnels from 2004 to 2006 ...........28 4. Transient stress periods for the subregional groundwater flow and solute transport
models from January 1, 1966, to December 31, 2018 ............................................................32 5. Simulated sources of recharge to the Upper Floridan aquifer ...........................................35 6. Measured and simulated discharges for the 1991 and 2006 data sets ..............................41 7. Percentage of nitrate-N removed in the unsaturated zone as recharging water
moves from land surface to the Upper Floridan aquifer ......................................................53 8. Simulated nitrate-N loading to the Upper Floridan aquifer and Wakulla Springs in
selected years .............................................................................................................................74 9. Results of the subregional groundwater flow model sensitivity analysis .........................81 10. Results of the subregional groundwater flow sensitivity analysis for stage in Spring
Creek Springs Group ..................................................................................................................82
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Conversion Factors, Datum, and Abbreviations
Multiply By To Obtain
mile (mi) 1.609 kilometer
square mile (mi2) 2.590 square kilometer
inch per year (in/yr) 25.4 millimeter per year
foot (ft) 0.3048 meter
foot per day (ft/d) meter per day
foot squared per day (ft2/d) 0.09290 meter squared per day
cubic foot per second (ft3/s) 0.2832 cubic meters per day
gallons per day 0.003785 cubic meter per day
million gallons per day (Mgal/d) 0.4381 cubic meter per day
acre 0.4047 hectare Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:
°C=(°F-32)/1.8
Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29).
Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).
Altitude, as used in this report, refers to distance above the vertical datum.
*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times foot of aquifer thickness [(ft3/d)/ft2]ft. In this report, the mathematically reduced form, foot squared per day (ft2/d), is used for convenience.
Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (µS/cm at 25°C).
Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) or micrograms per liter (µg/L).
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Abbreviations and Acronyms
bls below land surface
kg/yr kilograms per year
kg N/yr kilograms of nitrogen per year nitrate-N nitrate-nitrogen
µS/cm microsiemens per centimeter
Mgal million gallons
Mgal/yr million gallons per year
mg/L milligrams per liter
MODFLOW Modular Three-Dimensional Finite-Difference Groundwater Flow Model
MT3D Modular Three-Dimensional Multi-Species Transport Model
OSDS onsite sewage disposal system
RASA Regional Aquifer-System Analysis
SEF Southeast Farm
SWF Southwest Farm
the City the City of Tallahassee
TKN total Kjeldahl nitrogen
USGS U.S. Geological Survey
UFA Upper Floridan aquifer
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Nitrate-N Movement in Groundwater from the Land Application of Treated Municipal Wastewater and Other Sources in the Wakulla Springs Springshed, Leon and Wakulla Counties, Florida, 1966-2018
by J. Hal Davis, Brian G. Katz, and Dale W. Griffin
2. Biosolidsdisposalbylandspreadingat14,000kg/yr(21percent),
3. Creeksdischargingintosinksat7,800kg/yr(11percent),and
4. TheSouthwestFarmsprayfieldat4,500kg/yr(6percent).Thetotalsimulatednitrate-NloadtoWakullaSpringsin
1987hadincreaseddramaticallyto306,000kg/yr.Themajorsourcesofnitrate-Nloadin1987weredeterminedtobe:1. TheSoutheastFarmsprayfieldat186,000kg/yr
(61percent),
2. Biosolidsat37,000kg/yr(12percent),and
3. Inflowtothestudyareaacrossthelateralmodelboundariesat36,000atkg/yr(12percent).Alloftheothersourceswere8percentorless.TheWakullaSpringsdischargecanchangerapidly,even
duringperiodsoflittleornorainfall.ThisrapidchangeisprobablytheresultofWakullaSpringsintermittentlycapturinggroundwaterthathasbeengoingtotheSpringCreekSpringsGroup.Thisspringgroupislocatedinamarineestuaryandisaffectedbytidallyinfluencedsaltwaterintrusion.Twomodelingscenariosweresimulatedandresultsarepresentedfor2007and2018inanefforttobrackettherangeofpossiblecurrentandfuturechangesintheflowofWakullaSprings.Inscenario1,itwasassumedthatWakullaSpringswasnotcapturingSpringCreekSpringsGroupflow.Inscenario2,itwasassumedthatWakullaSpringswascapturingSpringCreekSpringsGroupflow.
AbstractTheCityofTallahasseebeganapilotstudyin1966at
theSouthwestFarmsprayfieldtodeterminewhetherdisposaloftreatedmunicipalwastewaterusingcenterpivotirrigationtechniquestouptakenitrate-nitrogen(nitrate-N)isfeasible.Basedontheearlysuccessofthisproject,anew,largerSoutheastFarmsprayfieldwasopenedinNovember1980.However,arecent2002studyindicatedthatnitrate-NfromtheseoperationsmaybemovingthroughtheUpperFloridanaquifertoWakullaSprings,thuscausingnitrate-Nconcentra-tionstoincreaseinthespringwater.Theincreaseinnitrate-Ncombinedwiththegenerallyclearspringwaterandabundantsunshinemaybeencouraginginvasiveplantspeciesgrowth.Determiningthelinkbetweenthenitrate-Napplicationatthesprayfieldsandincreasednitrate-Nlevelsiscomplicatedbecausethereareothersourcesofnitrate-NintheWakullaSpringsspringshed,includingatmosphericdeposition,onsitesewagedisposalsystems,disposalofbiosolidsbylandspreading,creeksdischargingintosinks,domesticfertilizerapplication,andlivestockwastes.
Groundwaterflowandfateandtransportmodelingwereconductedtosimulatetheeffectofallofthenitrate-NsourcesonWakullaSpringsfromJanuary1,1966,throughDecember31,2018.Thetotalsimulatednitrate-NloadtoWakullaSpringsin1967wasarelativelymodest72,000kilogramsperyear(kg/yr).Themajorsourcesofthenitrate-Nloadin1967weredeterminedtobe:1. Inflowtothestudyareaacrossthelateralmodel
boundariesat31,000kg/yr(43percent),
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2 Nitrate-N Movement in Groundwater, Leon and Wakulla Counties, Florida, 1966-2018
Undertheassumptionsofscenario1,thetotalsimulatednitrate-NloadtoWakullaSpringsin2007was222,000kg/yr.Themajorsourcesofnitrate-Nloadweredeterminedtobe:1. TheSoutheastFarmsprayfieldat111,000kg/yr
50percent),
2. Inflowtothestudyareaacrossthelateralmodelboundariesat44,000atkg/yr(20percent),and
3. Onsitesewagedisposalsystemsat38,000kg/yr(17percent).
Alloftheothersourcescontributed6percentorless.Undertheassumptionsofscenario2,thetotalsimulatednitrate-NloadtoWakullaSpringswas320,000kg/yr.Themajorsourcesofnitrate-Nloadweredeterminedtobe:1. TheSoutheastFarmsprayfieldat111,000kg/yr
(35percent),
2. Onsitesewagedisposalsystemsat83,000kg/yr(26percent),
3. Inflowtothestudyareaacrossthelateralmodelboundariesat52,000atkg/yr(16percent),and
4. Creeksdischargingintosinksat31,000kg/yr(10percent).
Alloftheothersourcescontributed8percentorless.Thenitrate-NloadstoWakullaSpringsfromthe
SoutheastFarmsprayfieldforscenarios1and2wereboth111,000kg/yr.TheseamountswerethesamebecausemostofthewaterfromtheSoutheastFarmsprayfieldwentintoWakullaSpringsinbothsimulations.Incontrast,thenitrate-Nloadsfromonsitesewagedisposalsystemsforscenarios1and2were38,000kg/yrand83,000kg/yr,respectively.TheadditionalwatercapturedbyWakullaSpringsinscenario2camefromanareathathadahighdensityofresidentialandcommercialsitesusingonsitesewagedisposalsystems.
Undertheassumptionsofscenario1,thetotalsimulatednitrate-NloadtoWakullaSpringsin2018willbe175,000kg/yr.Themajorsourcesofnitrate-Nloadforscenario1areanticipatedtobe:1. Inflowtothestudyareaacrossthelateralmodel
boundariesat48,000atkg/yr(28percent),
2. TheSoutheastFarmsprayfieldat42,000kg/yr(24percent),
3. Onsitesewagedisposalsystemsat51,000kg/yr(29percent),and
4. Fertilizerat18,000kg/yr(10percent).Alloftheothersourceswillcontribute5percentorless.Undertheassumptionsofscenario2,thetotalsimulatednitrate-NloadtoWakullaSpringsin2018willbe
305,000kg/yr.Themajorsourcesofnitrate-Nloadforscenario2areanticipatedtobe:1. Onsitesewagedisposalsystemsat119,000kg/yr
(39percent),
2. Inflowtothestudyareaacrossthelateralmodelboundariesat57,000atkg/yr(19percent),
3. TheSoutheastFarmsprayfieldat43,000kg/yr(16percent),
4. Creeksdischargingintosinksat31,000kg/yr(10percent),and
5. Fertilizerat32,000kg/yr(10percent).Alloftheothersourceswillcontribute6percentorless.
Thesimulatednitrate-NloadfromtheSoutheastFarmsprayfieldtoWakullaSpringsduring2007through2018decreasesfrom111,000kg/yrto42,000kg/yrinscenario1anddecreasesfrom111,000kg/yrto43,000kg/yrinscenario2.Bothscenariosshowthesedecreasesbecauseofthesimulatedplannedreductionintheconcentrationofnitrate-Ninthewastewaterusedforirrigationfromapproximately12milligramsperliter(mg/L)in2007to3mg/Lin2018.Incontrast,thesimulatednitrate-NloadfromonsitesewagedisposalsystemstoWakullaSpringsfrom2007through2018increasesfrom38,000kg/yrto51,000kg/yrinscenario1,andincreasesfrom83,000kg/yrto119,000kg/yrinscenario2.Bothscenariosshowincreasesrespectivetotheincreasesinpopulationandresidentialandcommercialsitesusingonsitesewagedisposalsystems.Inaddition,thesimu-latednitrate-NloadtoWakullaSpringsfrom2007through2018frominflowtothestudyareaacrossthelateralmodelboundariesincreasesfrom44,000kg/yrto48,000kg/yrinscenario1,andincreasesfrom54,000kg/yrto57,000kg/yrinscenario2.Bothscenariosshowincreasesduetoincreasingnitrate-NlevelsupgradientinLeonCounty.
IntroductionKarsticaquifersandtheirassociatedspringsare
particularlyvulnerabletonitratecontaminationfromvariousanthropogenicactivitiesatlandsurface.Publicconcernaboutincreasednitrate-nitrogen(nitrate-N)levelsfromlandapplicationsinnorthernFloridaisunderstand-able,particularlyasWakullaSpringsisamajorgroundwaterdischargepointfortheUpperFloridanaquifer(UFA),whichservesasthesourceofpublicwatersupplyformuchofLeonandWakullaCounties(fig.1;Katzandothers,2009).Increasedloadingofnitrate-NtoreceivingwatersinmanypartsofFloridahasresultedindetrimentaleffectstoaquaticecosystems,includingaproliferationofnuisanceaquaticvegetationandacceleratedalgalgrowth(FloridaSpringsTaskForce,2000).WakullaSpringsisaffectedasthese
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Introduction 3
Figure 1. Study area and potentiometric surface of the Upper Floridan aquifer during late May through early June 2006. Monitoring well details are included in table 1 of the appendix.
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SpringCreek Gage
St. MarksSpring
WakullaSprings
SpringCreekSpring
GEORGIA
FLORIDA
ALAB
AMA
0 25 MILES
25 KILOMETERS0
LEON COUNTYWAKULLA COUNTY CO
UNTY
JEFF
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EXPLANATION
0 5 MILES
5 KILOMETERS0
RESIDUALS DISPOSAL AREASPRAYFIELD LOCATION—Southeast farm (SEF) and southwest farm (SWF)SPRINGSHEDS—For Wakulla, St. Marks, and Spring Creek.REGIONAL MODEL BOUNDARYSUBREGIONAL MODEL BOUNDARY—and study area boundaryPOTENTIOMETRIC CONTOUR—Shows altitude at which water would
have stood in tightly cased wells. Contour interval is 4 feet.CODY SCARPDIRECTION OF GROUNDWATER FLOWMAPPED SUBMERGED CAVESCENTER PIVOT LOCATIONGAGING STATIONMONITORING WELLSINK—With creek inflow.SPRING LOCATION
Lost CreekGage
WakullaRiverGage
Fisher CreekGage
SopchoppyRiverGage
SEFSprayfield
SWFSprayfield
Airportsite
Council Site
Petty siteYoung site
Strickland site
Spring CreekSpring
WakullaSprings
St. MarksSpring
Turf Sink
AmesSink
Burnt MillSink
JumpCreekSink
Lost CreekSink
Black Creek Sink
Fisher Creek Sink
HallBranchSink
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TALLAHASSEE
Gul f o f Mex ico
Base from U.S. Geological Survey digital data, 1:24,000, datum nad83Albers Equal-Area Conic Projection,Standard parallels 29 30’ and 45 30’, central meridian -83 00’° ° °
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4 Nitrate-N Movement in Groundwater, Leon and Wakulla Counties, Florida, 1966-2018
higherlevelsofnitrate-N,incombinationwiththegenerallyclearspringwaterandabundantsunshine,maybeencourag-inginvasiveplantspeciesgrowth.Nitrate-NconcentrationsinWakullaSpringshavevariedduringthelastseveraldecades.Nitrate-Nwasabout0.2milligramperliter(mg/L)intheearly1960s;ithadincreasedtomorethan1.0mg/Lbythe1980s;itdeclinedtoabout0.8mg/L(Cheletteandothers,2002)inthe1990s;anditfurtherdeclinedtobetween0.5to0.7mg/L(fig.2;Katzandothers,2009)inthe2000s.Theincreaseofnitrate-NatWakullaSpringswasrelativetoanincreaseofpopulationsforLeonandWakullaCountiesfrom1965toabout1990;however,nitrate-Ndecreasedfrom1990to2007eventhoughpopulationgrowthcontinuedforbothcounties.
Nitrate-NsourcesintheareasurroundingWakullaSpringswereinventoriedbyCheletteandothers(2002).Thesourcesandpercentloadingatlandsurfaceduringtheperiod1990through1999weredeterminedtobe:theCityofTallahassee(theCity)wastewatertreatmentfacilities(40percent),atmosphericdeposition(26percent),biosolidsfromwastewatertreatment(15percent),commercialfertil-izerapplication(7percent),onsitesewagedisposalsystems(OSDS,6percent;OSDSaregenerallyknownasseptictanks),creeksdischargingintosinks(4percent),andlivestockwastes(2percent).Biosolids,asusedinthisreport,refertothesolidresidueproducedasaresultofsewagetreatment.
TheCheletteandothersreport(2002)indicatedthattheCitywastewatertreatmentfacilitieswerecontributing40percentofthenitrate-NloadtoWakullaSprings;howevertheiranalysiswasbasedonamassbalanceapproachthatdidnotdirectlytietheincreaseinnitrate-NinWakullaSpringstothewastewatertreatmentfacilities.Toreducethenitrate-Nload,thewastewatertreatmentfacilitieswouldrequireexpensiveupgrades.Beforeconsideringtheseupgrades,theCitywantedtobecertainthatitsfacilitieswereatleastpartiallyresponsiblefortheproblem.Forthisreason,theCityandtheU.S.GeologicalSurveybeganthiscooperativestudytodetermineifnitrate-NfromtheCitywastewatertreatmentfacilitieswereaffectingWakullaSpringsandtowhatdegree.
Figure 2. Nitrate-N concentration in Wakulla Springs from 1966 through 2007 compared to Leon and Wakulla County population. (Data sources: Population data is from the U.S. Census Bureau (2005), nitrate-N concentrations are from the U.S. Geological Survey and Jamie Shakar, City of Tallahassee, written commun., 2005).
NITRATE-N CONCENTRATIONTOTAL POPULATION–Dashed where estimated
EXPLANATION
1965 75 85 95 05 15
1.0
NIT
RATE
-N C
ONCE
NTR
ATIO
N,
IN M
ILLI
GRAM
S PE
R LI
TER
0
0.5
1.5
2.0
70 80 90 2000 10 2020YEAR
100,000
200,000
300,000
400,000
500,000
TOTA
LPO
PULA
TION
OFLE
ONAN
DW
AKUL
LACO
UNTI
ES
Purpose and Scope
ThisreportdocumentsthedevelopmentofagroundwaterflowmodelthatsimulatesthemovementofgroundwatertoWakullaSpringsandotherlocalspringsfromtheperiod1966through2018.Next,thereportdescribesthedevelopmentofafateandtransportmodelthatsimulatestheevolutionofnitrate-NconcentrationsintheUFAandspringsduringthesameperiod.Finally,thereportpresentstheresultsofeachofthesesimulations.Alsoincludedaredetailsaboutpreviousworkintheregionandthespecificstudyarea.Thereportdiscussesthecompilationofavailablenitrate-Nandotherdata,andthecollectionofadditionalwater-qualitydatatofillinthegapspriortocharacterizingthegroundwatersystem.Eachofthedeterminedsourcesofnitrate-Ninthestudyareaisreviewed.
Previous Investigations
HendryandSproul(1966)investigatedthegeologyandgroundwaterresourcesofLeonCounty.TheydescribedthegeologyandhydrologyoftheUFA,overlyingunits,andthegeneralwaterquality.Miller(1986)describedthegeologyoftheFloridanaquifersystemthatunderliesallofFloridaandpartsofGeorgiaandSouthCarolina.Insomeareas,MillerdividedtheFloridanaquifersystemintotheUFAandtheLowerFloridanaquifer;however,onlytheUFAwasdeterminedtobepresentwithinthisstudyarea.Millermappedthetop,bottom,andthicknessoftheUFAanddescribedthegeologyoftheformationsthatcomprisetheaquiferaspartoftheRegionalAquifer-SystemAnalysis(RASA)oftheU.S.GeologicalSurvey(USGS).BushandJohnston(1988)simulatedgroundwaterflowintheentireFloridanaquifersystemusingafinite-differencemodelaspartoftheRASA.BasedonMiller’sdetermination,theysimulatedonlythepresenceoftheUFAinthestudyareaforthisreport.Duringtheirinvestigation,model-derivedtransmissivitiesweredeterminedfortheUFA,aswellasratesofrechargeanddischarge.Arelativelycoarsegridwithspacingof8×8miles(mi)wasusedforthesesimulations.Themajorspringswithinthestudyareaofthisreportweresimulated,butthecoarsegridspacingrestrictedtheamountoffinedetailthatcouldbeincludedinthemodeling.Ground-waterflowtoWakullaSprings,St.MarksRiversprings,andtheSpringCreekSpringsGroupwassimulatedinstudiesbyDavis(1996)andDavisandKatz(2007).Thesetwostud-iessimulatedtheentirerechargeareaforthesespringsusingamuchfinermodelgridthanBushandJohnston(1988)andrefinedthemodel-derivedtransmissivitiesandratesof
-
Introduction 5
rechargeanddischarge.ThemodeldocumentedbyDavisandKatz(2007)wasusedastheregionalmodelinthisreport,withsomeminormodification.
Description of the Study Area
Thestudyarea(alsoreferredtointhisreportasthesubregionalmodelarea)coversabout500squaremiles(mi2)andextendsfromtheCodyScarpsouthtotheGulfofMexico(fig.1).ThestudyareaislocatedwithintheCoastalPlainphysiographicprovince(Brooks,1981),wherethetopo-graphyischaracterizedbyrollinghillsandland-surfacealtitudesthatrangefrom0toabout200feet(ft)justnorthoftheCodyScarp.Southofthescarp,theland-surfacealtitudesaregenerallylessthat50ftandarecharacterizedbyclosedbasinstypicalofkarstterrains.Theclimateishumidsub-tropicalwithrelativelyhighrainfall.Theaverageannual
temperatureinTallahasseeis67oFandtheaverageannualprecipitationisabout66inchesperyear(in/yr).TheaverageyearlypotentialevapotranspirationfortheTallahasseeareawasestimatedtobeabout46in/yr(Smajstrlaandothers,1984).
Background and Approach
Disposalofwastewaterinamannerthatdoesnotcauseenvironmentalproblemsisalwaysachallenge.Priorto1966,theCitydisposedoftreatedwastewaterbydischargingittoalocallake,butthenitrate-Ninthewastewatercausedalgalblooms.Topreventthis,theCitybeganchangingitsdis-posalmethod.In1966,theCitybeganapilotprojectcalledtheSouthwestFarm(SWF)sprayfieldthatusedcenterpivotirrigationtechniques(figs.1and3).Inthefirstyearofopera-tion,theCitysprayed91milliongallonsperyear(Mgal/yr)
Figure 3. Location of A, Southeast Farm sprayfield and B, Southwest Farm sprayfield and airport biosolids disposal area.
!
!
TALLAHASSEE
2
63
7
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11
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SJ8
SJ6
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Center Pivot Operational Start Dates1-7
8, 9, 11, 1210, 1314-16
November 1980March 1982March 1986April 1999
15
EXPLANATION
12
SE53
Turf Sink
A B
SJ2
SE53SE22
SJ9
SJ7
SJ5
SJ4SJ3 SJ1
SJ10
Southwest FarmSprayfield
Airportsite
TALLAHASSEE
LS25
SF02Southeast FarmSprayfield
Base from U.S. Geological Survey digital data, 1:24,000, datum nad83Albers Equal-Area Conic Projection,Standard parallels 29 30’ and 45 30’, central meridian -83 00’° ° °
SPRAYFIELD LOCATIONWATER-LEVEL CONTOUR—Shows
groundwater elevation in feet, June 2006CODY SCARPCENTER PIVOT—Location and numberMONITORING WELL—Location and numberSINK—With creek inflow
0 2 MILES
2 KILOMETERS00 2 MILES
2 KILOMETERS0
1
1
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6 Nitrate-N Movement in Groundwater, Leon and Wakulla Counties, Florida, 1966-2018
ofwastewater(Chelletteandothers,2002)onthe16-acresite.From1966through1980,theflowvolumeincreasedtoanestimated2,522Mgal/yrandthesiteexpandedtocover118.5acres(JamieShakar,CityofTallahassee,writtencom-mun.,2005).AfterNovember1980,thevolumeofwastewaterdisposedofbytheSWFsprayfielddecreasedtoanestimated112Mgal/yrbecausethenew,largerSoutheastFarm(SEF)sprayfieldbecameoperational(JamieShakar,CityofTallahassee,writtencommun.,2005).ThenewsprayfieldbeganoperationinNovember1980withcenterpivots1-7(fig.3).InMarch1982,centerpivots8,9,11,and12beganoperation;inMarch1986,centerpivots10and13beganoperation;andinNovember1999,centerpivots14-16beganoperation(JamieShakar,CityofTallahassee,writtencommun.,2005).In1981,thefirstfullyearofoperation,theCitysprayed2,824Mgal/yrofwastewater(JamieShakar,2005,CityofTallahassee,writtencommun.);from1981through2005,theflowvolumeincreasedtoanestimated7,154Mgal/yr.
TheinvestigationintotheeffectsofthelandapplicationoftreatedmunicipalwastewateronwaterqualityintheUFAandWakullaSpringsiscomposedoftwoparts.Thefirstpartconsistsofextensivewater-qualitysamplingattheSEFsprayfield,WakullaSprings,andotherlocalspringsthathasbeendocumentedbyKatzandothers(2009).Thesecondpartoftheinvestigationdescribedhereinpresentstheresultsofgroundwaterflowandsolutetransportmodelingsimulationsthatwereconductedtodeterminethecauseofincreasednitrate-NconcentrationsinWakullaSpringsfromtheyears1966through2007,andtoestimatefutureconcen-trationsthrough2018.TheenddateofDecember31,2018,wasselectedbecausetheplannedreductionsinnitrate-NconcentrationsattheSEFandSWFsprayfieldswillhavehadsufficienttimetotravelthroughthegroundwaterflowsystemandtobeevidentinthenitrate-Nconcentrationsoccurringinlocalsprings.
Geohydrologic Setting of the Wakulla Springs Springshed
Thestudyareaincludesthesouthernpartsofthethreemajorspringsheds(fig.1):WakullaSprings,theSt.MarksRiversprings,andtheSpringCreekSpringsGroup.ThesespringsareregionalgroundwaterdischargepointsfortheUFAofnorthernFloridaandsouthernGeorgia.Ground-waterflowinthestudyareaisshapedbythekarsticsubsurfaceconditionsandthosefeaturesresultingfromkarstificationatlandsurface.Limestonesedimentscom-prisingtheaquiferunderlyingthestudyareahavesecondarypermeabilityfeaturesthatstronglyinfluencetransporttimesofcontaminantsthroughthesystem.
Thissectiondescribesthedevelopmentoflong-termwater-leveltrends,recharge,andsecondaryporosityastheyrelatetothelocalgeologyandhydrologyoftheWakullaSpringsspringshed.Thesefactorsprovidetheframeworkforthemodelneededtogainabetterunderstandingofgroundwaterflowandtransportconcepts.
Geologic Setting
ThestudyareaisunderlainbysedimentaryrocksofTertiarythroughQuaternaryagethatconsistoflimestone,dolostone,clay,andsandofvaryingdegreesoflithification(Miller,1986).TheserockunitsgenerallydipsouthwardtowardtheGulfofMexico.Alistofgeologicunitsandtheprincipalhydrogeologicunits(aquifersandconfiningunits)withcorrespondingmodellayersareshowninfigure4.Forreference,themodellayercorrelationsfortheregionalmodelbyDavisandKatz(2007)areincluded.Thegeologicdescrip-tionsgiveninthissectionarebasedonworkbyMiller(1986)unlessotherwisecited.
ThePaleoceneClaytonFormationunderliestheentirestudyareaandconsistsofmassivecalcareousmarineclay.TheEocenesedimentscanbesubdividedfromoldesttoyoungestintotheOldsmarandAvonParkFormationsandtheOcalaLimestone,allconsistingofpermeablelimestone.TheOli-goceneSuwanneeLimestoneisgenerallypermeabletoverypermeable.TheMiocenesedimentscanbesubdividedintotheChattahoocheeFormation,theSt.MarksFormation,andtheHawthornGroup.TheChattahoocheeFormationisprimarilyadolostonecontainingquartzsand,clay,calcite,limestone,chert,mica,heavyminerals,phosphate,andfossils(Huddles-tun,1988).TheSt.MarksFormationisapredominantlyfine-tomedium-grained,siltytosandylimestonethathasunder-gonedegreesofsecondarydolomitization(HendryandSproul,1966).ThepermeabilityoftheSt.MarksandChattahoocheeFormationscanrangefromhighlypermeabletorelativelyimpermeable.TheHawthornGroupispredominantlysandandclay;subordinatecomponentsincludedolomite,dolos-tone,calcite,limestone,phosphorite,phosphate,silicaintheformsofclaystone,chert,andsiliceousmicrofossils,feldspar,heavyminerals,carbonaceousmaterialandlignite,zeolites,andfossils(Huddlestun,1988).ThePliocenesedimentsarerepresentedbytheMiccosukeeFormation,whichismostcom-monlysandyandsiltyclay.SedimentsoftheHawthornGroupandtheoverlyingclay,silts,andsandyclaysoftheMiccosu-keeFormationformalow-permeabilityhydrogeologicunitthatispresentonlyintheextremenorthernpartofthestudyarea.
Pleistocenesedimentsconsistofmedium-tocoarse-grained,tan,white,andbrownsandthatlocallycontainstraceamountsofcarbonaceousmaterialandshellfragments.TheHolocenedepositsincludethinsandandgravelaccumulationsdepositedmostlyadjacenttostreams,estuaries,andlagoons.
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Geohydrologic Setting of the Wakulla Springs Springshed 7
Figure 4. Geologic units, hydrogeologic units, and model layers in north-central Florida.
SYS-
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GEOLOGIC UNITMODEL LAYERS
Davis and Katz (2007) Sub-regional model(This study)
HYDROGEOLOGICUNIT
Undifferentiateddeposits
Undifferentiatedterrace and shallow
marine deposits
Miccosukee Formation
HawthornGroup
Chattahoochee andSt. Marks Formations
SuwanneeLimestone
ClaytonFormation
OcalaLimestone
OldsmarFormation
Avon ParkFormation
Water-tableaquifer
HawthornClays
Low-permeabilitysediments
No-flowboundary
No-flowboundary
Upper Floridanaquifer
Layers 3 and 4 Layers 1 and 2
Layer 1 Notsimulated(Generally
not present)
Notsimulated(Generally
not present)Layer 2
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8 Nitrate-N Movement in Groundwater, Leon and Wakulla Counties, Florida, 1966-2018
Hydrologic Setting
TheUFAispartoftheFloridanaquifersystemthatispresentinFloridaandpartsofGeorgia,SouthCarolina,andAlabama;itisutilizedformunicipal,industrial,agricultural,anddomesticwatersupply.Wheretransmissivitiesarehigh,theUFAgenerallyyieldslargequantitiesofpotablewater;wheretransmissivitiesarelow,thewaterqualityisgenerallyalsolowbecauseofhighlevelsofdissolvedsolids.BushandJohnston(1988)concludedthatcarbonaterocksoftheUFAarenearlyalwayscharacterizedbyanunevendistributionofpermeability.Thewater-bearingopeningsconsistofoneormoreofthefollowing:1. Openingsinlooselycementedfossilhashesthatare
similartotheintersticesofsands,
2. Mosaicsofmanyfracturesandsolution-widenedjoints,and
3. Solutioncavitiesranginginsizefromlessthan1fttotensoffeetorgreater.
Largesolutioncavitiesgenerallyarepresentnearlargespringsandsinkholes,wheredissolutionofthelimestoneisgreatest.Inareasawayfromthelargesolutionopenings,thefirsttwoconditionsdominate.ThepermeabilityoftheUFAisdirectlyrelatedtothethicknessandlithologyoftheover-lyinglow-permeabilitysediments.Thinnerandmorepermeableoverlyingsedimentsallowgreaterratesofinfiltrationandincreaseddissolutionofthelimestone.Theremovaloftheselow-permeabilitysedimentsfromsomeareasduringPleistocenetimeislargelyresponsibleforthecurrentdistributionofkarst,andthus,thecurrentdistributionoftransmissivity.ValuesoftransmissivitydeterminedbyaquifertestsfortheUFAvarygreatlyinthestudyarea,rangingfrom1.3×103to1.3×106feetsquaredperday(ft2/d)(Davis,1996).
ThestructureoftheUFAwasdescribedbyMiller(1986)andtheremainderofthissectionisbasedonhisworkunlessotherwisecited.ThealtitudeofthetopoftheUFAisabout50ftabovetheNationalGeodeticVerticalDatumof1929(NGVD29)inthenortheastcornerofthestudyareaanddipstoabout-100ftbelowNGVD29inthesouthwestcorner(fig.5).ThealtitudeofthebaseoftheUFA(fig.6)wasmodi-fiedfromMiller(1986)basedonnewinformationcollectedduringthewelldrillingpartofthisstudy(thisisdiscussedintheDataCollectionandFieldMethodssection).Inbrief,theboringassociatedwithwellSJ-7indicatedthatthebaseofthefreshwaterflowsystemwasabout400ftNGVD29,soMiller’s(1986)mapwasrecontouredinthisarea.ThebaseoftheUFAdipsto-1,800ftNGVD29onthewesternsideofthestudyareabecauseofapaleochannelthatexistedduringtheearlyTertiaryandwasdescribedbyHuddlestun(1988).ThethicknessoftheUFAwasdeterminedbysubtractingthealtitudeofthebaseoftheUFAfromthatofthetop(fig.7).
Groundwater Flow
Onthewesternsideofthestudyarea,thepotentiometricsurfaceissteepestduetolow-permeabilitylimestonedepositedwithinadeepwaterpaleochannel(Huddlestun,1988).Inthecentralandeasternpartsofthestudyarea,thepotentiometricsurfaceslopesgentlytothesouthandsouth-east;thegentleslopeiscausedbyveryhighpermeabilitiesduetodissolutionofthelimestone.TheUFAwithinthestudyareaisinastateofdynamicequilibriuminwhichtherehavebeennoknownlong-termchangesinthepotentiometricsurface;butwaterlevelsdofluctuateseasonallyandyearlyinresponsetovariationsinrainfall(DavisandKatz,2007).BushandJohnston(1988)foundnoevidenceforanetdeclinebetweentheestimatedpredevelopmentpotentiometricsurfaceandtheobservedpotentiometricsurfaceinMay1980.
GroundwaterflowstoWakullaSpringsbyoneofthemostextensivesubmergedcavesystemsintheUnitedStates,withapproximately37miofmappedcavepassage(Loperandothers,2005;fig.8).CavedivershaveenteredthesubmergedcavesystemthroughWakullaSpringsandnumeroussinkholesintheWakullaSpringsspringshed.Identifyingindividualcavepassagesastunnelswithalphabeticletterdesignationsisastandardnamingconventionestablishedbycaveexplorers.Thiscavesystem,fororientationanddescriptionpurposesinthisreportasshowninfigure8,isassumedtostartatthespringventandinitiallyheadssouthwardwhereitbranchesintotheA-andK-tunnels.TheA-andK-tunnelseventuallymergetoformtheO-tunnel,whicheventuallyconnectstotheQ-tunnel.TheQ-tunnelcontinuesheadingtowardtheSpringCreekSpringsGroup,atleasttothepointwhereadivingexplorationteamhadtoturnaround.TheB-tunnelinitiallytrendseastwardthenturnsnorthwardinthegeneraldirectionoftheSEFsprayfield;theC-tunnelislocatedclosetotheB-tunnelandtrendstowardsouth.TherelativelyshortD-tunnelheadsnorthward.TheextensiveR-tunnelconnectsneartheA-K-Otunneljunction;theR-tunnelconnectswithothertunnelsthatextendseveralmilesnorthwestwardpassingthroughseveralsinkholes.
Tracertestshavebeenconductedusingdyeinjectiontechniquesatseveralsitestodeterminethedirectionandvelocityofgroundwaterflow(ToddKincaid,Hazlett-Kincaid,Inc.,writtencommun.,2006;fig.8).DyeinjectedintoFisherCreekSinkwasdetectedinEmerald,UpperRiver,andTurnerSinksandWakullaSprings(fig.8);themeasuredtraveltimefromFisherSinktoWakullaSpringswasabout10days(thestraightlinedistanceis5.7mi);dyeinjectedintoAmesSinktraveledtoWakullaSpringsinabout20days(thestraightlinedistanceis5.6mi)(ToddKincaid,Hazlett-Kincaid,Inc.,writtencommun.,2006).Thetraveltime,asusedinthisreport,referstothelengthoftimethatittakesforadye(orothertracer)totravelfromtheinjectionpointtoapointwhereitisdetected.DyeinjectedintoTurfSink,locatedattheSEF(fig.1),arrivedatWakullaSpringsinabout40days(11.7mistraightlinedistance)indicatingadirectconnectionbetweentheSEFsprayfieldandWakullaSprings(ToddKincaid,
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Geohydrologic Setting of the Wakulla Springs Springshed 9
Figure 5. Top of the Upper Floridan aquifer used to set the top of model layer 1. (Contours modified from Miller 1986.)
319
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LEON COUNTYWAKULLA COUNTY CO
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0 5 MILES
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Spring CreekSpring
WakullaSprings
TALLAHASSEE
Gul f o f Mex ico
St. MarksSpring
-50
0
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EXPLANATIONRESIDUALS DISPOSAL AREASPRAYFIELD LOCATION—Southeast farm (SEF) and
southwest farm (SWF)SUBREGIONAL MODEL BOUNDARYTOP OF THE UPPER FLORIDAN AQUIFER—In feet
above and below NGVD 1929. Contour interval 50feet
-50
CODY SCARPMAPPED SUBMERGED CAVESCENTER PIVOT LOCATIONMONITORING WELLSINK—With creek inflowSPRING LOCATION
Base from U.S. Geological Survey digital data, 1:24,000, datum nad83Albers Equal-Area Conic Projection,Standard parallels 29 30’ and 45 30’, central meridian -83 00’° ° °
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10 Nitrate-N Movement in Groundwater, Leon and Wakulla Counties, Florida, 1966-2018
Figure 6. Base of the Upper Floridan aquifer used to set the bottom of model layer 2. (Contours modified from Miller, 1986.)
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LEON COUNTYWAKULLA COUNTY CO
UNTY
JEFF
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0 5 MILES
5 KILOMETERS0
Spring CreekSpring
WakullaSprings
TALLAHASSEE
Gul f o f Mex ico
St. MarksSpring
-1,200-800-600
-400
-1,00
0-1,20
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EXPLANATIONRESIDUALS DISPOSAL AREASPRAYFIELD LOCATION—Southeast farm (SEF) and
southwest farm (SWF)SUBREGIONAL MODEL BOUNDARYBASE OF THE UPPER FLORIDAN AQUIFER—In feet
below NGVD 1929. Contour interval 200 feet-600
CODY SCARPMAPPED SUBMERGED CAVESCENTER PIVOT LOCATIONMONITORING WELL—and numberSINK—With creek inflowSPRING LOCATION
SJ-7
SJ-7
Base from U.S. Geological Survey digital data, 1:24,000, datum nad83Albers Equal-Area Conic Projection,Standard parallels 29 30’ and 45 30’, central meridian -83 00’° ° °
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Geohydrologic Setting of the Wakulla Springs Springshed 11
Figure 7. Thickness of the Upper Floridan aquifer.
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LEON COUNTYWAKULLA COUNTY CO
UNTY
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0 5 MILES
5 KILOMETERS0
Spring CreekSpring
WakullaSprings
TALLAHASSEE
Gul f o f Mex ico
St. MarksSpring
1,700
1,400
500
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1,1001,400
1,100
1,10
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#
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EXPLANATIONRESIDUALS DISPOSAL AREASPRAYFIELD LOCATIONCENTER PIVOT LOCATIONMAPPED SUBMERGED CAVESSUBREGIONAL MODEL BOUNDARY
CODY SCARPTHICKNESS OF UPPER FLORIDAN
AQUIFER—in feet feet below NGVD1929. Contour interval 300 feet
SINK—with creek inflowSPRING
#
500
Base from U.S. Geological Survey digital data, 1:24,000, datum nad83Albers Equal-Area Conic Projection,Standard parallels 29 30’ and 45 30’, central meridian -83 00’° ° °
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12 Nitrate-N Movement in Groundwater, Leon and Wakulla Counties, Florida, 1966-2018
Figure 8. Location of the Wakulla Springs cave system.
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SPRING LOCATION
SINKHOLE — where standing wateror marshy conditions are indicated
SINK — with creek inflow
MAPPED SUBMERGED TUNNELS —letter is tunnel designation
DIRECTION OF FLOW IN TUNNEL
EXPLANATION
WakullaSprings
Groundwaterdivide postulatedby Kincaid
LEON COUNTYWAKULLA COUNTY
Upper River Sink
Turner Sink
Emerald Sink
WakullaSprings
AmesSink
JumpCreekSink
Black Creek Sink
FisherCreek Sink
WakullaRiver Gage
INSET MAPINSET MAP
Base from U.S. Geological Survey digital data, 1:24,000, datum nad83Albers Equal-Area Conic Projection,Standard parallels 29 30’ and 45 30’, central meridian -83 00’° ° °
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Geohydrologic Setting of the Wakulla Springs Springshed 13
Hazlett-Kincaid,Inc.,writtencommun.,2006).Onlyasmallproportionoftheinjecteddyeswasrecoveredduringthetracertests,whichsuggeststhepossibilitythatsomeoftheground-watermaybebypassingWakullaSprings.
Groundwaterflowinthetunnels(conduitflow)thatconnecttoWakullaSpringsiscomplexandisnotcompletelyunderstood.TheflowintheR-tunnelisprobablythesimplesttounderstand.TheR-tunnelispartoftheextensivecavesys-temthattrendsnorthwestfromWakullaSprings.UpperRiverSinkoccurswherethiscavesystembreacheslandsurface;theflowinthissinkhasbeenmeasuredseventimesintheperiodfrom1932to1977andaveraged165ft3/s(Rosenauandoth-ers,1977).Thisfindingindicatesthatthecavesystem(includ-ingtheR-tunnel)iscarryingasubstantialquantityofwater.WheretheR-tunnelreachestheA-K-Otunneljunction,theflowcouldbesubstantiallyhigherifthetunnelsaregainingwaterallalongtheirlength.FlowatthejunctionoftheRandA-K-O-tunnelshasthepossibilityofgoingnorthtoWakullaSprings,southtowardtheSpringCreekSpringGroup,orboth.Sometimes,cavediversswimmingsouthwardintheA-tunnelfromthespringentrancehaveobservedthatthegroundwaterflowinthecaveisnorthwardtowardthespringvent,butreversessomewhereinthevicinityoftheR-tunnelconnectionandcanflowsouthwardtowardtheSpringCreekSpringsGroup(fig.8).Agroundwaterdivide(fig.8)inthisareawaspostulatedasearlyas1999byKincaid(1999).
Kincaidfurthernotedthatthisdivideisnotstationaryandcanmoveasgroundwaterconditionschange.Iftheground-waterdivideweretoshifttothesouth,thenuponreachingtheA-K-O-tunneljunction,alloftheflowintheR-tunnelwouldflownorthwardtoWakullaSprings;conversely,ifthedivideweretoshifttothenorth,thenalloftheflowintheR-tunnelwouldgosouthwardtowardtheSpringCreekSpringsGroup.Ifthedividewerelocatedasshowninfigure8,theR-tunnelflowwouldsplitwithsomegoingnorthwardandsomegoingsouthward,whichappearstohavebeenthecasewhenKincaidpostulateditsexistence.Inarecenttracertest,adyewasintroducedintoLostCreekSink(fig.1);thisdyewaslaterdetectedatsomeoftheSpringCreekSpringsGroupandatWakullaSprings(ToddKincaid,Hazlett-Kincaid,Inc.,writtencommun.,2008).ThisfindingsuggeststhattheflowintheQ-tunnelcanreversecompletelyandflownorthwardtowardWakullaSprings.
Rapiddissolutionofthelimestoneisoccurringwithinthestudyareaasevidencedbytheseextensivecavesys-tems.CavemapsofWakullaSpringsshowthatmanyofthecavesliebetween300and400ftbelowlandsurface(bls)(althoughsomearemuchshallowerandevenbreachthelandsurfaceassinkholes).Across-sectionalconceptualmodelofground-waterflowforthestudyareaisshowninfigure9.Inthisconceptualmodel,itwasassumedthatdissolutionofthelimestoneisoccurringatalllevelsoftheaquifer,butis
Figure 9. Generalized geologic cross section and model layers.
NGVD1929
NGVD1929
FEET FEET
EXPLANATIONSANDHAWTHORN GROUPUPPER FLORIDAN AQUIFERNO-FLOW BOUNDARYSPECIFIED HEAD BOUNDARYGENERALIZED DIRECTION OF GROUNDWATER FLOW
South
Vertical scale greatly exaggerated
500
-500
-1,000
-1,500
Cody Scarp Gulf ofMexico
Low-PermeabilitySediments
LAYER1
LAYER 2
500
-500
-1,000
-1,500
North
Leon
Coun
tyW
akul
laCo
unty
-
14 Nitrate-N Movement in Groundwater, Leon and Wakulla Counties, Florida, 1966-2018
probablyoccurringmostactivelyintheshallowerpartwheretherechargingrainwaterfirstencounterslimestone.Thesands,silts,andclaysthatoverliethelimestonetendtofilltheshal-lowdissolutioncavitiesfromabove;thedeeperdissolutioncavitiesaresomewhatprotectedbytheirdepthandarelesslikelytoinfill.Infillingoftheshallowerpartsoftheaquiferresultsinoveralllowerhydraulicconductivityandlowergroundwatervelocities.Incontrast,thelowerpartoftheaquiferhashigherhydraulicconductivitiesandhighergroundwatervelocities.
WakullaRiverdischargehasbeenmeasuredsporadicallysince1929,andapermanentgagewasinstalledin2004.TheWakullaRivergageislocatedapproximately3midownstreamfromthespring(fig.1).EssentiallyalloftheflowmeasuredatthegagecomesfromWakullaSprings,withonlyasmallamountcomingfromotherspringsthatflowintotheWakullaRiver.Theaveragedischargemeasuredatthatgagefortheperiod1929to2008was559ft3/s(table1)withastandarddeviationof242ft3/s.AgagehasbeeninoperationontheSt.MarksRiversince1956andislocatedapproximately1midownstreamfromtheheadspring(fig.1);theaveragedischargeattheSt.Marksgagefrom1956to2008is697ft3/swithastandarddeviationof350ft3/s.DischargeattheSpringCreekSpringsGrouphasonlybeenmeasuredthreetimesinthepast(recentlyagagewasinstalled,buttheratingcurveshavenotbeenestablished).TheSpringCreekSpringsGroupislocatedinatidalestuaryandislogisticallydifficulttomeasurebecauseitrequiresa13-hourmeasurementperiodtocoveronefulltidalcycle.ThedischargeforallthreeofthemajorspringshasonlybeenmeasuredoncesimultaneouslyandthatwasinNovember1991(table1)duringalowrainfallperiodwhenthewaterclarityinallofthespringswasgood.Duringlowrainfallperiods,thewaterinthespringsandriversbecomesveryclear.Forexample,thebottomofWakullaSprings,more
than100ftdeep,cansometimesbeseenfromtheglassbottomboatsduringtheseconditions.Lackofclarityindicatesthatsubstantialquantitiesofsurfacewaterareenteringtheaquiferthroughsinkholes.Duringheavyrainfallperiods,darkbrown(tannicacidstained)surfacewaterwillflowintosinkholesandtravelthroughthecavestothesprings,causingthespringdischargestoriseandtheclaritytofalltoafewfeetorless.
TheUFAintheregionsurroundingthestudyareashowsnolong-termchangesinthepotentiometricsurface(BushandJohnston,1988;DavisandKatz,2007),sothevolumeofgroundwaterflowingsouthwardtowardthespringsshouldhavebeenrelativelyconstant.However,thedischargeatWakullaSpringsappearstohaveexperiencedalong-termincreasebetween1928and2008(fig.10).WakullaSpringsislocatedupgradientfromSpringCreekSpringsGroup,soitispossiblethattheflowinWakullaSpringshasincreasedbytakingflowawayfromtheSpringCreekSpringsGroup.Therearetwopossiblereasonsforthelong-termincreaseinWakullaSpringsflow.First,southFloridaexperienceda9-in.sealevelrisefrom1932to2007duetothewarmingoftheseawatertemperaturesinthewesternpartofthenorthAtlantic(ScienceandTechnologyCommittee,2007).IfthissamesealevelriseoccurredinnorthFlorida,thenadditionalheadpressurewouldhaveoccurredattheSpringCreekSpringsGroup,possiblyresultinginmoreofthewaterintheR-tunnelflowingnorthwardtoWakullaSprings.Second,theevolutionofthesubmergedcavepassagesmayhaveallowedWakullaSpringstocapturemoreflowinthesamewaythatonerivercancaptureflowfromanotherriverthrougherosionprocesses.
Rapidshort-termchangesareoccurringatWakullaSpringsinadditiontotheincreasesinlong-termdischarges.Someofthesechangescouldbecausedbyherbicidetreat-mentsusedtokillhydrilla,aninvasiveplantthathascolonizedtheWakullaRiver.Forexample,immediately
River gage
Measured discharge in
November 1991 (ft3/s)
Measured discharge during late May to early
June 2006 (ft3/s)
Average discharge
for period of record (ft3/s)
St.Marks 602 560 697a
Wakulla 350 750 559b
SpringCreekSpringsGroup 307 na na
Total 1,259 ≥1,310 naaPeriodofrecordis1956to2008.bPeriodofrecordis1929to2008.
Table 1. Measured discharges for Wakulla and St. Marks Rivers and the Spring Creek Springs Group.
[Rivergagelocationsshowninfigure1;ft3/s,cubicfeetpersecond;N/A,datanotavailable;≥,greaterthanorequalto;na,notapplicable]
-
Geohydrologic Setting of the Wakulla Springs Springshed 15
beforethetreatmentonApril18,2006,thedischargeinWakullaSpringswasabout350ft3/s,butitincreasedtoabout750ft3/safterthetreatment(fig.11).Hydrillagrowsinthick,aeriallyextensivematsthatextendfromthebottomoftherivertothesurfaceandcanrestrictriverflow;theherbicidetreatmentkillstheplants,thusremovingthisrestriction.Thisincreaseinflowofabout400ft3/soccurredduringadryperiodwhenlittleornosurfacewaterwasrechargingtheUFAthroughsinkholes.EvidenceforthelackofsurfacewaterflowingintothesinksresultedfromexaminingthedischargerecordoftheSopchoppyRiver,whichisadjacenttothestudyarea(gagelocationisshownonfig.1).TheSopchoppyRiveristheclosestrivertothestudyareawithanextensiverecordofdischargemeasurements(since1961).DuringtherapidincreaseinWakullaSpringsflowduringApril2006,flowintheSopchoppyRiverwasatoneofthelowestlevelssince1966(dischargewaslessthan10ft3/s)duetobelowaverage
rainfall.TheSopchoppyRiver,LostCreek,FisherCreek,andBlackCreekallhavesimilarcatchmentareas,soLostCreek,FisherCreek,andBlackCreeklikelyhadverylowflowsorwerecompletelydryduringtheincreaseindischargeatWakullaSprings.Figure11illustratesthesimilarityinflowpatternbetweenLostCreekandtheSopchoppyRiver.
AtWakullaSprings,thesameoccurrenceoflow-flowconditionspriortotheherbicideapplicationandapproximatedoublingofflowafterthetreatmentwasrepeatedforApril2007andApril2008.TherapidchangeinflowinWakullaSpringsduetothehydrillatreatmentsdemonstrateshowsmallchangesinthehydraulicconditionscanresultinlargeshiftsingroundwaterflowinthestudyarea.WhenthehydrillacolonizedtheWakullaRiver,itreplacedthenativeeelgrass.Theeelgrassalsogrewinthickmatsthatextendfromthebottomoftherivertothesurfaceandcouldhaveactedasarestrictiontoflow.Therefore,thehydrillatreatmentscan
Figure 10. Wakulla Springs discharge from 1928 to 2008.
Figure 11. Wakulla Springs, Sopchoppy River, and Lost Creek discharges from January 2005 to August 2008.
1920 40 60 80 2000
10,000
1,000
100
1030 50 70 90 2010
WAK
ULLA
SPR
INGS
DIS
CHAR
GE,
IN C
UBIC
FEE
T PE
R SE
CON
D
YEAR
Linear regression
r =0.152
200
400
600
800
1,000
1,200
1,400
1,600
2005 2006 2007 2008
1,800
0J F M AM J J A S O N D J F MAM J J A S O N D J F M AM J J A S O N D J F MAM J J
DISC
HARG
E, IN
CUBI
C FE
ET P
ER S
ECON
D
Water-level measurements taken in late Mayand early June for model calibration
Herbicide appliedto kill Hydrilla
EXPLANATIONSOPCHOPPY RIVER DISCHARGE 10-day
running averageLOST CREEK DISCHARGE
—
—10-dayrunning average
WAKULLA SPRINGS DISCHARGE 10-dayrunning average
—
-
16 Nitrate-N Movement in Groundwater, Leon and Wakulla Counties, Florida, 1966-2018
removeanunwantedinvasivespecies,butthisactioncreatesanunnaturalstateoflowerdensityvegetationintheriverthatdoesnotrestrictflowanddoesnotrestorehistoricconditions.However,hydrillatreatmentsbeganin2002,sotheseshort-termchangesarenotthecauseofthelong-termincreaseinflowinWakullaSprings.
WakullaSpringscanrapidlytransitionfromlow-flowtohigh-flowconditionsduetoseveralcircumstances.Afteranherbicidetreatmentforhydrilla,flowincreasesinWakullaSpringssothatmost,orall,oftheflowintheR-tunnelgoestothisspring.ThisoccurrencewouldreducetheflowatSpringCreekSpringsGroup.BecauselessfreshwatergoestotheSpringsCreekSpringsGroup,thehydraulicheadatthespringsdropsslightly(thespringsareinanestuary)andallowsmoresaltwatertobepushedbackintothespringvents,thusfillingtheventswithhigherdensitysaltwater.TherehasbeenlimitedexplorationofthecavesattheSpringCreekSpringsGroup,buttheWakullacavesareknowntoexceed350ftindepth.AssumingthattheSpringCreekSpringGroupcavesaresimilar,asubstantialverticalcolumnofsaltwatercanbepushedbackintothecavesystem.Thesaltwatercancauseahigherequivalentfreshwaterhead(fig.12).Ifthecaveis300ftdeepandfilledwithpureseawater,thentheequivalentfreshwaterheadintheSpringCreekSpringsGroupwillbe7.5ft,whichexceedstheheadof5ftinWakullaSprings.
Inthespringmonths,growinghydrillabeginstoobstructflowintheWakullaRiver,causingmorewaterintheR-tunneltodiverttotheSpringCreekSpringsGroup.IftheadditionalfreshwaterflowtotheSpringCreekSpringsGroupisenoughtopushoutthesaltwater,thentheSpringCreekSpringGroupcanbeginflowingagainandmaycontinuetoflowassum-ingthatasubstantialamountofwaterfromtheR-tunnelwasflowingsouth.Ifsealevelrises,theheadattheSpringCreekSpringsGroupmayriseandpushmoresaltwaterintotheSpringCreekSpringsGroup,thuscausingWakullaSpringstomaintainhigherflowsforlongerperiodsthaninthepast.Othercauses,suchasblockageinthecaves,canreducetheflowintheSpringCreekSpringsGroup.Cavesinthestudyareacancarryasedimentbedload(justlikeariverdoes)andthiscantemporarilyblockaconduit.LostCreekflowsintotheLostCreekSinkabout5minorthwestoftheSpringCreekSpringsGroupandcanbeasourceofsediments,ascanverticalmigrationdownwardofthesurfacesediments.
Data Collection and Field MethodsFielddatawerecollectedtouseformodelcalibration
purposesandconsistedofwater-levelmeasurements,riverdis-chargemeasurements,geologiccoring,monitoringwellinstal-lation,andgroundwater-andsurface-waterqualitysampling.Nitrate-Nloadingatlandsurfacewasthendeterminedforeachofthesevenmajorsources:1. Irrigationusingwastewaterandfertilizerapplication
attheSEFandSWFsprayfields,
2. Atmosphericdeposition,
3. EffluentdischargesfromOSDS,
4. Disposalofbiosolidsbylandspreading(thiswasdiscontinuedin2005),
5. Creeksdischargingintosinks,
6. Fertilizerapplication(separatefromthatappliedatthesprayfields),and
7. Livestockwastes.
Figure 12. Hydrostatic balance between freshwater and saltwater illustrated by a U-tube (modified from Todd, 1980).
Freshwater
Saltwater
h
z
Thickness ofsaltwater (z),
in feet
Height of freshwaterabove saltwater (h),
in feet2.55.07.5
10.0
100200300400
-
Data Collection and Field Methods 17
Well and Core Samples
Groundwater-levelmeasurementsweremadein108wellsinlateMaytoearlyJune2006todefinethepotentiometricsurfaceoftheUFA(app.;fig.1).SouthoftheSEFsprayfield,10newwellswereinstalledaspartofthisstudytoinfillgapsinthewaterlevelandwater-qualitycoverageandtocollectgeologiccores(fig.3;wellsSE-22andSE-53,SJ-1to10).Characteristicsofallwellsaregivenintheappendix.Surface-watergagesmaintainedbytheUSGSontheWakullaandSt.MarksRiverscontinuouslymeasureddischarges(gagelocationsshownonfig.1).
Water-qualitysamplingwasconductedbytheCityandUSGSforanextensivelistofcompoundstodeterminetheeffectoftheSEFsprayfieldontheUFAandWakullaSprings.ThisworkisbeingdocumentedbyKatzandothers(2009)andonlythedatausedformodelcalibrationpurposeswillbediscussedinthisreport.Thewells,springs,andothersitessampledbytheUSGS,samplingdates,andthenitrate-Nandchloridemeasurements,areincludedintable2.Inadditiontowater-qualitysamplingconductedspecificallyforthisstudy,theCitycollectsroutinesamplesfrommanyofthewellsattheSEFsprayfieldaspartoftheiroperatingpermit,andthiswater-qualitydatasetalsowasusedformodelcalibration.
TheUSGShassporadicallysampledWakullaSpringsbeginninginthe1960sandthesedatawereusedforthisstudy.
ThelithologyandthicknessoftheUFAwasnotwelldescribedsouthoftheSEF,soaspartofthemonitoringwelldrillingprocessacontinuouscorewascollectedfromlandsurfaceto476ftblsatwellSJ-7well(fig.3).Theopenholewasgeophysicallyloggedforfluidtemperature,fluidresistivitynaturalgamma,spontaneouspotential,8,16,32,64-inch(in.)formationresistivity,anddiameterusinga3-armcaliper.Thelogsshowedasubstantialchangeinaquiferpropertiesoccurringintheintervalfromabout400ftblstothebottomat476ftbls.Atthesedepths,theresistivitylogsshowedalowerresistivityinthewatersurroundingtheborehole,indicatinghigherdissolvedsolidsintheforma-tion.Thehigherresistivitywater(aboveabout400ftbls)wasinterpretedtobepartoftheactivefreshwaterflowsystemandthelowerresistivity(below400ftbls)wasinterpretednottobepartofthefreshwaterflowsystem.Intheintervalfromlandsurfacetoabout400ftbls,thevolumeofrockrecoveredinthecorebarrelwasoftenlessthan50percent(andsometimeszero),withnumerousdissolutioncavitiesbeingreportedbythedriller.Belowabout400ftbls,thecoresshowedlittletonodissolutioninthelimestone,andthecorerecoveryincreasedto90percentormore,indicatingmuchlowereffective
Table 2. Concentrations of nitrate-N and chloride in water samples from wells, springs, and Tallahassee sprayfield effluent.
[μS/cm;microsiemenspercentimeter,oC;degreesCelsius,mg/L;milligramsperliter;
-
18 Nitrate-N Movement in Groundwater, Leon and Wakulla Counties, Florida, 1966-2018
Figure 13. Nitrate-N loading to land surface and the Upper Floridan aquifer from 1966 through 2018.
A
B C
D E
F G
H I
1965 75 85 95 05 1570 80 90 2000 10 20201965 75 85 95 05 1570 80 90 2000 10 2020
YEAR YEAR
NIT
RATE
–N,I
NKI
LOG
RAM
SPE
RYE
AR
NIT
RATE
–N, I
NKI
LOG
RAM
SPE
RYE
AR
NIT
RATE
–N, I
NKI
LOG
RAM
SPE
RYE
AR
NIT
RATE
–N, I
NKI
LOG
RAM
SPE
RYE
AR
800,000
600,000
400,000
200,000
0
EXPLANATIONSEF Sprayfield
BoundariesLivestock
FertilizerCreeks
BiosolidsOnsite sewage disposal system
AtmosphericSWF Sprayfield600,000
400,000
200,000
0
600,000
400,000
200,000
0600,000
400,000
200,000
0
600,000
400,000
200,000
0600,000
400,000
200,000
0
600,000
400,000
200,000
0600,000
400,000
200,000
0
600,000
400,000
200,000
0
TOTAL NITRATE LOAD TO LAND SURFACE—Beforeentering the unsaturated zone
TOTAL NITRATE LOAD TO THE UPPER FLORIDANAQUIFER—Amount of land surface load that makesit through the zone to reach theunsaturated
-
Data Collection and Field Methods 19
upperlimitonthemassofnitrate-NthatcouldreachtheUFA.Nitrate-Nisconsumedbyarangeofbiologicalprocesses(suchasplantuptakeormicrobialprocesses)intheunsaturatedzone;themassthatactuallyreachestheUFAwillusuallybelessthanthemassloadedatlandsurface.Aneighthsourceofnitrate-Ntothestudyareaistheresultofgroundwaterflowingacrosstheboundaries(althoughthisisnotaloadingtolandsurface).Theloadfromeachsourcewasdeterminedforeachyearfrom1966throughabout2006andextrapolatedthrough2018basedonpopulationgrowthwhereapplicable.Figure13summarizesthemassofnitrate-Nloadingperyearateachsource;italsoshowsthemassfromeachsourcethatmakesitthroughtheunsaturatedzonetoreachtheUFA,asdeterminedfromfateandtransportmodeling(discussedinalatersectionofthisreport).
Southeast and Southwest SprayfieldsThemassofnitrate-Ninthewastewatereffluentused
forirrigationandfertilizerapplicationistrackedbytheCityforbothsprayfields.Thetotalyearlymassofnitrate-NloadedtolandsurfaceattheSEFwascalculatedbysummingthemassappliedinthewastewater,themassinrainfall,andthemassappliedasfertilizer.AttheSEF,theloadpeakedatabout600,000kg/yrin1986(fig.13A)whenfertilizerapplicationwashighest;since1986,theloadhasdeclinedtoabout320,000kg/yr.Thisdeclinewasduetoareductionandeventualeliminationoffertilizerusage.Afurtherdeclinetoabout91,000kg/yrisanticipatedby2013,basedonplannedimprovementsatthetreatmentplantthatwillreducethewastewaternitrate-Nconcentrationfromabout12mg/Lto3mg/L.
Themassofnitrate-Nloadedatlandsurfacewasconvertedtoaconcentrationbydividingbythenetrecharge(netrechargeisthesumoftheirrigatedvolumeplusrainfallvolumeminusthepotentialevapotranspiration).TheSEFsprayfieldnitrate-Nconcentrationatlandsurfacepeakedatabout20mg/Linthemiddletolate1980s(fig.14A).Theserelativelyhighvalueswereacombinationofthewastewatereffluentconcentration,rangingfrom10to15mg/L,andtheheavyapplicationoffertilizer.Dilutionbyrainfallreducedthenitrate-Nconcentrationssomewhatandwasgreatestduringheavyrainfallyears.Asteadydeclineinnitrate-Nconcentrationsoccurredfromthemid-1980suntil2006;this
Figure 14. Nitrate-N and chloride concentrations and recharge rates at the Southeast Farm (SEF) and Southwest Farm (SWF) sprayfields. (Data from Jamie Shakar, City of Tallahassee, written commun., 2005).
A
1965 75 85 95 05 1570 80 90 2000 10 2020
YEAR
CHLO
RIDE
CON
CEN
TRAT
ION
,IN
MIL
LIGR
AMS
PER
LITE
RN
ITRA
TE–N
CON
CEN
TRAT
ION
,IN
MIL
LIGR
AMS
PER
LITE
R
EXPLANATION
RECH
ARG
E,IN
INCH
ESPE
RYE
AR
1,600
1,200
800
400
0
NITRATE CONCENTRATION IN WASTEWATER EFFLUENTNITRATE CONCENTRATION AT LAND SURFACENITRATE CONCENTRATION REACHING THE UPPER FLORIDAN
AQUIFERCHLORIDE CONCENTRATION IN WASTEWATER EFFLUENTCHLORIDE CONCENTRATION AT LAND SURFACE AND
RECHARGING THE UPPER FLORIDAN AQUIFER It is lessthan the effluent concentration because of dilution by rainfall
NET RECHARGE RATE
—
SEF Sprayfield
C SEF Sprayfield
B SWF Sprayfield1,600
1,200
800
400
0
1,600
1,200
800
400
0
70
60
50
40
30
20
10
0
20
15
10
5
0
20
15
10
5
0
porosity.Basedonthisdiscovery,thebaseofthefreshwaterflowsystemwasassumedtobeabout400ftblssouthoftheSEFsprayfield,andwasthesupportingreasonforrevisingMiller’s(1986)mapofthebaseoftheUFAasdiscussedinthepreviousHydrologicSettingsection.
Nitrate-N Loading and Concentrations at Land Surface from Various Sources
Nitrate-Nloadingatlandsurfaceinthisreportreferstonitrate-Nappliedatornearlandsurfaceandisconsideredtobethemassenteringtheunsaturatedzone.Itrepresentsan
-
20 Nitrate-N Movement in Groundwater, Leon and Wakulla Counties, Florida, 1966-2018
wasaresultofanongoingreductioninfertilizerusage.After2006,thenitrate-Nconcentrationisanticipatedtodeclinefurtherbecauseofthecompleteeliminationoffertilizerapplicationandimprovementsatthewastewatertreatmentplant.Theconcentrationofnitrate-Ninthewastewatereffluentisanticipatedtobereducedto3mg/Lby2013;withrainfalldilution,theconcentrationatlandsurfacewillbeabout2.5mg/L.ThenetrechargerateattheSEFsprayfieldrangedfromalowof83.4in/yrin1983to140.8in/yrin2006,andisanticipatedtoreach175in/yrin2018(fig.14A)duetotheincreasingvolumesofwastewaterresultingfrompopulationgrowth.
TheSWFsprayfieldnitrate-Nloadatlandsurfacewasinitiallylowin1966whendisposalfirstbeganandpeakedatabout140,000kg/yrin1980.Thenitrate-NloadabruptlydecreasedaswastewatereffluentwasdivertedtothenewlyopenedSEFsprayfieldandhasbeenunder10,000kg/yrsince1980(fig.13B).Thisfacilitywasapilotprojecttotestirriga-tiontechniques.Wastewaterwassprayedonthelandsurface,thusresultinginhighrechargeratesbecausethelandareaavailablewasrelativelysmall.
Thenitrate-NconcentrationsatlandsurfaceattheSWFsprayfieldwerewithinthe10to15mg/Lrangeduringthehighestrechargeyearsof1966through1980(fig.14B);from1980through2007,theconcentrationwasgenerallylessthan5mg/L.Thenitrate-Nconcentrationinthewastewatereffluentfrom1966through2007generallystayedwithinthe10to15mg/Lrange;irrigationratesweresohigh(exceeding1,000in/yrattimes)thatdilutionbyrainfallwasminor.Incontrast,irrigationratesfrom1980through2007werelow(between20and75in/yr)anddilutionbyrainfallwasimportant,yieldingnitrate-Nconcentrationsatlandsurfaceofabout5mg/Lorless.Theconcentrationinthewastewatereffluentisanticipatedtobeabout3mg/L;theconcentrationatlandsurfacewillbeabout0.7mg/Lbasedonassumedirrigationrateswithrainfalldilution.
Atmospheric Deposition Atmosphericdepositionisoneofthelargestsourcesof
nitrate-Nloadingtolandsurface.Thedissolvedinorganicnitrogen(nitrate-Nplusammonia)rangedfrom0.15mg/Lto0.28mg/Landaveraged0.22mg/Lin1999(Chelletteandothers,2002).Assumingalong-termaveragerainfallof60in/yr,aconcentrationof0.22mg/L,andatotalareaof463mi2,theloadatlandsurfacewasestimatedtobeabout400,000kg/yr(fig.13C);however,plantsuptakemostofthisnitrate-Nthatisthinlyspreadoveralargearea.
Effluent Discharges from Onsite Sewage Disposal Systems
Theyearlynitrate-NloadingtolandsurfacefromOSDSswasdeterminedbyestimatingthetotalnumberofOSDSsinthestudyareaandmultiplyingthatnumberbythenitrate-NloadperOSDS.TheactualnumberofOSDSsinLeonandWakullaCountieswasavailablefortheyears1970to2005
(fig.15);however,thelocationswereavailableonlyin2005(fig.16).
Therewere39,043OSDSsinLeonCountyin2005thatincluded8,026inthestudyarea.ForyearswhentheactualnumberofOSDSsinLeonCountywasnotavailable,thenumberwasestimatedbyusingthecountypopulationnumberfromtheU.S.Censusdataandapplyingtheproportionof1OSDSforevery6.838countyresidents(thiswastheactualproportionin2005).Inthestudyarea,therewere0.2056OSDSsforeveryOSDSinthecounty(alsotheproportionin2005)(fig.15A).Chelletteandothers(2002)estimatedthattherewere2.42peopleperOSDSinLeonCounty,withawateruseof55gallonsperpersonperday.Thenitrate-NconcentrationintheeffluentfromOSDSsishardtoassess.AccordingtoaliteraturereviewbyOtisandothers(1993),totalnitrogenintheeffluentinfluentrangesfrom35to100mg/L.TheU.S.EnvironmentalProtectionAgency(1980)estimatedthatthetotalnitrogenconcentrationinOSDSeffluentrangesfrom25to100mg/L.Forthisstudy,thenitrate-Nconcentrationintheeffluentatthedrainfield(theconcentrationafterallformsofnitrogenareconvertedtonitrate-N)wasassumedtobe60mg/L.
Figure 15. Actual and estimated number of onsite sewage disposal systems in A, Leon and B, Wakulla Counties from 1966 to 2018.
ONSITE SEWAGE DISPOSAL SYSTEM—Shows the actualnumber of systems for the county
ESTIMATED ON-SITE-DISPOSAL SYSTEM—Shows theestimated number of systems for the county
ESTIMATED ON-SITE-DISPOSAL SYSTEM WITHIN THESTUDY AREA—Shows the estimated number ofsystems within the study area for the county
EXPLANATION
B Wakulla County
A Leon County60,000
40,000
20,000
0
1965 75 85 95 05 1570 80 90 2000 10 2020
YEAR
NUM
BER
OF O
NSI
TE S
EWAG
E DI
SPOS
AL S
YSTE
MS,
ACTU
AL A
ND
ESTI
MAT
ED
20,000
15,000
10,000
5,000
0
-
Data Collection and Field Methods 21
Figure 16. Locations of onsite sewage disposal systems in Leon and Wakulla Counties in 2005.
319
98
61
267
98
365
319
267
61
363
27
LEON COUNTYWAKULLA COUNTY CO
UNTY
JEFF
ERSO
N
0 5 MILES
5 KILOMETERS0
Spring CreekSpring
WakullaSprings
TALLAHASSEE
Gul f o f Mex ico
St. MarksSpring
EXPLANATIONRESIDUALS DISPOSAL AREASPRAYFIELD LOCATIONMODEL-SUBREGIONAL BOUNDARYCODY SCARP
MAPPED SUBMERGED CAVESCENTER PIVOT LOCATIONOSDS—onsite sewage disposal systemSPRING LOCATION
Base from U.S. Geological Survey digital data, 1:24,000, datum nad83Albers Equal-Area Conic Projection,Standard parallels 29 30’ and 45 30’, central meridian -83 00’° ° °
-
22 Nitrate-N Movement in Groundwater, Leon and Wakulla Counties, Florida, 1966-2018
Figure 17. Land surface nitrate-N concentration and concentration recharging the Upper Floridan aquifer from biosolids disposal for the A, Tallahassee airport, B, Southwest Farm (SWF) sprayfield, and C, Council, D, Petty, E, Strickland, and F, Young farm sites (shown on fig. 1).
TOTAL NITRATE LOAD AT LAND SURFACE Before entering thezone
TOTAL NITRATE CONCENTRATION REACHING THE UPPER FLORIDANAQUIFER Amount of land surface load that makes it through the
zone to reach the aquifer
―
―
unsaturated
unsaturated
EXPLANATION
1965 75 85 95 05 1570 80 90 2000 10 20201965 75 85 95 05 1570 80 90 2000 10 2020
YEAR YEAR
NIT
RATE
–N,I
NM
ILLI
GRA
MS
PER
LITE
R
C Council Farm biosolids
A Airport biosolids200
150
100
50
0
E Strickland Farm biosolids
D Petty Farm biosolids
B SWF Sprayfield biosolids
F Young Farm biosolidsNIT
RATE
–N,I
NM
ILLI
GRA
MS
PER
LITE
R
200
150
100
50
0
200
150
100
50
0
200
150
100
50
0
200
150
100
50
0
200
150
100
50
0
-
Data Collection and Field Methods 23
ThenumberofOSDSsintheWakullaCountypartofthestudyareawascalculatedusingthesamemethodasforLeonCounty(fig.15B).In2005,therewere11,334OSDSsinWakullaCountythatincluded9,714OSDSsinthestudyarea,thusgivingaproportionof0.8571OSDSsinthestudyareaforeveryOSDSinWakullaCounty.Chelletteandothers(2002)estimated2.57peopleperOSDS,awateruseof55gallonsperperson,andanitrate-Nconcentrationintheeffluentof60mg/L.WakullaCountyisintheprocessofconstructingsanitarysewers,sotheestimatednumberoffutureOSDSsmaybetoohighifasubstantialnumberofadditionalsitesareconnected.Inaddition,WakullaCountyhaspassedanordinancerequiringadvancedOSDSs,whichwillreducetheconcentrationofnitrate-Ngoingtothedrainfieldsandwillreducetheactualnitrate-Nload.IfthisordinanceresultsinallthenewOSDSsbeingadvanced,andtheconversionofasub-stantialnumberofexistingOSDSs,thentheactualloadfromOSDSswillbelessthantheloadcalculatedhere.
Thetotalnitrate-Nloadatlandsurface(theloadatthedrainfield)inbothcountieswascalculatedbymultiplyingthenumberofOSDSs,thenumberofpersonspersystem(2.42forLeonCountyand2.57forWakullaCounty),awateruseof55gallonsperperson,andanitrate-Nconcentrationof60mg/L.Theresultwas40,000kg/yroftotalnitrate-Nin1966,increasingtoabout230,000kg/yrin2006.About350,000kg/yroftotalnitrate-Nisanticipatedby2018fig.13D).
Disposal of Biosolids by Land SpreadingFrom1966to2005,theCitydisposedofwastewater
biosolidsbylandapplication,whichconsistedofspreadingathinlayeracrossalargearea.MostofthelandapplicationoccurredattheCityairportsite(fig.1);however,smallervolumesweredisposedoffrom1996to2005atfoursites(Council,Petty,Strickland,andYoungfarms;fig.1).Atotalof37,000kgN/yrwasappliedin1966atallsitesco