Nitrate-N Movement in Groundwater from the Land ... · Davis, J.H., Katz, B.G., and Griffin, D.W. — — USGS/SIR 2010-5099

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


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

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.

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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

iv

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

v

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 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

vii

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).

ix

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


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),

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

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.

319

98

61

267

98

365

319

267

61

363

27

16

32

44

5256606

46872

48

4036

2824

20

12

8

8

12

16

20

4

28

24

SpringCreek Gage

St. MarksSpring

WakullaSprings

SpringCreekSpring

GEORGIA

FLORIDA

ALAB

AMA

0 25 MILES

25 KILOMETERS0

LEON COUNTYWAKULLA COUNTY CO

UNTY

JEFF

ERSO

N

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

4

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’° ° °


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.

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Center Pivot Operational Start Dates1-7

8, 9, 11, 1210, 1314-16

November 1980March 1982March 1986April 1999

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EXPLANATION

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SE53

Turf Sink

A B

SJ2

SE53SE22

SJ9

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SJ5

SJ4SJ3 SJ1

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Southwest FarmSprayfield

Airportsite

TALLAHASSEE

LS25

SF02Southeast FarmSprayfield


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

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1


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.

Geohydrologic Setting of the Wakulla Springs Springshed 7

Figure 4. Geologic units, hydrogeologic units, and model layers in north-central Florida.

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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


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,


Figure 5. Top of the Upper Floridan aquifer used to set the top of model layer 1. (Contours modified from Miller 1986.)

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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



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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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

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Figure 7. Thickness of the Upper Floridan aquifer.

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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



Figure 8. Location of the Wakulla Springs cave system.

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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



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

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Cody Scarp Gulf ofMexico

Low-PermeabilitySediments

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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]


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

—


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;


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


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


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


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


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



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


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

Documents

Nitrate-N Movement in Groundwater from the Land ... · Davis, J.H., Katz, B.G., and Griffin, D.W. — — USGS/SIR 2010-5099