interactions between ground water and surface water in the suwannee river basin, florida

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATIONVOL. 33, NO.6 AMERICAN WATER RESOURCES ASSOCIATION DECEMBER 1997

INTERACTIONS BETWEEN GROUND WATER AND SURFACE WATERIN THE SUWANNEE RIVER BASIN, FLORIDA'

Brian G. Katz, Rodney S. DeHan, Joshua J. Hirten, and John S. Catches1

ABSTRACT: Ground water and surface water constitute a singledynamic system in most parts of the Suwannee River basin due tothe presence of karst features that facilitate the interactionbetween the surface and subsurface. Low radon-222 concentrations(below background levels) and enriched amounts of oxygen-18 anddeuterium in ground water indicate mixing with surface water inparts of the basin. Comparison of surface water and regionalground water flow patterns indicate that boundaries for groundwater basins typically do not coincide with surface water drainagesubbasins. There are several areas in the basin where groundwater flow that originates outside of the Suwannee River basincrosses surface water basin boundaries during both low-flow andhigh-flow conditions. In a study area adjacent to the SuwanneeRiver that consists predominantly of agricultural land use, 18 wellstapping the Upper Floridan aquifer and 7 springs were sampledthree times during 1990 through 1994 for major dissolved inorganicconstituents, trace elements, and nutrients. During a period ofabove normal rainfall that resulted in high river stage and highground water levels in 1991, the combination of increased amountsof dissolved organic carbon and decreased levels of dissolved oxygenin ground water created conditions favorable for the natural reduc-tion of nitrate by denitrification reactions in the aquifer. Asaresult, less nitrate was discharged by ground water to the Suwan-nec River.(KEY TERMS: geochemistry; water quality; ground water hydrolo-gy; ground water/surface water interactions; hydrologic tracers; iso-topes; watershed management.)

INTRODUCTION

Interactions between ground water and surfacewater typically result in a single dynamic flow systemin many watersheds in Florida. Ground water andsurface water are hydraulically connected throughnumerous karst features (such as sinkholes, conduit

systems in the underlying limestone, and springs)that facilitate the exchange of water between the sur-face and subsurface, particularly where the UpperFloridan aquifer is unconfined (Figure 1). Uniqueproblems can arise in protecting both ground waterand surface water quality in karst areas because ofthe direct and rapid transport of recharge throughconduits to the subsurface and through resurgence bysprings. In some areas, recharge from unknowndrainage pathways to areas of discharge may con-tribute to chemical and biological contamination ofwater supplies. Such contamination in karst areashas been documented in many studies (White, 1993).

Legislation enacted in 1993 mandated that theFlorida Department of Environmental Protection(FDEP) develop and implement measures to protectthe functions of ecosystems in the State. Watershedmanagement is one of the main components of a pro-gram designed to protect and manage ecosystems.The FDEP has identified several key objectives toeffectively address comprehensive watershed manage-ment issues (Barnett et at., 1995): (1) more coordinat-ed management of ground water and surface waterresources; (2) more effective partnerships with local,regional, State, and Federal government agencies; (3)coordination of ground water and surface water moni-toring efforts to assess the quality and quantity of thewater resources and delineate the boundaries ofthree-dimensional watersheds; and (4) the develop-ment and maintenance of comprehensive statewidedata bases for water resource information and moni-toring networks oriented toward targeted watersheds.

1Paper No. 97010 of the Journal of the American Water Resources Association (formerly Water Resources Bulletin). Discussions are openuntil August 1, 1998.

2Respectively, Research Hydrologist, U.S. Geological Survey, 227 N. Bronough St., Suite 3015, Tallahassee, Florida 32301; Senior ResearchScientist, Florida Geological Survey, 903 W. Tennessee St., Tallahassee, Florida 32304; Law Engineering, Inc., 605 E. Robinson St., Suite 230,Orlando, Florida 32801; and Research Assistant, University of Florida, Department of Geology, 1112 Thrlington Hall, Gainesville, Florida32611.

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1237 JAWRA

Katz, DeHan, Hirten, and Catches

_I L

Figure 1. Generalized Hydrogeologic Section in the Suwannee River Basin Showing Karst FeaturesThat Facilitate the Exchange of Water Between the Surface and Subsurface.

The Suwannee River basin in Florida was one ofseveral watersheds nationwide selected for a pilotstudy by the Intergovernmental Task Force on Moni-toring Water Quality (ITFM). The study was designedto evaluate the effectiveness of current monitoringprograms, coordinated among Federal, State, andlocal agencies, in addressing key issues related tomonitoring water resources. Information gaps werefound to exist in State and Federal monitoring pro-grams and it was recommended by the ITFM thatthese gaps be addressed by developing an integrated,voluntary, nationwide strategy for water-quality mon-itoring (ITFM, 1995). The watershed approach hasbeen recommended as a highly effective way to man-age water resources because this approach integratesground water and surface water systems (ITFM,1995). Several other independent studies by nationaland international groups have recognized the need forintegrated monitoring of the various hydrologic com-ponents of a watershed for evaluating ecosystemhealth (United Nations Economic Commission forEurope, 1993; Interagency Ecosystem ManagementTask Force, 1995; Council of State Governments,1995).

This paper evaluates hydrologic and water-qualitydata for the Suwannee River basin in Florida to bet-ter understand the interactions between groundwater and surface water within the context of water-shed management concerns. Three critical watershedmanagement issues are addressed in the paper: (1)the degree of hydrochemical interaction betweenground water and surface water in the basin, (2) thecoincidence of ground water subbasins and surfacewater subbasins under different hydrologic condi-tions, and (3) the ability of natural processes toreduce elevated concentrations of nitrate in the UpperFloridan aquifer. The approaches presented in thispaper provide a framework for evaluating the impor-tance of the interactions between ground water andsurface water in a watershed context that can beextrapolated to other watersheds within Florida andnationwide where ground water and surface watersystems are intimately linked.

JAWRA 1238 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

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____ DIRECTION OFGROUND-WATER FLOW

GROUND-WATERDISCHARGE TOA SPRING/RIVER

__ I

I I I I I I I I I I I I I

Interactions Between Ground Water and Surface Water in the Suwannee River Basin, Florida

STUDY AREA

The Suwannee River basin in Florida consists of anarea of 11,000 km2 (Figure 2). The basin is character-ized by karstic wetland and lowland topography, asmall number of tributary streams, and an abundanceof Upper Floridan aquifer springs. Land uses in thebasin are predominantly forest, agriculture, and wet-land (Ham and Hatzell, 1996). The basin can be sub-divided into five subbasins (Figure 2) based onsurface drainage patterns: the Suwannee River basin

300

290

above the Withiacoochee River (Upper SuwanneeRiver subbasin), the Withlacoochee River subbasin,the Alapaha River subbasin, the Santa Fe River sub-basin, and the lower Suwannee River subbasin. Theclimate in the basin is subtropical and is character-ized by long, warm summers and mild winters. Rain-fall averages 132 cm per year (Crane, 1986); however,there are large variations between locations and fromyear to year. Approximately 50 percent of the averageannual rainfall occurs from June through September,but the shorter rainy season from late Februarythrough late April typically produces some of the

Figure 2. Study Area, Suwannee River Basin in Florida, Location of Monitoring Sites for GroundWater Levels and Background Ground Water Quality, and Surface Water Subbasins.

JOURNAL OF THE AMERICAN WATER RESOURCES AssociATioN 1239 JAWRA


highest stages for the Suwannee River and its tribu-taries.

Hydrogeologic Framework of the Suwannee RiverBasin

The major hydrogeologic units in the SuwanneeRiver basin include the surficial aquifer system, theintermediate confining unit, and the Upper Floridanaquifer (Table 1). The surficial aquifer system, whichconsists of undifferentiated sands and clays that arepost Miocene in age, ranges in thickness from 3 to 10m but may reach 15 to 18 m in the easternmost por-tion of the basin (Scott, 1991). The intermediate con-fining unit is composed of siliclastic sediments ofMiocene age and is present only under the NorthernHighlands in the northeastern part of the SuwanneeRiver basin in Florida. The thickness of the interme-diate confining unit in this geographic region mayexceed 100 m (Scott, 1988). The Upper Floridanaquifer, the uppermost part of the Floridan aquifersystem, consists of limestone and dolomite of Eoceneage. The Upper Floridan aquifer is the primary sourcefor industrial, agricultural, and municipal use in theSuwannee River basin in Florida. The base of potablewater in the Upper Floridan aquifer ranges fromapproximately 300 m below land surface in the south-ern part of the study area to more than 380 m belowland surface in the northern part of the study area(Ceryak et al., 1983). Solution features (sinkholes,solution conduits, springs) characteristic of karstareas provide the opportunity for direct hydraulic andgeochemical interactions between surface water andground water in the Suwannee River basin.

The hydrogeology within the Suwannee River basinis directly related to the physiography. The northernpart of the basin is located in the Northern High-lands, an area characterized by land surface altitudesgreater than 30 m, reaching elevations up to 70 m.Surface water features are common in the NorthernHighlands because clayey sediments of the intermedi-ate confining unit underlie the surficial aquifer sys-tem and retard the infiltration of rainwater. Thewater levels of surface water features reflect thewater table in the surficial aquifer system. The south-ern part of the Suwannee River basin lies in the GulfCoastal Lowlands physiographic division, which ischaracterized by land altitudes that are less than 30m and the presence of carbonate rock at or near landsurface. The Highlands and Lowlands are separatedby a topographic break referred to as the Cody Scarp(Pun and Vernon, 1964). With the exception of theSuwannee River, every river or stream that originatesin the Northern Highlands disappears undergroundas it crosses this transition zone. The Suwannee Riverremains above ground in the Gulf Coastal Lowlandsbecause it has incised the limestone of the UpperFloridan aquifer. The base flow for the Suwannee andSanta Fe rivers (Figure 2) is supplied by artesianspring flow.

Hydrology of the Suwannee River

The Suwannee River originates in the OkefenokeeSwamp area of south Georgia and flows southward forapproximately 390 km to the Gulf of Mexico. Near theriver's headwaters, the channel is incised in sandyclays and clayey sands that overlie the carbonate rock

TABLE 1. Generalized Stratigraphic Column for Suwannee River Basin(modified from Scott, 1991).

System Series Formation Hydrostratigraphic Unit

Quaternary Pleistocene and Holocene Undifferentiated Siliclastic SedimentsSurricial Aquifer System

Tertiary

Pliocene Cypresshead Formation

Miocene Hawthorn Group Intermediate Confining Unit

Oligocene Suwannee Limestone

CC

n0s:aa-a

.C0

UpperFloildanAquifer

Eocene Ocala LimestoneAvon Park FormationOldsmar Formation



(limestone and dolostone) of the Upper Floridanaquifer. Surface runoff draining swamps, flatwoods,and lakes provides the flow for this portion of theriver. In contrast, near the intersection of Hamilton,Columbia, and Suwannee Counties (Figure 2), theriver channel deepens and the river is incised intocarbonate rocks. Limestone outcrops and shoals arecommon along this part of the river. South of the con-fluence with the Santa Fe River, the basin is largelyinternally drained. Approximately nine first magni-tude (average discharge of 2.8 m3/s or more) and 63second magnitude (average discharge of 0.28 to 2.8m3/s) springs discharge into the Suwannee River inthe lower part of the basin (Rosenau et at., 1977).Together, these springs contribute about 100 m3/s tothe Suwannee River, which represents about 30 to 50percent of the average annual flow in the river, andstrongly influence the water quality of the river (Hullet at., 1981; Crane, 1986). The contribution of groundwater sources to the water quality of the SuwanneeRiver is discussed in greater detail in a later section.

Water-quality conditions in most sections of theSuwannee River are generally very good (Hand et at.,1990). Nutrient concentrations of the Suwannee Riverand its tributaries for the period 1970 to 1991 werelow, with median nitrate concentrations generally lessthan 0.4 mg/L as N. Median total phosphorus concen-trations were generally less than 0.5 mg/L as P (Hamand Hatzell, 1996), except in reaches of the upperSuwannee River, above the confluence with the With-lacoochee River (Figure 2), where the river and itstributaries are affected by discharges from wastewa-ter treatment plants, livestock feedlots, paper mills,and phosphate mines (Hand et at., 1990). Naturallyoccurring phosphate deposits result in relatively highbackground phosphorus concentrations (greater than1 mgfL as P) in the upper Suwannee River (Ham andHatzell, 1996). Increasing development along theSuwannee River corridor accompanied by an increasein septic tank fields can have an adverse impact onriver water quality due to increased nitrogen loading.

Monitoring Programs in the Suwannee River Basin

Historically, ground water and surface water sys-tems in the Suwannee River basin have been moni-tored separately under specific programs, with theexception of a small number of studies of localizedareas. During the past 30 years, a considerableamount of hydrologic data (such as river stage andground water level) have been collected as part ofextensive surface water and ground water networks.

The Suwannee River Water Management District,(SRWMD) in cooperation with the FDEP and U.S.Geological Survey (USGS), maintains rainfall, water

level, and water-quality monitoringnetworks in theSuwannee River basin. These networks were estab-lished to evaluate present surface water and groundwater conditions and to determine short and long-term trends and conditions. As part of a surface waternetwork, lake water levels are being measured regu-larly at 17 sites, river stage and discharge are moni-tored at 18 sites, and daily rainfall is recorded at 34sites. Surface water quality samples are collectedmonthly, bimonthly, or quarterly at 52 sites by theSRWMD as part of the Surface Water Improvementand Management (SWIM) program, which was estab-lished in 1989. Currently, ground water levels arebeing measured at 328 sites in the basin, whichincludes monthly measurements at 43 wells and con-tinuous measurements (using water-level recorders)at 32 wells.

Since 1987, information on ground water qualityhas been collected as part of the Florida GroundWater Quality Monitoring Network Program(FGWQMNP), which was established to delineate thebaseline or background water quality of the majoraquifer systems within Florida and to determine theeffects of various land use activities on ground waterquality. The 107 wells designed to monitor back-ground water quality of the principal aquifers in theSuwannee River watershed were selected to avoidknown areas of ground water contamination. Waterfrom these wells is sampled every three years formajor ions, nutrients, trace elements, and selectedorganic compounds (Maddox et al., 1992). Also as partof the FGWQMNP, the effects of land-use practices onground water quality are being studied at a mixedurban-industrial site and at an agricultural studyarea in the Suwannee River basin.

As part of the National Water Quality AssessmentProgram (NAWQA) study of the Georgia-FloridaCoastal Plain, the USGS has sampled water from sixwells in its regional background network for the surfi-cial aquifer system (Crandall and Berndt, 1996).Seven sites on the Suwannee River in Florida and onesite on the Santa Fe River are being sampled byNAWQA for bed material, water quality, and biologi-cal species (Ham and Hatzell, 1996).

METHODS AND APPROACH

Methods of collection and analysis of ground watersamples, along with data screening procedures andquality assurance information for ground water quali-ty data have been previously described (Katz, 1992;Maddox et at., 1992). Chemical analyses of water from107 wells tapping the poorly confined parts ofthe Upper Floridan aquifer (Figure 2) include data



collected and compiled for the FGWQMNP (Katz,1992; Maddox et al., 1992). Methods of collection andanalysis of water samples from the Suwannee Riverare described by Hull et al. (1981) and EnvironmentalServices and Permitting, Inc. (1995).

Maps of the potentiometric surface of the UpperFloridan aquifer were constructed using water-leveldata for 226 wells that were measured during high-flow conditions in April and May 1984 and usingwater-level data for 325 wells measured during low-flow conditions in December 1990 and January 1991.The potentiometric contours that were constructedfrom water-level data were generalized to representsynoptically the altitude to which water would rise intightly cased wells. No attempts were made toaccount for differing depths of wells, non simultaneousmeasurements of water levels, variable effects ofpumping, and changing climatic influence.

Descriptive and nonparametric statistics were usedto summarize the concentrations of selected chemicalconstituents. Boxplots were used to present the medi-an, the 10th, 25th, 75th, and 90th percentiles, andprobable outliers of nitrate concentrations in groundwater from an agricultural study area in LafayetteCounty. Nonparametric statistical techniques wereused for analysis of water-quality data, because thesedata generally were not normally distributed, andthese techniques do not require equal variances.Hypothesis tests were used to determine if observeddifferences in concentrations or water levels are dueto random variability or statistically significant popu-lations. Spearman's Rho statistic was used to deter-mine the degree of correlation between Alapaha Riverstage and water levels in a nearby well and thedegree of correlation of pH and specific conductancewith ground water levels. This nonparametric statis-tic measures the strength of an increasing or decreas-ing relation between two variables (Iman andConover, 1983). All observations were given equalweight so that any extreme values will not have a dis-proportionate effect on the correlation. A positive ornegative relation between two variables was consid-ered significant at an alpha (cx) value, or level of sig-nificance, of 0.05 or less.

RESULTS AND DISCUSSION

Water-Quality and Water-Level Evidence for GroundWater/Surface Water Interactions in the Basin

The water quality of the Suwannee River at anygiven location is affected by discharge of water fromswamps and wetlands, the surficial aquifer system

and the Upper Floridan aquifer, point sources, andnonpoint sources. During periods of low flow, much ofthe water in the Suwannee River in the upper part ofthe basin originates from tributary streams and dis-charge of water from the surficial aquifer system. Inthe lower Suwannee River subbasin, river flow origi-nates from springs. Springs that normally dischargeinto the Suwannee River and its tributaries may tem-porarily become sinks when high-flow conditionscause flow to reverse (Giese and Franklin, 1996).Flood peaks of some streams in the Suwannee Riverbasin were significantly attenuated as a result of sub-stantial subsurface storage provided by the karsticnature of the Upper Floridan aquifer (Giese andFranklin, 1996).

The relative influence on water quality from vari-ous sources that contribute water to the SuwanneeRiver are indicated by downstream changes in riverwater chemistry. Based on an analysis of water-quali-ty data collected during 1968-1977 (Hull et al., 1981)and 1989-1995 (Hornsby and Mattson, 1996), pH, spe-cific conductance, and concentrations of calcium (Ca),magnesium (Mg), bicarbonate (HCO3), and sulfate(SO4) clearly show an increase with distance down-stream (Table 2). For example, the mean pHincreased from 4.20 at Benton, Florida, to 5.88 atSuwannee Springs, Florida, and to 7.31 at Wilcox,Florida (Hornsby and Mattson, 1981). Accompanyingthese increases, the concentrations of sodium (Na),potassium (K), and nitrate (NO3) increased slightly,while the concentrations of total organic carbon(TOC), total organic nitrogen, and dissolved irondecreased in a downstream direction (Table 2). Trendsin water chemistry are consistent with the relativecontribution of water from surface water and groundwater sources.

The effect of increasing ground-water discharge ina downstream direction on the chemistry of water inthe Suwannee River in a downstream direction isshown in a Durov diagram (Figure 3). This diagram issimilar to a quadrilinear Piper diagram with theexception that the intersection points from the twoPiper triangles (showing the relative contribution of aparticular cation or anion to the sum of cations oranions, in milliequivalents per liter) are plottedagainst two additional variables (pH and TOC, in thiscase) in adjacent scaled rectangles. For instance, nearthe headwaters of the river (near Benton, Florida),there is no dominant cation or anion in river water,which results in a mixed water type. The low pH, lowdissolved solids (Table 2), and relatively high concen-trations of TOC (Figure 3) in the river water nearBenton, Florida, indicate the relatively large contribu-tion of water from tributary streams and runoff'originating in swamps and wetlands. Further down-stream, the contribution from ground water increases,



TABLE 2. Mean Concentration, Number of Samples, and Range of Concentrations (in parentheses) of SelectedWater-Quality Constituents Determined at Four Successive Downstream Locations (see Figure 2) on the Suwannee River.

[Concentrations are expressed as milligrams per liter unless otherwise noted. SC denotes specific conductance;pS/cm denotes microsiemens per centimeter; Alk denotes alkalinity, expressed as CaCO3; TOC denotes total

organic carbon; TON denotes total organic nitrogen as nitrogen (N); P denotes phosphorus; p.g/L denotes microgramsper liter. Data are summarized from 1989-1995 (Hornsby and Mattson, 1996) and represent total concentrations

of chemical constituents with the exception of TON and dissolved Fe, which are from 1968-1977 (Hull et ci., 1981).]

Station Name and Distance inSuwannee

Kilometers From Mouth of River

Benton Springs Branford WilcoxConstituent 315 242 123 55.5

pH 4.2074; (3.06-7.43)

SC, pS/cm 6874: (43-160)

TOC 5174; (2 1-76)

Alk 6.474; (0.4-23)

Ca 1.766; (0.1-10.6)

Mg 0.9666; (0.1-4.4)

Na 3.666; (0.1-6.1)

K 0.6566; (0.2-1.7)

CI 7.666; (0.4-12)

So4 5.266; (1-17)

NO3 asN 0.0574; (0.0 1-0.22)

TON 0.67816; (0.37-0.94)

P04-ortho as P 0.05774; (0.004-0.19)

Fe,tg/L 4605; (280-630)

5.8874; (3.85-7.92)

- 7.1574; (5.56-8.15)

7.3173; (6.28-8.16)

12274; (42-355)

20973; (29-447)

23474; (48-500)

4174; (6.3-81)

1674; (2.3-37)

1574; (2.0-36)

3074; (0.4-150)

8374; (3.9-150)

9374; (14-160)

1266; (2.444)

2866; (3.4-54)

3366; (5.9-57)

3.266; (1.0-13)

5.766; (1.1-21)

5.266; (1.2-9.6)

5.066; (2.9-12)

5.366; (2.6-9.9)

4.966; (3.3-11)

0.7366; (0.1-3.5)

1.166; (0.5-3.0)

1.366; (0.5-10)

7.566; (1.2-12)

7.166; (3.0-11)

7.566; (1.3-13)

1366; (1.0-43)

1466; (1.0-32)

1566; (1.0-27)

0.1574; (0.0 1-0.72)

0.5774; (0.05-1. 1)

0.5474; (0.05-1.0)

0.7346; (0.11-1.5)

0.4483; (<0.02-1.4)

0.3544; (0.01-0.93)

0.7574; (0.05-3.9)

0.1274; (0.04-0.33)

0.1073; (0.05-0.27)

41012; (110-660)

21032; (<3-570)

17011; (<3-350)

particularly near Branford, Florida, where the wateris dominated by Ca and HCO3 ions resulting aCa-HCO3 water type. Accompanying this change inwater type is a substantial decrease in TOC and anincrease in pH. The chemistry of the river water nearBranford, Florida, is controlled largely by springwater discharge to the river, and the similarity in

water chemistry of the river water and spring waternear Branford, Florida, is shown by the closeness ofthe points representing these waters in Figure 3.

A substantial increase in the concentration ofNO3 in the Suwannee River near Branford, Fla.,(Table 2) probably is the result of the contribution ofspring water with elevated NO3 concentrations. This


SURFACE WATER CHEMISTRY

E +

0

Q).'- •E X-Suwannee River near

Deep Creek tributary

Suwannee River near

Suwannee River near

Suwannee River near

Benton, Fla.

Suwannee Springs,

Branford, Fla.

Wilcox, Fla.

Fla.

GROUND WATER CHEMISTRY

Surficial aquifer system

Upper Floridan aquifer

0 Spring near Wilcox, Fla.

V Spring near Branford, Fla.

00Co

0c'j

C-0Cz>I-0'1-1Im

rn

C-)

1

m

'iiC/)0C0mC',

C),U,0C)

0z

aN

CD

CD

aCD-

ciaC)

CDCI,

Zcrow

wz<C/)

oI-

4 6 8

pH60

30

0

Figure 3. Durov Plot Showing Differences in the Major-Ion Composition, pH, andTotal Organic Carbon Concentrations of River Water and Ground Water.

-JLU>LU-J

_i<UJw>(I)LU.JW

>UJ

I

connection is supported by the inverse relationbetween NO3 concentrations and stage in the Suwan-nee River (Hornsby and Mattson, 1996). Large sea-sonal fluctuations in NO3 concentrations, pH, and

wI-

-w0—ZI-.F-LU00

ZWO00)_W

OUJw0

specific conductance of river water have beenobserved at several locations (Hornsby and Mattson,1996). Low concentrations of these constituents dur-ing high-flow conditions from February through April



(A)

(B)

Ia

Figure 4. (a) Relation of Alapaha River Stage and Water Level in Alapaha Fire Tower Well, 1989-1994, and(b) Variation in pH and Specific Conductance in Water from Alapaha Fire Tower Well, 1989-1994.

1989 1990 1991 1992 1993 1994

Katz, Dellan, Hirten, and Catches

in 1994 and again in 1995 is due to dilution from alarge contribution of surface runoff generated duringspring rainfall.

In some locations in the basin, the hydraulic link-age between ground water and surface water is evi-denced by the correlation of ground water and surfacewater levels. For example, variations in the stage ofthe Alapaha River are highly correlated (r 0.87)with water levels in the Upper Floridan aquifer (Fig-ure 4). Ground water levels in nearby Alapaha FireTower well increased significantly (a 0.001) withincreasing stage in the Alapaha River during 1989-94.

Further evidence for the connection betweenground water and surface water is indicated by watersampled from the Alapaha Fire Tower well, whichrepresents a mixture of river water and ground water.The relative proportions of ground water and surfacewater in the mixture can be evaluated qualitativelybased on changes in specific conductance of watersamples from the well (Figure 4). During high flowconditions, surface water enters the aquifer, and theconductance of water in the aquifer decreases. This isdue to the mixing of river water having lower medianspecific conductance (39 tS/cm, Hull.et al., 1981), withaquifer water having a higher median specific conduc-tance (310 p.S/cm, Maddox et al., 1992). A negativecorrelation between water levels and specific conduc-tance was found; however, the relation is not statisti-cally significant. There was a significant (a = 0.007)positive correlation (r = 0.41) between pH and waterlevels in the aquifer. Based on differences in the medi-an pH of river water, 5.6 (Hull et al., 1981) and that ofthe aquifer, 7.1 (Maddox et aL, 1992), one mightexpect that pH would be a useful indicator of mixingof ground water and surface water. However, the rela-tion between water levels and the pH of water fromthe Alapaha Tower well is not as straightforward asthe relation between specific conductance and waterlevels because the range in pH values for river water,4.7 to 7.2 (Hull et al., 1981) overlaps the range of pHvalues measured in water from the Upper Floridanaquifer in this area, 5.3 to 9.4 (Katz, 1992).

Hydrologic tracers, such as naturally occurringenvironmental isotopes including radon, (222Rn,Ellins et al., 1991; Kincaid, 1994); and oxygen(180/160), hydrogen (D/H), and carbon (13C/12C) (Katzet al., 1995a, 1995b), can be used to quantify moreprecisely the mixing proportions of river water andwater from the Upper Floridan aquifer for variousflow conditions. Environmental isotopes and syntheticorganic compounds have been used in a number ofprevious studies to establish and quantify the linkagebetween the river and aquifer where the interactionsbetween ground water and surface water are muchmore complex. For example, naturally occurring

radionuclides, such as uranium (238U and 2U), radi-um (226Ra), and 222Rn, were used to trace groundwater influx to rivers and streamflow losses to groundwater. These studies rely on the mobility of U, Ra,and Rn, which is controlled by different geochemicaland physical processes leading to their separation orfractionation in ground water and surface water sys-tems (Cowart and Burnett, 1994; Kraemer, in press).For example, 222Rn is a gas and, hence, its concentra-tion in ground water can be two to three orders ofmagnitude higher than the concentration in surfacewater.

In a study along a 2-km reach of the Santa FeRiver (a tributary to the Suwannee River), measure-ments of 222Rn and SF6 (sulfur hexafluoride, avolatile synthetic gas that was injected as a tracerand used to estimate and correct for radon loss) inground water and in surface water revealed thatincreases in river discharge were accompanied by cor-responding increases in ground water discharge tosprings, loss of streamfiow to ground water, and inputto springs from resurgent streamfiow (Kincaid, 1994).One particularly noteworthy finding was that eventhough the regional potentiometric-surface map of theUpper Floridan aquifer shows that the Santa FeRiver is a gaining stream, streamflow is actuallybeing lost to the Upper Floridan aquifer in manyplaces along the river. Siphons that are visible at thesurface also provide direct evidence that stream wateris being diverted to the subsurface. As much as 55percent of spring discharge at this study area wassupplied by resurgent surface water that originated inthe the overlying Santa Fe River, and not water fromthe Upper Floridan aquifer (Kincaid, 1994).

Differences in the 234U/238U activity ratio (UA.R)and uranium (U) concentrations in ground water andsurface water were used to determine the source andamount of recharge for different parts of the aquiferand ground water contributions to the SuwanneeRiver (Crane, 1986). Most sampled sites producedwaters with low activity ratios (less than 1.0) andhigh U concentrations (0.5 to 10 p.g/L). These charac-teristics typically are associated with areas where theUpper Floridan aquifer is unconfined and whererecent and intense dissolution of aquifer minerals isoccurring, such as in places where material overlyingthe Upper Floridan aquifer has been breached bysinkholes. For example, three water samples from aSuwannee River tributary basin (including thesprings near Branford, Florida, and wells upgradient.of the springs), had very low activity ratios (0.57) andhigh U concentrations (1.76 p.g/L). Many springs hadactivity ratios greater than 0.75, which Craneattributed to a mixture of ground water from areas ofhigh recharge with ground water from areas of littleor no recharge. The Suwannee River has UAR values



and U concentrations that are anomalous when com-pared to those of other river systems of the world(Crane, 1986). The anomalous values result from mix-ing of some surface runoff (high UAR, low U concen-tration) with large amounts of ground water flow fromsprings and seeps (generally low UAR and high Uconcentrations).

The input of water to the Suwannee River fromlarge springs south of Branford was traced using226Ra (Burnett et al., 1990). Stream stations north ofBranford tended to have a lower mean concentrationof 226Ra (0.189 disintegrations per minute per liter,dpmlL) compared to the mean concentration for sta-tions south of Branford (0.270 dpmlL). Even though226Ra has a strong affinity for adsorption on aquiferminerals, the concentration of 226Ra in ground wateris generally several times to several orders of magni-tude higher than in surface waters. Several first-order magnitude springs have relatively high 226Raconcentrations (0.155 to 0.917 dpmtL) and the concen-tration of 226Ra in these springs progressivelyincreased in a downstream direction. This trend wasattributed to the increasing contribution of waterfrom deeper parts of the Upper Floridan aquifer thatsupply spring water to the lower reaches of the river(Burnett et al., 1990).

A study of the hydraulic connections between theSuwannee River and the unconfined Upper Floridanaquifer along a 75-km reach from Ellaville to Bran-ford, revealed that the direction of thehydraulic gra-dient in the aquifer is toward the river duringlow-flow conditions and away from the river duringhigh-flow conditions (Hirten, 1996). The effect of ele-vated river stage on the potentiometric surface of theUpper Floridan aquifer can extend outward from theriver to at least 3 km in this area during high-flowconditions based on a high correlation (r2 = 0.90)between river stage and water levels measured inwells located less than 3 km from the river (Hirten,1996). Water levels measured in wells located morethan 3 km from the river also fluctuated in concertwith river stage; however, Hirten (1996) noted'thatthere was a lag time of approximately 25-45 daysbetween a rise in river stage and a rise in groundwater levels. Geochemical data for ground water nearthe river are very limited, but do indicate the possibil-ity that river water mixes with ground water over dis-tances of much less than 3 km (Hirten, 1996). Impactson water quality from river water mixing with waterin the Upper Floridan aquifer are probably restrictedto smaller, localized areas compared to those areaswhere ground water levels are affected by high riverstage. Dilution effects may account for the localizedimpact of river water on aquifer water quality.

Comparison of Ground Water and Surface WaterBasin Boundaries

The accurate delineation of karst drainage basinsrepresents a considerable challenge because of com-plex patterns of surface water and ground water flow.Studies have shown that in karst systems, surfacewater basins typically do not coincide with corre-sponding ground water basins (Gunay, 1988). Sinkingstreams can reappear in different basins becauseground water divides are controlled by aquifer proper-ties as well as by topographic conditions. Carbonateoutcrop areas and joint and fracture patterns must be1accurately mapped in great detail to determine thecontribution from autogenic recharge, which consistscif diffuse infiltration into fractured limestone andinternal runoff into sinkholes (White et al., 1995).

Two-dimensional (areal) boundaries for surfacevvater and ground water subbasins in the SuwanneeFtiver basin were compared for two different hydrolog-i conditions: a relatively dry period with low-flowcnditions during December 1990 to January 1991and a wet period with high-flow conditions duringAril to May 1984. Patterns of ground water flowwfere determined from potentiometric-surface maps oftlie Upper Floridan aquifer constructed for low flowairid high flow conditions. It was assumed that theaLuifer regionally was isotropic and that flow lineswtere perpendicular to equipotential lines. Thesegiound water flow patterns were superimposed onsub-face water basin boundaries (delineated based ontopography) for the major subbasins within theSiwannee River basin. Generally, the comparison ofground water flow patterns with surface water basinboundaries indicate that ground water basins do notcoincide with surface water drainage subbasins,except in some parts of the lower Suwannee Riverbsin and the Santa Fe River subbasin (Figure 5)wliere ground water and surface water are well con-ncted. The patterns of ground water flow were verysiirnilar during low-flow and high-flow conditions;therefore, only the potentiometric surface map con-stiucted for high flow conditions is shown (Figure 5).THere are several areas in the basin where groundweter that originates outside of the Suwannee Riverb$in crosses surface water basin boundaries duringboth low flow and high flow conditions. For example,in Hamilton County, during high-flow conditionswater levels increased by nearly 6 m compared tolevels measured during low-flow conditions; however,during both low- and high-flow conditions, groundwater flow lines cross surface water drainage bound-aries, particularly where a potentiometric surfacehigh exists in eastern Hamilton and western


300

290

Katz, DeHan, ]Elirten, and Catches

Figure 5. Potentiometric Surface of th Upper Floridan Aquifer and Ground Water FlowPatterns During High Flow Cond(itions Relative to Surface Water Subbasins.

Columbia County (Figure 5) Also, the area of a poten-tiometric surface high (represented by the 27-rn con-tour) enlarges during high-flow conditions frornasmall area in northeast Taylor County to a much lar1g-er area that includes central Madison County (Figure5). Ground water basin boundaries do not coincidewith surface water basin boundaries in Madison, Tay-br, and Jefferson Counties.

It is important to note that the ground water leveldata used for constructing potentiornetric surfacemaps were collected as part of a program that was

designed to delineate the regional potentiometric sur-face of the Upper Floridan aquifer rather than todelineate ground water basins. The wells in this net-work are open to different depths in the aquifer andthe open intervals of these wells probably interceptboth localized and regional ground water flow sys-tems; therefore, the wells in the present network(approximately 250 wells in the basin) are not ideallysuited to define two-dimensional boundaries forground water basins accurately in most areas ofthe Suwannee River basin. No attempt was made to


Interactions Between Ground Water and Surface Water in the Suwannoc River Basin, Florida

separate shallow and deep ground water flow systemsbased on the depth or open interval of a well.

To define ground water and surface water drainageareas more accurately on a local scale, the hydraulicconnections between discharge areas (such as springs)for the Upper Floridan aquifer and for surface waterneed to be determined for different flow conditionsusing tracer techniques involving dyes, naturallyoccurring isotopes and, in some cases, human explo-ration (cave diving). In one such study in the SantaFe River basin, the degree of interconnection amongsprings that discharge from Ginnie Springs Park tothe Santa Fe River was studied using rhodamine dyetracing experiments. Based on the dispersion of dye tomore than one spring, Wilson and Skiles (1988) con-cluded that there is an extensive network of three-dimensional braided conduits in the aquifer systemand unique ground water drainage divides do notexist within a few hundred meters of the spring dis-charge points.

Natural Remediation of Nit rate in Ground Water

High nitrogen loading from wastes generated bypoultry and dairy farms, and fertilizers applied tocropland along the Suwannee River in Lafayette andSuwannee Counties has resulted in elevated NO3 con-centrations in the Upper Floridan aquifer (Andrews,1994; Ceryak and Hornsby, 1996). Water of the UpperFloridan aquifer flows toward and discharges to theSuwannee River in this area and the discharges frommany springs and seeps in the riverbed affect the con-centration of NO3 in the river (Horn sby and Mattson,1996).

Ground water flow and its effect on water qualityare being evaluated in a 73-km2 study area adjacentto the Suwannee River in Lafayette County (Figure 6)as part of the FGWQMNP. The study area consistsmainly of agricultural land use, such as dairy andpoultry farms, cropland, and silvaculture. In April-

Figure 6. Lafayette County Agricultural Study Area With Potontiomotric Surfaceof Upper Floridan Aquifer and Location of Sampling Sites.


EXPLANATIONSTUDYAREABOUNDARY

DIRECTION OF GROUND.WATER FLOW

POTENTIOMETRICCONTOUR--In metersabove sea level


May 1990, March 1991, and June 1994, water sam-ples from 18 wells tapping the Upper Floridan aquiferand seven springs that discharge into the SuwanneeRiver were analyzed for NO3 and other chemical con-stituents. The principal source of NO3 in water dis-charged by springs to the Suwannee River wasattributed to a combination of leachate from livestockwastes and septic tanks, based on measured nitrogen-isotope ('5N/'4N) ratios of NO3 in water samples col-lected in May 1993 (Andrews, 1994).

In the study area, the dominant regional groundwater flow direction is north to northeast toward theSuwannee River (Figure 6). However, during the 1991sampling period, heads in the Upper Floridan aquifernear the Suwannee River were lower than water lev-els in the river creating the potential for water to flowfrom the Suwannee River into the Upper Floridanaquifer. Ground water levels in 1991 increased by asmuch as 6 m at some wells following three months ofabove normal rainfall, compared to water levels in1990 and 1994 (near normal rainfall).

Nitrate concentrations in ground water variedwidely for sampling periods in 1990, 1991, and 1994(Figure 7). For instance, the median and range (inparentheses) of NO3 concentrations in water samplesfrom the Upper Floridan aquifer (in mg/L as nitrogen)were: April-May 1990, 1.52 (< 0.02-17); March 1991,0.20 (< 0.05-9.5); and June 1994, 2.0 (< 0.02-22.). Thevariation in the distribution of NO3 concentrationswere associated with fluctuations in water levels inthe Upper Floridan aquifer. Substantially lower con-centrations of NO3 were measured in water from 76percent of wells sampled in 1991 (when ground waterlevels were high) compared with ground water sam-ples collected in 1990 and 1994.

The large decrease in NO3 concentrations inground water sampled in 1991 probably results fromdenitrification reactions. The process of denitrifica-tion involves the transformation of NO3 by bacteria(present in the soil and aquifer material) to nitrogengases, such as N2O and N2. This process serves as amajor sink for nitrogen and can result in substantial-ly lower amounts of NO3 in aquifers (Korom, 1992).For denitrification to occur in an aquifer, four condi-tions are required (Firestone, 1982): (1) the presenceof NO3 and other N oxides as terminal electron (e-)acceptors, (2) the presence of heterotrophic orautotrophic bacteria, (3) e-donors, such as organic car-bon, or other reduced species such as Fe2 and sul-fide, and (4) an anaerobic environment or one withrestricted 02 availability.

The overall reaction involving denitrification in acarbonate aquifer can be represented by the followingexpression:

4NO3 + 5CH20 + 5CaCO3 + 4H ----> 2N

+ 10HC03 + 5Ca2 + 2H2O

In the above expression, an organic substrate repre-sented by CH2O serves as an electron donor. Interme-diate products, such as nitrite (N02) and nitrogengases (NO and N20) are produced but are not shownin the above reaction. In this overall denitrificationreaction, hydrogen ions are consumed, resulting in anincrease in pH. Assuming that dissolution of calcite(in limestone matrix comprising the Upper Floridanaquifer) occurs, bicarbonate and calcium ions are pro-duced by the reaction. Other indicators of reduced oranoxic conditions would include an increase in dis-solved iron and manganese concentrations.

To positively confirm that denitrification reactionshave occurred it would be desirable to measurechanges in specific indicator constituents, such asnitrous oxides and the isotopic composition of nitro-gen species along flow paths. However, analyses fordissolved nitrogen gases and nitrogen isotopes werenot included as part of the monitoring network pro-gram. Therefore, for this study, the potential for deni-trification reactions is evaluated (below) by comparinghydrologic data and changes in the concentration ofselected chemical species for ground water sampled inthe Lafayette County study area during 1990 and1991.

The following hydrologic and geochemical informa-tion provides evidence that conditions existed in theUpper Floridan aquifer during March 1991 thatwould promote denitrification. Measured groundwater levels increased in all sampled wells; forinstance, some water levels increased by as much as6.4 m following three months of above normal rainfallin 1991. The hydraulic gradient decreased from0.0019 in 1990 to 0.0012 in 1991. It is likely that amore sluggish flow system resulted from a dammingeffect by the river on water discharging from theUpper Floridan aquifer. Also, heads in the UpperFloridan aquifer near the river were lower than waterlevels in the river, probably resulting in the develop-ment of a "saddle" in the potentiometric surfacewhereby water from the river could flow into parts ofthe aquifer. Accompanying these hydrologic changeswere substantially lower concentrations of NO3 mea-sured in water from 71 percent of wells sampled in1991 compared to 1990. River water with high TOCconcentrations (Table 2) could provide organic carbonneeded for denitrification; however, organic carbonderived from the limestone may serve as another e-donor source (Foster et al., 1985). Furthermore, thelower NO3 concentrations likely were due to denitrifi-cation rather than dilution because concentrations of



Figure 7. Differences in (A) Ground Water Levels and (B) Nitrate-N Concentrations in Ground Water,Lafayette County Study Area, During April-May 1990, March 1991, and June 1994.


24

22

20

18

16

14

12

10

8

6

A

EXPLANATION(17) Number of observations

* Data vatues outside the10th and 90th percentiles

90th percentile

75th percentile

Median

25th percentile

10th percestite

(A)

(B)

(17)

II

(16)*

*

(15)*l-w>uJ<JLU

UJ(J)

I—0<cI

0(j)ZaOF—uJ

wI-

z-J

0w

cr<

-J-J

F

April-May March June1990 1991 1994

24

22

20

18

16

14

12

10

8

6

4

2

0

EXPLANATION ()(26) Number of observations

* Data values outside the10th and 90th percentiles

90th percentile *75th percentile

Median

25th percentile

10th percentile

(17)*:

Li.

April-May March June1990 1991 1994

(I)-J-J

wLLF

Oozw00cr'LU00

chloride and other major ions did not show a corre-sponding decrease from 1990 to 1991. For instance,chloride concentrations increased in water samplesfrom 75 percent of the wells in which NO3 concentra-tions decreased from 1990 to 1991. Water from 71 per-cent of wells sampled also showed a decrease in redoxpotential based on measured Eh, and an increase inmeasured pH, Ca2, HC03, and DOC (Figure 8).

The increase in pH, Ca2, and HC03 is consistentwith the overall denitrification reaction presentedearlier. Additional evidence for more reducing condi-tions in ground water in 1991 compared to 1990 is theincrease in dissolved iron in water from 61 percent ofthe wells that yielded water with decreased NO3 con-centrations. Also, an increase in sulfate concentra-tions in water from approximately 60 percent of thewells sampled indicates the possibility that sulfide isalso an e-donor for the denitrification. In a study ofthe fate of NO3 in ground water beneath several dairyfarms in Suwannee and Lafayette Counties, Andrews(1994) noted that denitrification was more likely tooccur in deeper rather than in very shallow groundwater because deeper ground water had considerablylower dissolved oxygen concentrations than shallowground water.

To reiterate, it is likely that a sluggish groundwater flow system in March 1991, the significantlyreduced amounts of dissolved oxygen in the aquifer,combined with increased amounts of organic carbon

and other reduced species that could serve as e-donors, created conditions favorable for the naturalreduction of NO3 in ground water by denitrification.As a result, less NO3 would be discharged by groundwater to the Suwannee River during high-flow peri-ods than would be discharged during low-flow periods.

SUMMARY AND CONCLUSIONS

The direct linkage between ground water and sur-face water in parts of the Suwannee River basin hasbeen demonstrated by analysis of hydrologic andwater-chemistry data collected during regionalassessments and localized studies. Naturally occur-ring tracers (isotopes, major and minor chemicalspecies), artificially introduced tracers (dyes andother synthetic compounds such as sulfur hexafluo-ride), and detailed data on ground water levels andriver stage provide important information on thehydrochemical interactions between ground waterand surface water. Complex ground water flow andmixing patterns can be revealed only through consis-tent monitoring with time and during varying hydro-logic conditions. Insufficient chemical and hydrologicinformation exists at present to determine the lateralextent of mixing of river water with water in theUpper Floridan aquifer during periods of high flow

Katz, DeHan,Hirten, and Catches

100 —

80 -

60 -

40 -

20 -

F1 INCREASE IN VALUE OR CONCENTRATIONFROM 1990 TO 1991

I I DECREASE IN VALUE OR CONCENTRATIONFROM 1990 TO 1991

pj_Ca HCO3 DOC Cl Eh

Figure 8. Changes in Water Level, Eh, pH, and Concentration of Selected Constituents inGround Water From 1990 to 1991 in Lafayette County Study Area.

WATER pHLEVEL



conditions except in very localized areas. Sampling ofground water and surface water needs to be coordi-nated and conducted during changing hydrologic con-ditions, especially during periods of high and low flow.

Existing information on ground water levels col-lected during low- and high-flow conditions indicatesthat ground water basin boundaries (delineated frompotentiometric-surface maps) generally do not coin-cide with boundaries for surface water subbasins andthat there is flow of ground water across surfacewater subbasin divides. To define areal and verticalground water basin boundaries more precisely, addi-tional wells would be needed to refine the map of thepotentiometric surface for the Upper Floridan aquiferand to obtain more detailed flow patterns. The use ofground water level data to evaluate the coincidence ofground water and surface water basin boundaries wascompromised somewhat because these data were col-lected by different agencies for programs with differ-ent monitoring objectives.

Reduction in nitrate concentrations in groundwater by denitrification processes is likely to occurnaturally during periods of high flow conditions (highwater levels in the Upper Floridan aquifer and highstages in the river and its tributaries). A period ofabove normal rainfall in 1991 resulted in high riverstage and high ground water levels. The combinationof a more sluggish ground water flow system duringthis period along with increased amounts of dissolvedorganic carbon (from river water) and less dissolvedoxygen in ground water created conditions favorablefor the natural reduction of nitrate in the Upper Flori-dan aquifer. Increases in pH and the concentrations ofcalcium, and bicarbonate in ground water were con-sistent with the denitrification process. More special-ized sampling of ground water along flow paths fornitrogen isotopes and nitrous oxide compounds wouldprovide much needed information to confirm and doc-ument the importance of naturally occurring denitrifi-cation reactions in parts of the watershed

The results of this study add to our understandingof hydrochemical interactions between ground waterand surface water across spatial and temporal scales.The study findings also improve our knowledge of theimportance of natural and anthropogenic factors onobserved changes in environmental conditions thai;are related to the linkage of ground water and surfacewater systems in a watershed. In order to protect,sustain or manage watershed ecosystems, it is critica.lto establish an integrated monitoring program wherevarious resources that comprise an ecosystem areevaluated both spatially and temporally (such as soil,ground water, surface water, sediments, the geologicframework, and atmospheric deposition).

It is unlikely that adequate State and Federalfunding will be available in the foreseeable future to

conduct delineation and mapping of zones of interac-tion between ground water and surface water inwatersheds throughout Florida and other states. Onesolution to this problem might be the establishment ofwatershed coalitions, involving the private sectorstakeholders, that could be responsible for generatingthe necessary resources for proper watershed assess-ments. The watershed coalitions will benefit theircommunities by not only identifying the threats to thehealth and function of watersheds, but also by being apart of the solution to existing problems.

ACKNOWLEDGMENTS

The authors acknowledge the joint funding for this study by theIntergovernmental Task Force on Monitoring Water Quality andthe Florida Department of Environmental Protection. The authorsalso thank Ron Ceryak and David Hornsby, Suwannee River WaterManagement District, for their generous sharing of water-qualitydata and hydrologic information. The following reviewers are alsorecognized for their constructive comments that led to an improvedmanuscript: J. B. McConnell, D. M. Schiffer, and three anonymousreviewers.

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Ceryak, R. and D. Hornsby, 1996. Groundwater Quality Report.Suwannee River Water Management District Report WR-97-02,31 pp.

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interactions between ground water and surface water in the suwannee river basin, florida

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