exchange of water between conduits and matrix in the...

Ž .Chemical Geology 179 2001 145–165www.elsevier.comrlocaterchemgeo

Exchange of water between conduits and matrix in theFloridan aquifer

Jonathan B. Martin), Randolph W. Dean 1

Department of Geological Sciences, UniÕersity of Florida, 241 Williamson Hall, P.O. Box 112120, GainesÕille, FL 32611-2120, USA

Abstract

Flow through carbonate aquifers may be dominated by conduits where they are present, by intergranular or fractureporosity where conduits are missing, or may occur in conduits and matrix porosity where both are well developed. In thelatter case, the exchange of water between conduits and matrix could have important implications for water management andhydrodynamic modeling. An extensive conduit system has been mapped by dye trace studies and cave diving exploration atthe Santa Fe SinkrRise system located in largely unaltered rocks of the Floridan aquifer of north-central Florida. In thisarea, the Santa Fe River flows underground at the River Sink and returns to the surface ;5 km to the south at a firstmagnitude spring called the River Rise. Limited data show that discharge is greater by 27–96% at the River Rise than at theSink and that the downstream increase in discharge is inversely related to discharge of the river. Natural SO2y

4

concentrations indicate that ;25% of the water discharging from the Rise originates from the Sink during low flow.Conversely, SO2y and other solute concentrations indicate that most of the water discharging from the Rise originates from4

the Sink during floods. Ar ;40% decrease in Naq and Cly concentrations over a 5 1r2-month period at a down-gradientwater supply well may reflect flow of dilute flood water from the conduits into and through the matrix at rates estimated tobe between 9 and 65 mrday. Calcium concentrations remain constant through time at the well, although flood waters have;90% lower Ca2q concentrations than ground water, perhaps reflecting dissolution of the matrix rocks. This apparentexchange of water between matrix and conduits is important for regional ground water quality and dissolution reactions.q 2001 Published by Elsevier Science B.V.

Keywords: Karst; Aquifers; Conduits; Matrix; Contaminants; Floridan

1. Introduction

Carbonate karst aquifers have long been concep-tualized as containing three types of porosity: inter-granular porosity within the matrix rocks, small aper-ture fracture porosity and large cavernous conduit

Ž .porosity e.g., Smart and Hobbs, 1986; White, 1999 .

) Corresponding author. Tel.: q1-352-392-6219; fax: q1-352-392-9294.

Ž .E-mail address: [email protected] J.B. Martin .1 Now at: CH2M Hill, 4350 W. Cypress Street, Suite 600,

Tampa, FL 33607, USA.

These types of porosity and their relative proportionswithin a karst aquifer can cause permeability to spanmany orders of magnitude resulting in both laminarand turbulent flow and widely ranging flow ratesŽe.g., Hickey, 1984; Wilson and Skiles, 1988; Hali-

.han et al., 1999 . Differences in flow rates as well asstorativity of conduit and intergranular porosity meanthat most flow is concentrated in conduits whilemost water is stored in the intergranular porosityŽ .Atkinson, 1977a,b .

Conduit porosity can be connected to surfacewater through cavernous opening such as sinkholesand sinking streams. These openings allow rapid and

0009-2541r01r$ - see front matter q 2001 Published by Elsevier Science B.V.Ž .PII: S0009-2541 01 00320-5

( )J.B. Martin, R.W. DeanrChemical Geology 179 2001 145–165146

extensive mixing of surface and ground water, causenatural changes in the chemical composition ofground water and increase its vulnerability to con-

Žtamination e.g., Field, 1988, 1993; White et al.,.1995; Memon and Prohic, 1998 . If surface water

flows rapidly through the conduit system, however,it will have little impact on water quality within theintergranular porosity, which commonly is the pri-mary water supply in karst areas because of its highspecific storage. In contrast, the exchange of waterbetween conduit and intergranular porosity couldaffect the quality of water in intergranular porosityand disturb chemical equilibrium established be-tween the water and surrounding rock. The extent ofthis exchange is thus critical to management of watersupplies, as well as the understanding of karst hydro-dynamics and of fluid–solid reactions within theaquifer.

Many well-studied examples of karst aquifers oc-cur in dense and recrystallized rocks, which restrictmuch ground water flow to conduits. The dominanceof conduit flow in these aquifers has lead to aresearch focus on changes in discharge at springsŽ .e.g., their AflashinessB and how these changes re-

Žflect flow through the subsurface e.g., Pitty, 1968;Shuster and White, 1971; Ternan, 1972; Smart andFord, 1986; Hess and White, 1988; Felton and Cur-rens, 1994; Padilla et al., 1994; Ryan and Meiman,

.1996; Halihan et al., 1998; White 1999 . For exam-ple, mathematical relationships between rainfall andspring discharge can be used in certain cases topredict recharge areas and variations in flow with

Žrecharge e.g., Dreiss, 1983, 1989a,b; Wicks and.Hoke, 1999 . One important control on the flashiness

is the volume of recharge in a particular area pro-Žvided by sinking streams Newson, 1971; Atkinson,

.1977b . Information on flow-through intergranular,fracture and conduit porosity can also be obtainedthrough sampling and observations of behavior of

Ž .water levels in wells Hickey, 1984; Shevenell, 1996 .In reality, a comprehensive approach using multipletechniques is required to understand and characterizeflow in all parts of the porosity systems of karst

Žaquifers Smart and Hobbs, 1986; Worthington,.1999 .

Both conduit and matrix flow could contributesignificantly to flow in carbonate aquifers that arecomposed of unaltered carbonate rocks with high

primary porosity or where carbonate rocks have ex-tensively developed secondary matrix porosity andr

Žor fracture systems e.g., Atkinson, 1977b; Beck,1986; Wilson and Skiles, 1988; Recker et al., 1988;Smart and Hobbs, 1986; Ford and Williams, 1989;Sanford and Konikow, 1989; Mylroie and Carew,

.1995; Shevenell, 1996 . In such aquifers, it will beimportant to include the matrix in models of bothregional and local ground water flow. In order toconstruct such models, the relative contributions ofmatrix and conduit flow to the total ground watersystem must be known, as well as the potential for,the controls on, and the extent of exchange of waterbetween the conduit and intergranular porosity of thematrix rocks. Although some estimates have beenmade for the flow of water from matrix rocks to the

Ž .conduits e.g., Newson, 1971; Atkinson, 1977b , fewattempts have been made to observe or quantify the

Žflow of water from conduits to the matrix e.g.,.Wilson and Skiles, 1988 .

This paper uses chemical composition of water ina sinking river, its resurgence and two nearby watersupply wells to make observations about exchange ofwater between conduits and matrix porosity. Thestudy area is along a 5-km section of the Santa Fe

ŽRiver in north-central Florida Skirvin, 1962; Hisert,.1994; Martin and Dean, 1999 . The time that water

from the sinking stream remains underground can bemeasured at high resolution using temperature data

Žfor water flowing into the Sink Martin and Dean,.1999 . This information on residence time allows the

collection of water samples from a single pulse ofriver water that passes through the conduits thatsource the resurgence. Changes in the chemical com-position of the water suggest that a large fraction ofwater discharging from the Rise is derived from thematrix during low flow. In contrast, during flooding,water appears to flow from the conduits and throughmatrix porosity for at least several kilometers downthe regional ground water gradient following floods.

2. Field area

2.1. General geologic and hydrogeologic back-ground

The post-Cretaceous lithostratigraphy of theFlorida platform can be crudely divided into two

( )J.B. Martin, R.W. DeanrChemical Geology 179 2001 145–165 147

major units: pre-Miocene carbonate-dominated rocks,which are further subdivided into several distinctformations, and Miocene and younger siliciclastic-dominated rocks, which in part comprise the

Ž . ŽHawthorn Group Fig. 1 Scott, 1988, 1992; Gros-.zos et al., 1992 . These stratigraphic units largely

control the hydrogeology of the region with theFloridan aquifer formed mostly of pre-Miocene car-

Žbonate rocks and other aquifers the Intermediate and.Surficial aquifers contained within overlying silici-

clastic rocks. The Hawthorn Group acts as an upperconfining unit to the Floridan aquifer, but has beenremoved by erosion from much of west-central

Ž .Florida Fig. 2 . The Hawthorn Group is breached inmany places where it is less than ;30-m thick,resulting in semi-confined conditions for the Flori-

Ž .dan aquifer Miller, 1986 .The erosional edge of the Hawthorn Group pro-

Žduces the greatest topographic relief in Florida ;25. Žm in 10 km , a feature called the Cody Scarp Puri

.and Vernon, 1964 . To the northeast of the scarp,numerous lakes and streams are perched on theconfining unit. Southwest of the scarp, surface wateris limited to scattered wetlands and only two major

Ž .streams, the Suwannee and Santa Fe rivers Fig. 2 .All streams that cross the Cody Scarp either sinkcompletely into the subsurface or sink into the sub-surface and re-emerge. The principal exception is theSuwannee River, which becomes a losing stream atthis boundary. The Cody Scarp is thus, a criticallocation for surface and ground water mixing and isan area of intense carbonate dissolution and karstifi-

Žcation Hunn and Slack, 1983; Upchurch and.Lawrence, 1984; Kincaid, 1994, 1998 .

Numerous springs occur along the scarp, acrossthe unconfined portion of the Floridan aquifer and

Žwhere the Hawthorn Group is breached Rosenau etal., 1977; Katz et al., 1997, 1999; Hornsby and

.Ceryak, 1999 . Most large springs are sourced byintermediate to long-flow paths, with some fractionof the discharge composed of water that has been in

Žthe subsurface longer than 20 years Katz et al.,.1999 . Small springs typically have shallow local

flow systems. A major spring system, ;20 km westof Santa Fe sink–rise system, shows little variationsin its discharge or chemical composition in responseto storm input or seasonality of precipitation, al-though it is connected to several ephemeral sinking

Fig. 1. Lithostratigraphic and hydrostratigraphic units of north-central Florida. Thickness of units is not implied in the diagram. Southwestof the Santa Fe Sink, the Miocene Hawthorn Group, is missing and the Holocene and Plio-Pleistocene sands are thin. Modified from ScottŽ . Ž .1988, 1992 and Groszos et al. 1992 .


Fig. 2. Schematic physiographic map of northern Florida showingthe location of the Santa Fe River sink–rise system. The HawthornGroup confines the Floridan aquifer in the darkly shaded region,but is missing in the lightly shaded region where the Floridanaquifer is unconfined. The Cody Scarp separates the unconfinedfrom the confined regions. Three major rivers in the area: theSanta Fe, Suwannee and Ichetucknee rivers are also shown.

Ž .streams through mapped conduits Hirth, 1995 . Thelack of variation in discharge and composition wasattributed to matrix flow dominating over conduit

Žflow regardless of the presence of conduits Martin.and Gordon, 2000 . Dissolution along the Cody Scarp

may thus depend on the fraction of undersaturatedsurface water that flows from conduits into small

Žfractures and intergranular porosity e.g., Upchurch.and Lawrence, 1984 . This mechanism also could

potentially be important for contamination of theŽ .ground water supplies e.g., Katz et al., 1999 .

2.2. Specific regional studies

The Santa Fe River flows westward from LakeSanta Fe for ;40 km until it reaches the CodyScarp, where it sinks into a 36-m deep sinkhole at

Ž .the River Sink Fig. 3 . Approximately 5 km southof the Sink, a first magnitude spring called the River

Rise, marks the location of the headwaters of thelower Santa Fe River. Numerous karst windows andsinkhole lakes occur between the River Sink andRise. Most sinkholes have depths that range between

Ž .;20 and 36 m Skirvin, 1962 . One such lake,Sweetwater Lake, has a strong current and representsa short segment of the river. Other lakes, mostnotably Downing Lake, are only a few meters deepand apparently, not connected to the conduit system.

A single dye trace study used the gas SF to6

connect the Sink with several of the intermediatekarst windows including Ogden Pond, Ravine Sink,Parener’s Branch Sink, Small Sink, New Sink, Jim’s

ŽSink, Two Hole Sink and Sweetwater Lake Hisert,.1994 . Gas from the initial injection was not recov-

ered at the River Rise, although a second injection inSweetwater Lake showed a connection betweenSweetwater Lake and the Rise. The trace was con-

3 Žducted at relatively high discharge of 42 m rs stageŽ ..of 13.5 m above sealevel masl and travel times

averaged 4.3 kmrday, indicating the presence ofconduit connections between the various sinkholesand karst windows. Subsequent work used tempera-ture as a natural tracer and confirmed the flow rates

Ž .measured during Hisert’s 1994 injected tracer studyŽ .Dean, 1999; Martin and Dean, 1999 . The tempera-ture study showed that the subsurface flow ratevaries as a function of river stage. Additional sourcesof water must flow from the River Rise; however,water was observed to discharge from the River Risewhen the upper Santa Fe River was temporarily

Ž .dammed for a construction project Skirvin, 1962 .No estimates have been made for the source of thiswater, or the controls on mixing between the differ-ent sources of water.

Exploration of the conduits located between theSink and the Rise was recently initiated by local

Ž .cave divers M. Poucher, personal communication .The exploration shows that a conduit enters Sweet-water Lake at its northern end. This conduit isconnected to a separate large conduit system that

Ž .enters the area from the east Fig. 3 . The easternconduit system is not sourced by any major perennialstreams and has only minor and periodic rechargefrom local storm water runoff. Consequently, theeastern conduit system should contain largely groundwater that enters the conduits from intergranular andfracture porosity. Conduits have also been mapped to


Fig. 3. Sketch map of the field area showing the locations of the Santa Fe River, its Sink and Rise, several intermediate karst windows,wetlands and the sampled water supply wells at the Sink and Rise. The named karst windows were connected to the Sink and Rise by dye

Ž . Ž .trace studies Hisert, 1994 . The direction of regional ground water flow is taken from Meadows 1991 . Conduits have been recentlyŽ .mapped by cave diving M. Poucher, personal communication . Cave exploration is on-going.

extend south from Sweetwater Lake and north fromthe River Rise. Although a connection has not yetbeen established between these conduits, the mostrecent maps show a separation of only ;150 m.

3. Methods

3.1. Field sampling

Water samples were collected from three loca-tions along the river: at the River Sink, at Sweetwa-

Ž .ter Lake and at the River Rise Fig. 3 . The RiverSink samples were collected ;40 m upstream fromthe Sink. The Rise samples were collected ;100 m

downstream from the two primary resurgence pointsŽ .Hisert, 1994 in order to allow for complete mixingof water from those springs. In addition, water wascollected from two local water supply wells, onelocated ;500 m upstream from the Sink and theother located ;2 km downstream from the Rise.Samples were collected at five discrete flow condi-tions, three at low-flow conditions, one during floodconditions and one during intermediate conditionsŽ .Table 1 .

Samples were collected from Sweetwater Lakeand from the Rise at times that were based on thelength of time for the flow through the subsurfacethat had previously been measured using temperature


Table 1Sampling times and river stage

Sample Sample date River stageaŽ .period masl

1 April 28–May 2, 1998 10.702 June 1–4, 1998 10.503 June 19–26, 1998 10.454 October 5, 1998 13.435 October 13–14, 1998 11.90

a maslsmeters above sea level.

Ž .as a tracer Martin and Dean, 1999 . The timing forcollection of the samples is important in order toprevent artifacts in the chemical composition ofdownstream samples that may result from temporalchanges in chemistry of the river water, for example,by dilution during major rainstorms. To further en-sure that each sample represents a single slug ofwater traveling from the Sink to Sweetwater Lake tothe Rise, several samples were collected at eachstation during each collection time. Two to foursamples were collected from the Sink over times upto 2 h in order to observe short-term variations in thechemical composition of the river water. Subse-quently, three to nine samples were collected atSweetwater Lake and the River Rise at time periodsranging from several hours to 4 days. The largestnumber of samples was collected during the lowest

Ž .stage 10.45 masl because of the long travel time.At this stage, water takes ;7 days to flow from theSink to the Rise, but at the highest stage sampledŽ .13.43 masl , water flows through the system in less

Ž .than a day Hisert, 1994; Martin and Dean, 1999 .Several samples were collected from the water sup-ply wells while in the field for each of the fivesampling trips. The wells were flushed until conduc-tivity, temperature and dissolved oxygen concentra-tions were constant before each of the samples wascollected. The number of well volumes purged dur-ing sampling is unknown.

Water samples from the river were collected froma small inflatable raft at a depth of at least 0.5 mbelow the surface from the middle of the river. Foreach collection, three separate samples were pre-served from each site: one for alkalinity and dis-

Ž . ysolved inorganic carbon DIC , one for NO and3

one for major dissolved components. The alkalinityand DIC samples were preserved in 60-ml BOD

bottles with ground glass stoppers that were sealedwith vacuum grease. They were poisoned with asaturated solution of HgCl in order to prevent2

microbial alteration. Alkalinity was measured withina few days after collection. Each sample for theNOy measurement was acidified with 100 ml3

H SO . Samples for the major components were2 4

preserved in 125-ml high-density polyethylene bot-tles. All samples were kept refrigerated until mea-sured.

3.2. Chemical analyses

When the samples were collected, pH, conductiv-ity and dissolved oxygen concentrations were mea-sured in the field. Temperature and pH were mea-sured using an Orion portable pH meter, modela250A. The electrode was calibrated once at thebeginning of each day of sampling. Conductivity anddissolved oxygen were measured with an Orionmodel a130 portable conductivity meter and a YSImodel a55 portable oxygen meter. Alkalinity wasdetermined by titrating samples with 0.1 N HCl tothe second carbonate end point. The end point was

Ždetermined using the Gran technique e.g., Drever,.1988 . Concentrations of DIC were measured with a

Coulometrics coulometer that was standardized withdissolved KHCO .3

Concentrations of Naq, Mg2q, Ca2q, Cly, NOy,3

and SO2y were measured using a Dionex ion chro-4

matograph model 500 equipped with an auto sam-pler. Typically, 27 samples were measured in tripli-cate during an individual run, and every fifth samplewas an internal standard of Santa Fe River water.Instrumental drift between runs was corrected byapplying a normalizing factor, NF, to the value ofeach run:

NFsAvg rAvgall run

where Avg is the average concentration of the ionall

in the internal standard for all internal standardsmeasured during this study and Avg is the averagerun

value of the ion for the internal standard within arun. The concentration for each sample within a runwas multiplied by the value of NF for that run. Thesevalues range from 0.87 to 1.09. Analytical error wascalculated for each ion individually after correctingits concentrations with the normalizing factor. The


Table 2Estimated precision

aŽ .Ion CV %yCl 1.2yNO 0.732ySO 1.04qNa 2.62qMg 2.3

2qCa 2.1

aCoefficient of variation.

overall error for each ion was calculated from theŽ .internal standard as the coefficient of variation CV

of the internal standard for all the runs:

CV%s StdrAvg )100Ž .where Std is the standard deviation and Avg is theaverage for all corrected measurements of the inter-nal standards. The estimated precision for individualmeasurements is reported in Table 2.

The accuracy of the measurements was checkedby calculating a charge balance using the geochemi-

Žcal speciation program PHREEQC Parkhurst and.Appelo, 1999 . Charges do not balance for many of

the samples but there are systematic differences inŽ .the imbalance among the samples Table 3 . The

samples that are closest to balancing are from theRiver Sink, Sweetwater Lake and River Rise duringlow-flow conditions. Samples that were collectedfrom the Sink, Sweetwater Lake and the Rise duringflood conditions show a strong excess positivecharge. For these samples, the imbalance may resultfrom analytical errors associated with making mea-surements of solutes near the detection limit. Incontrast to the samples from the flood, samples fromboth the Rise and Sink wells show a strong excessnegative charge. Although some of the charge imbal-ance for samples from the wells may reflect analyti-cal errors, the imbalance also may result from thepresence of unmeasured species. The samples werenot filtered and consequently may have high metalconcentrations derived from scaling in the wells.

4. Results

4.1. Precipitation and riÕer stage

Precipitation, river stage and discharge records forthe year of the study are shown in Fig. 4. Precipita-

tion is measured at the entrance of O’Leno StatePark, approximately 3 km west of the River Sink.The stage of the River is measured approximately500 m upstream from the River Sink. Precipitationdata and stage are collected daily by park staff andreported to the Suwannee River Water Management

Ž .District SRWMD . Discharge measurements are col-Ž .lected by the US Geological Survey USGS ;5 km

downstream of the River Rise near the town of HighŽ .Springs, FL Franklin et al., 1998, 2000 . Numerous

springs flow into the river between the Rise and theUSGS gauging station, however, significantly in-creasing the discharge at the gauging station com-

Ž .pared to the Rise. For example, Hisert 1994 showedthat discharge increased from 27.4 to 47.4 m3rsfrom the rise to a location ;4 km downstream fromthe rise.

Over the study period, from January 1998 throughJanuary 1999, a total of ;150 cm of rain fell at theO’Leno station, which is typical annual rainfall forthe region. The distribution through time of precipi-tation is unusual, however, with a large amount ofrain occurring during the normally dry winter monthsŽ .Fig. 4 . More than 40 cm of rain fell in Februaryand March 1998, which are normally the driestmonths of the year. With little loss to evapotranspira-tion during this time, these abnormally heavy winterrains caused major flooding of the river. An addi-tional 42 cm of rain fell during the normally wetseason between June and August but had little im-pact on the river stage because much of the precipita-tion is lost through evapotranspiration during the

Žwarm summer months e.g., Martin and Gordon,.2000 . At the end of September and beginning of

October, Hurricane Georges passed over the regionand generated 14 cm of rain in a 4-day period,causing a second flood during the year. Both theFebruary flood and the flood from Hurricane Georgesshow smooth recession curves in the hydrographŽ .Fig. 4 .

The maximum stage of the river occurred onFebruary 20, 1998 and reached an elevation of 15.41masl, which corresponds to a discharge of 253 m3rsat the USGS gauging station. The maximum stage ofthe second flood occurred on October 5, 1998 andwas 13.42 masl, which corresponds to a discharge of94 m3rs at the USGS gauging station. When theriver reaches an elevation of ;14.3 masl, it over-

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Table 3Analytical data

Ž .aa bŽ .Location Sample n Stage T 8C s Cond. s DO s pH s Alk. s DIC s

Ž . Ž . Ž . Ž . Ž .period masl mSrcm mgrl mM mM

Sink well 1 1 NA 23.6 542 2.45 7.2 4.63 5.43Ž .Groundwater 2 1 NA 24.0 540 6.25 7.4 4.05 4.60

3 2 NA 24.4 0.9 542 1 2.60 0 7.4 0.04 3.90 0.03 5.08 0.084 1 NA 25.3 542 2.44 7.4 4.09 5.025 1 NA 23.7 540 2.36 7.4 4.12 4.96

Rise well 1 1 NA 21.0 413 1.89 7.4 3.54 3.61Ž .Groundwater 2 2 NA 22.9 0.2 397 2 1.63 0.40 7.5 0.05 3.01 0.08 3.50 0.05

3 5 NA 21.1 0.0 357 18 1.43 0.04 7.5 0.07 2.80 0.17 3.44 0.024 1 NA 21.0 331 1.30 7.4 2.79 3.305 1 NA 20.8 330 1.16 7.4 2.81 3.26

Sink 1 4 10.70 20.9 0.00 282 0.5 4.68 0.09 7.3 0.06 1.85 0.05 1.89 0.022 2 10.50 24.4 0.20 375 1.0 6.84 0.32 7.8 0.10 2.25 0.01 2.49 0.093 2 10.45 24.4 0.10 406 5.8 5.76 0.20 7.8 0.04 2.34 0.39 2.804 2 13.43 24.7 0.90 54.3 0.2 4.66 0.03 5.7 0.02 0.05 0.06 0.22 0.005 3 11.90 23.5 0.21 70.5 0.0 4.83 0.02 5.9 0.03 0.05 0.03 0.24 0.00

Sweetwater 1 9 10.70 21.0 0.07 386 1.3 3.02 0.03 7.3 0.02 2.14 0.02 2.24 0.02Lake 2 4 10.50 23.0 0.05 486 1.4 2.81 0.08 7.5 0.04 2.37 0.04 2.75 0.02

3 4 10.45 23.7 0.45 481 8.8 2.71 0.09 7.5 0.06 2.39 0.21 2.96 0.024 3 13.43 25.3 0.06 56.4 0.1 4.47 0.01 5.9 0.01 0.16 0.23 0.26 0.025 3 11.90 23.6 0.06 92.8 0.1 3.86 0.03 6.3 0.03 0.20 0.05 0.37 0.00

Rise 1 8 10.70 21.0 0.05 406 0.6 2.08 0.02 7.3 0.03 2.44 0.04 2.49 0.022 4 10.50 22.4 0.05 473 1.3 2.25 0.08 7.4 0.04 2.61 0.15 2.82 0.023 4 10.45 22.5 0.00 452 8.8 1.86 0.43 7.5 0.03 2.26 0.04 2.98 0.024 3 13.43 25.3 0.00 57.3 0.0 4.26 0.02 5.9 0.08 0.00 0.00 0.25 0.005 3 11.90 23.7 0.06 100 0.6 3.46 0.01 6.4 0.01 0.26 0.03 0.44 0.01

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Ž .b

Location Sample Cl s NO s SO s Na s Mg s Ca s Error SI3 43Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .period mM =10 mM mM mM mM mM %

Sink Well 1 0.38 70.5 0.059 0.35 0.08 1.16 y30.0 y0.13Ž .Groundwater 2 0.38 67.9 0.054 0.35 0.08 1.31 y19.4 0.07

3 0.38 1 71.0 0.0 0.054 0.000 0.36 0.00 0.08 0.00 1.19 0.54 y21.6 0.024 0.39 70.6 0.055 0.36 0.08 1.18 y24.1 0.055 0.39 69.3 0.053 0.37 0.08 1.33 y19.4 0.08

Rise well 1 0.36 0.2 0.018 0.27 0.05 0.90 y29.3 y0.17Ž .Groundwater 2 0.32 1 1.1 0.0 0.016 0.000 0.25 0.01 0.05 0.00 0.94 0.02 y20.6 y0.09

3 0.27 10.0 1.3 0.0 0.014 0.001 0.25 0.03 0.04 0.00 0.97 0.03 y15.7 y0.134 0.22 0.2 0.012 0.20 0.04 0.92 y18.0 y0.255 0.21 1.6 0.012 0.19 0.04 0.91 y18.9 y0.26

Sink 1 0.27 2 37.4 0.3 0.211 0.000 0.27 0.02 0.33 0.02 0.94 0.02 4.4 y0.542 0.26 0 21.9 0.2 0.336 0.000 0.28 0.00 0.50 0.01 1.28 0.01 9.5 0.203 0.26 2 13.7 0.3 0.392 0.002 0.29 0.00 0.56 0.00 1.41 0.00 11.5 0.254 0.15 0 0.0 0.0 0.022 0.001 0.17 0.01 0.08 0.00 0.17 0.00 46.6 y4.275 0.27 4 0.3 0.4 0.024 0.000 0.24 0.01 0.09 0.00 0.22 0.00 40.0 y4.00

Sweetwater 1 0.34 3 42.7 0.6 0.466 0.006 0.33 0.01 0.39 0.01 1.25 0.02 2.3 y0.38Lake 2 0.37 1 36.3 0.1 0.683 0.005 0.37 0.00 0.55 0.01 1.63 0.01 7.1 y0.02

3 0.38 7 31.6 0.9 0.772 0.007 0.39 0.01 0.59 0.01 1.72 0.01 7.7 0.014 0.08 17 0.0 0.0 0.026 0.002 0.16 0.00 0.07 0.00 0.17 0.00 37.4 y3.585 0.27 1 1.6 0.2 0.064 0.000 0.25 0.00 0.11 0.00 0.33 0.02 30.1 y2.84

Rise 1 0.34 2 52.1 0.8 0.516 0.021 0.33 0.03 0.40 0.02 1.45 0.02 2.2 y0.272 0.37 2 52.3 0.4 0.675 0.005 0.37 0.01 0.52 0.01 1.67 0.01 4.3 y0.083 0.38 3 48.5 0.7 0.746 0.002 0.38 0.01 0.57 0.01 1.73 0.01 9.4 y0.024 0.16 8 0.0 0.0 0.024 0.000 0.16 0.00 0.08 0.00 0.21 0.00 55.4 y4.865 0.27 2 0.8 0.7 0.073 0.001 0.25 0.00 0.12 0.00 0.38 0.00 29.6 y2.57

NAsNot available.Ž .SIssaturation index with respect to calcite calculated using PHREEQC and alkalinity Parkhurst and Appelo, 1999 .

a Number of samples that were measured and reported as an average concentration.bs sstandard deviation of measured if n)2. If ns2, s represents the difference between the two measured values.


Fig. 4. Records of A, precipitation; B, river stage; and C, discharge from January 1, 1998 to January 31, 1999. Although winter is typicallythe dry season, abnormally large rains occurred in February and March 1998. The time of the sample collection trips are shown by thearrows in B and C. The horizontal line in B represents the water level of the river that allows overland flow to by-pass the River Sink. Datashown in C are collected by the USGS at a gauging station located ;5 km downstream from the Rise.

flows the banks that surround the River Sink andflows overland between the Sink and Rise. Thiscondition lasted for ;13 days during 1998, fromFebruary 18 to March 2. The river stage during theOctober flood was insufficient to cause overflow andconsequently, during this flood, all river water wasdiverted into the Sink.

4.2. Chemical composition

Chemical compositions of the water samples arereported in Table 3 as averages of all of the individ-

ual samples from one collection interval at any indi-vidual site. Variation in concentrations of multiplesamples is listed as standard deviation where morethan two samples were collected and as differencesin the values, if only two samples were collected.The data are reported as averages because theirconcentrations for the most part change little over

Ž .short e.g., several hours sampling times. The largestvariations occur when the concentrations are low andapproach the detection limit of the measurement

Ž yduring the flood samples e.g., NO during sample3


.period 5 . Although stage data are collected onlyŽ .daily e.g., Fig. 4 , our observations of the river

indicate that stage changes little over periods ofhours. Consequently, the concentrations would notbe diluted in this time frame by flood waters.

4.2.1. Ground waterWater collected from the Sink and Rise wells are

presumed to represent the composition of groundwater in the vicinity of the well. On average, watercompositions differ between the two wells, however,

Ž . q y 2y Ž . q yFig. 5. A Concentration of Na , Cl and SO through the 6 months of the study at the Sink well. B Concentration of Na , Cl and4

SO2y through 5 days in June at the Sink well. Squares represent Naq concentrations, circles represent Cly concentrations and triangles4

represent SO2y concentrations. The steady decrease in these conservative elements appear to reflect mixing with dilute flood waters that4

flow from conduits to the matrix rocks.


indicating that the composition of the ground watervaries regionally over distances of at least severalkilometers. In general, the solute concentrations arelower at the Sink well than at the Rise well. Forexample, as a measure of the total ion concentra-tions, conductivity averages 541 mSrcm at the Sink

Ž .well but only 365 mSrcm at the Rise well Table 3 .The largest difference between the wells occurs inthe concentration of NOy, which is ;70 times more3

concentrated in the Sink well than in the Rise well.The concentration of NOy is locally important be-3

cause it represents a potential contaminant in groundŽ .water in this largely agrarian region Andrews, 1994 .

Although the concentrations of NOy are below the3

drinking water standards set by the EPA, concentra-tions have been shown to be increasing through time

Ž .in several springs in the region Katz et al., 1999 .As NOy reaches critical concentrations, it has the3

potential to alter ecosystems in springs and springruns.

The two wells also differ in how concentrations ofsome solutes change through time. The compositionsof all solutes at the Sink well are essentially constantfor all five sampling periods. In contrast, the concen-trations of several solutes decrease at the Rise well

Ž .over the 6 months that they were sampled Fig. 5A .The largest decreases in concentrations are shown bydissolved oxygen, Cly, SO2y and Naq. These so-4

Ž . Ž .Fig. 6. Concentrations of various solutes versus river stage for the River Sink circle , Sweetwater Lake square and the River RiseŽ . y qdiamond . The arrows in the figures for Cl , Na , and conductivity represent the concentration of ground water as represented by the Sinkwell. With the exception of oxygen, all solutes show decreasing concentrations with increasing stage reflecting dilution by rainwater.


lutes decrease by 30–40% from the first samplingperiod at the end of April 1998 to the final samplingperiod 5 1r2 months later in the middle of October

Ž .1998 Table 3 . The temporal variation in concentra-tion in the well was also measured over a 5-dayperiod during sampling trip 3. These five samplesshow a continuous decrease in the concentrations ofSO2y and Cly through time, similar to the longer4

Ž .term variations Fig. 5 . With the exception of thefirst day sampled, Naq concentrations also exhibit asimilar decrease through time in concentration. Othercomponents, including conductivity and alkalinityalso decrease over the full 6-month sampling period,but by only ;10–20%, and these components vary

Ž .little over the 5-day sampling period Table 3 .These percentage changes are close to the analyticaluncertainty of the measurements and thus, may notbe significant. Other components, including pH andMg2q and Ca2q concentrations, remain approxi-mately constant through time.

4.2.2. Surface waterThe concentrations of all components differ

greatly through time at the River Sink and Rise andat Sweetwater Lake. With the exception of oxygen,all components show a large decrease in concentra-

Ž .tion from base flow to flood conditions Fig. 6 . Thisdecrease in concentration likely results from dilutionby rainwater, which is more than an order of magni-

Žtude dilute than the river water e.g., Maddox et al.,.1992; Gordon, 1998 . Concentrations of most com-

ponents show an inverse correlation with river stageŽ .but the correlation is not linear Fig. 6 . The compo-

nents that exhibit the closest linear relationship withstage are Cly and Naq. Conductivity, alkalinity,Ca2q, Mg2q and SO2y concentrations are non-lin-4

ear with stage, exhibiting similar concentrations insamples collected at a stage of 11.90 and at 13.43masl. The oxygen concentration shows the leastamount of correlation with stage of all of the compo-nents. At low stage, water from Sweetwater Lakeand the River Rise has the lowest measured oxygenconcentration, while water at the River Sink has the

Ž .highest measured concentration Fig. 6 . In contrast,at high stage, all three sampling locations have ap-proximately the same oxygen concentration and thisconcentration is intermediate between the highest

values measured at the Sink and lowest values mea-sured at Sweetwater Lake and the Rise at low stage.

5. Discussion

Regional scale mapping shows that the potentio-metric surface of the upper Floridan aquifer slopesto the southwest through north-central FloridaŽ .Meadows, 1991 . Consequently, the regional flowof ground water should be from the northeast to the

Ž .southwest through the study area Fig. 3 . No de-tailed potentiometric surface maps have been con-structed in the area; however, such maps wouldprovide important information about the local direc-tion of flow of the ground water. Nonetheless, ifground water that enters the conduits is derived fromthe northeast of the area, then the Sink well does notlie on a flow path that would source the easternconduit network that flows to Sweetwater Lake andthe River Rise. Both the source of the water from thenortheast and the Sink well are located on the featheredge of the Hawthorn Group, which trends northwest

Ž .through the area Fig. 2 . As a result, the Sink well islocated in a setting that is geologically similar to thesource of regional ground water for the area and mayhave chemical compositions similar to those of thewater feeding the conduits.

The regional southwest orientation of the flowpaths also indicates that water leaking from conduitswould be expected to flow to the southwest. If waterleaves the conduits and becomes part of the regionalground water, it might be intercepted by the Risewell, which is located directly down the regionalgradient from conduits that feed the River Rise.There have previously been case studies document-ing the infiltration of water from the matrix and

Žfractures into conduits Newson, 1971; Atkinson,.1977a,b . Fewer studies document the loss of water

Žfrom conduits to the matrix e.g., Wilson and Skiles,.1988 but the loss of water from conduits to the

matrix in the Santa Fe SinkrRise area has anecdo-tally been described by local farmers who claim thattheir well water becomes muddy following majorfloods on the river. The following sections describechemical changes along the river flow path andinterpret them in terms of gain of water from thesurrounding matrix and loss of water to the matrix.


5.1. Ground water contribution to conduits

5.1.1. Discharge measurementsThe concentration of solutes in water at Sweetwa-

ter Lake and the River Rise should be controlled bythe relative proportions of two water sources: onefrom the Sink, which represents surface water, andanother from the conduit system to the east, whichpresumably represents ground water because of thelack of significant surface runoff into the systemŽ .Fig. 3 . The relative contributions of surface waterand water from the eastern conduits should be seenas differences in discharge at the River Sink andRise. Along with the gauging station downstreamfrom the River Rise, discharge of the Santa Fe Riveris measured daily at a location 26 km upstream from

Ž .the River Sink Franklin et al., 2000 . River dis-charge changes greatly between these two gauging

Ž .stations, however, e.g., Hisert, 1994 , limiting theirusefulness in comparing changes in discharge at theSink and Rise. There is no rating curve available forthe gauging station that is located ;1 km upstreamfrom the Sink that was used to collect stage datashown in Fig. 4B. Because of the large change indischarge over short distance of the river, dischargeat the Sink and Rise should be compared with mea-surements made at locations within a few hundredmeters to the Sink and the Rise.

Four discharge measurements have been madeimmediately upstream from the Sink and down-

Ž .stream from the Rise Table 4 . These measurementsindicate that discharge consistently increases fromthe Sink to the Rise presumably reflecting watercontributed to the Rise from the eastern conduits.

The absolute volume of additional water at the Riseincreases with increasing discharge of the river, butthe fraction of this additional water relative to thetotal discharge decreases with the discharge. Thehighest discharge measurement listed in Table 4 ismore than an order of magnitude less than the maxi-

Ž 3 .mum discharge 292 m rs that has been measuredŽat the downstream gauging station Franklin et al.,

.2000 , suggesting that discharge at the Sink and theRise has not yet been compared during flooding. It ispossible that during high discharge, there may belittle, or no contribution from the eastern conduits tothe rise.

5.1.2. Low-flow conditionsDuring low flow, downstream variations in solute

concentrations reflect mixing, although the chemicalcompositions indicate that the mixing is not straight-forward two end-member mixing between water withthe composition of the Sink well and water from the

Ž .Sink Fig. 7 . Deviations from two end-membermixing are likely caused by differences in the com-position of water at the Sink well and water in theeastern conduits. Nonetheless, Cly concentrationsincrease at Sweetwater Lake and the Rise to approxi-mately the same value as at the Sink well. Dissolvedoxygen also follows this pattern, although at Sweet-water Lake, the concentration is 0.1–0.5 mgrkghigher than at the Sink well. At the lowest river stageŽ . q10.45 masl , the Na concentration is greater by

Ž .0.03 mM ;8% than at the Sink well. In contrast,the SO2y and Ca2q concentrations are clearly differ-4

ent at Sweetwater Lake and the River Rise than atthe Sink well and the River Sink. Sweetwater Lake

Table 4Discharge measurements at Santa Fe Sink and Rise

Ž . Ž .Date Stage Q Sink Q Rise Increase % increase Reference3 3 3Ž . Ž . Ž . Ž .masl m rs m rs m rs

2r23–24r61 NA 16.25 22.23 5.98 27 15r7r61 NA 5.57 10.87 5.30 49 211r11r61 NA 4.47 7.75 3.28 42 23r21r00 10.23 0.13 3.39 3.26 96 3

Ž .1: Florida Water Resources Division reported in Skirvin, 1962 .Ž .2: Skirvin 1962 .

3: This study.NA, Not available.


ŽFig. 7. Concentrations of various solutes versus distance from the Sink well for five different sampling periods: three during low flow solid. Ž . Žsymbols and two during a single flood open symbols . The data for each sample location Sink well, Sink, Sweetwater Lake, Rise and Rise

.well are indicated by the labeled arrows. The downstream change in concentrations, the differences in concentrations between low-flow andflood conditions and the variations of concentrations at the Rise well appear to reflect mixing of water between conduits and matrix. See textfor explanation.

and the Rise have SO2y and Ca2q concentrations4

that are up to 0.71 and 0.53 mM greater than at theŽ .Sink well ;70% and 85%, respectively . Their

SO2y concentrations are ;50% and Ca2q concen-4

trations are ;20% greater than the concentration ofthe water at the Sink. Regardless of the differentcompositions of water in the eastern conduits and theSink well, the different compositions of water at theSink from those at Sweetwater Lake and the Risesuggest that little of the water discharging to Sweet-water Lake and the River Rise is derived from theRiver Sink at low-flow conditions.

The chemical measurements suggest that mostmixing occurs to the north of Sweetwater Lake at

low-flow conditions because there is only a smallchange in the chemical composition between Sweet-water Lake and the Rise, but a large change between

Ž .the Sink and Sweetwater Lake Fig. 7 . Chemicalmeasurements from additional sinkholes north ofSweetwater Lake may be useful to determine theextent of mixing between different sources of water.Some ground water must enter the system betweenSweetwater Lake and the Rise to cause the increasein SO2y concentration between these two locations.4

The exact relationship between discharge and mixingwill require high-resolution measurements throughtime of chemical composition and discharge mea-surements. Such information would allow calcula-


tions of the absolute volumes of river and groundwater discharging from Sweetwater Lake and theRise, thereby constraining the exchange of waterbetween the matrix and conduits.

5.1.3. Flood conditionsThe downstream changes in chemical composi-

Žtions are different during flooding river stage of.11.90 and 13.43 masl than low-flow conditions

Ž .Fig. 7 . When the river stage is elevated, water atSweetwater Lake and at the River Rise retain com-positions that are similar to the composition at theRiver Sink. This similarity in composition occurs forall components with the exception of dissolved oxy-gen. Dissolved oxygen decreases from the Sink tothe Rise by 0.4 mgrkg at the highest stage and by1.4 mgrkg at intermediate stage. These decreases inconcentration of dissolved oxygen may reflect oxida-tion reactions along the flow path rather than mixing.Although SO2y concentrations appear to be elevated4

in ground water in the eastern conduits over those inthe Sink well or at the River Sink, they remainconstant from the Sink to the Rise at the highest

Ž .flood stage sampled 13.43 masl . At the intermedi-Ž . 2yate stage 11.90 masl , SO concentrations increase4

from 0.024 mM at the Sink to 0.064 and 0.073 mMat Sweetwater Lake and the Rise, respectively, ap-parently reflecting addition of SO2y-rich water from4

the eastern conduits.There are at least two explanations for the invari-

ant downstream composition during flooding. Flood-ing may change the composition of the water in theeastern conduits and become similar to the composi-tion of water at the sink. If this is the case, there isno way to separate the two sources of water chemi-cally. Such a change in composition could occurthrough the rapid addition of rainwater through sink-holes. The lack of extensive surface drainage nearthe eastern conduit system argues against flow ofsurface water into the conduit system that are com-parable in magnitude to flow from the Santa FeRiver. Alternatively, flow from the River Sink mayprevent water from entering Sweetwater Lake andthe River Rise from the eastern conduits. The 27%increase in discharge at the River Rise comparedwith the Sink at highest discharge conditions shownin Table 4 indicates that flow from the easternconduits is not completely blocked at these discharge

levels. High resolution measurements of chemicalcomposition and discharge through time, as well asbetter constraints on the spatial and temporal varia-tion in water of the eastern conduits will be requiredin order to determine the proportions of water fromthe eastern conduits relative to water from the sink.

5.1.4. Mixing calculationsChanges in the downstream SO2y concentrations4

could be used to estimate the relative proportions ofwater from the Sink and the eastern conduits thatdischarges from the Rise. Sulfate is the best solutefor this calculation because it appears to have thelargest difference in concentrations between the riverwater and water in the eastern conduit, thereby mini-mizing problems associated with not knowing theexact concentrations of water in the eastern conduits.We assume that SO2y concentrations are at least4

0.772 mM for ground water in the eastern conduits.This value is the highest concentration measured at

Ž .either Sweetwater Lake or the River Rise Table 3and thus, should represent the sample with the high-est fraction of water from the eastern conduits. Thetrue concentration of SO2y in the ground water is4

likely to be higher than this number, however, be-cause the sample probably mixed with an unknownamount of SO2y-poor river water. The following4

calculations of mixing thus represent minimum frac-tions of river water in Sweetwater Lake and the Rise.

The calculations are made assuming two end-member mixings according to:

SO sX SO q 1yX SO 1Ž . Ž . Ž . Ž . Ž .4 4 4SW S GW

where X represents the fraction of River water, SO4

represents the concentration of SO2y in surface4Ž . Ž .water SW , the Sink S , or ground water from the

Ž . Ž .eastern conduit GW . Solving Eq. 1 for X gives:

SO y SOŽ . Ž .4 4SW GWXs 2Ž .

SO y SOŽ . Ž .4 4S GW

This equation indicates that at intermediate stageŽ .11.90 masl , Sweetwater Lake and the River Risecontain 25% and 26% of ground water with elevatedSO2y concentrations. These values are similar to the4


proportional increase in discharge from the Sink tothe Rise at the highest discharge conditions shown in

Ž .Table 4. At low stage 10.70 masl , Sweetwater Lakeand the River Rise contain 46% and 54% of groundwater with elevated SO2y concentrations. These val-4

ues are similar to the percentage increase in dis-charge from the Sink to the Rise at the intermediate

Ž .discharge conditions Table 4 . The lowest dischargemeasurements and those with the highest increase indischarge from the Sink to the Rise were made at

Ž .near record low river stage of 10.23 masl Table 4but no chemical compositions of the water at thisstage are available. Furthermore, it is impossible todetermine the fraction of ground water that were

Ž .sampled at the lowest stage 10.45 masl becausethese SO2y concentrations are used as a proxy for4

the SO2y concentration in the eastern conduits. Cal-4

culations made under these conditions would indi-cate that all water discharging at the Rise originatesfrom the eastern conduit system.

5.2. Loss of water to matrix

The compositions of all components in water atthe Sink well stay constant for all five sampling

Ž . qperiods Table 4, Fig. 7 . In contrast, the Na andCly concentrations at the Rise well decrease steadilyover a period of 5 1r2 months between the first and

Ž . ylast sampling trips Fig. 5 . Sodium and Cl can beconsidered conservative solutes in the Floridanaquifer because there are no fluid–solid reactionsthat would change their concentrations. Their only

Žsource would be from marine aerosols Maddox et.al., 1992; Grubbs, 1998 . Consequently, the observed

decrease in Naq and Cly concentrations at the Risewell may reflect dilution of these components fromwater that entered the matrix during the winter flood-

Ž .ing Fig. 4 .It is unlikely that the dilution resulted from the

flooding in October caused by Hurricane Georgesbecause of the monotonic decrease in concentrationsthroughout the sampling period including the short5-day decrease observed at low-flow conditions in

Ž .June Fig. 5 . Although no samples were collectedduring the winter flood, water in the river during thistime is likely to have Naq and Cly concentrationsthat are at least as dilute as the water in the riverduring the October flood with concentrations at the

River Sink as low as 0.17 and 0.15 mM, respectivelyŽ .Table 3 . These concentrations are ;40% lowerthan the concentrations in the Rise well during theinitial sampling period but only 10–15% lower thanthe concentration at the Rise well during the finalsampling period. The Naq and Cly concentrationscould be even lower than these values during thewinter floods because discharge then was signifi-

Ž .cantly larger than during the October flood Fig. 4 .In addition, the winter floods resulted from precipita-tion from continental weather systems, which wouldhave lower Naq and Cly concentrations than marineprecipitation from Hurricane Georges that caused the

ŽOctober flood e.g., Berner and Berner, 1987; Dr-.ever, 1988 . The lack of change in the composition

of the Rise well also suggests that the dilution isderived from variations in composition of the portionof the regional ground water system that is locateddown gradient from the Sink.

A possible range of flow rates from the conduitsto the Rise well can be estimated by assuming thatthe dilution results from the loss of flood water fromconduits to the matrix during the winter floods. The

Ž .maximum flooding stage of 15.41 masl occurred onŽFebruary 20, 1998 but a second smaller peak stage

.of 13.73 masl occurred later, on March 23, 1998.The shortest observed time for the dilution to occurwould be between the second peak and the firstsampling trip, which would be a period of 38 days.The dilution could have begun before the first sam-pling, but the dilution certainly continued until theend of sampling in mid-October. The longest ob-served time for dilution to occur would be from theinitial flood on February 20th through the last sam-pling period on October 13th, a time of 236 days.The straight line distance between the Rise well andthe northern and southern ends of the conduit con-necting Sweetwater Lake and the River Rise rangesfrom 2 to 3 km. Assuming that the dilution occurredover a time range of 38–236 days and that the waterhad to flow a distance of 2500 m, flow rates from theconduits to the well would range from 9 to 65mrday. Flow rates could be greater than these val-ues if the dilution began prior to the initial samplingtrip. Similarly, the Rise well could have continued tobe diluted following the termination of sampling.Nonetheless, if the observed dilution results fromloss of flood water from the conduits, these rates


bracket possible flow rates through the matrix of theFloridan aquifer.

Loss of conduit water to matrix has previouslybeen documented along the lower Santa Fe riverŽ . Ž .Wilson and Skiles, 1988 . Wilson and Skiles 1988used rhodamine WT to measure flow rates rangingfrom 184 to 1411 mrday over distance up to 1 km.These rates are one to two orders of magnitudegreater than those calculated above on the basis ofthe decrease in Naq and Cly concentrations at theRise well. If flow rates to the Rise well are similar to

Ž .those measured by Wilson and Skiles 1988 , theinitial breakthrough at the Rise well would haveoccurred in 1.7–13.5 days assuming a flow pathlength of 2500 m. The smooth decrease in concentra-

Ž .tions over a period of several months Fig. 5 sug-gests that the breakthrough of dilute flood waterwould not occur in such a short amount of time. Thecomposition of water in the well would be expectedto quickly return to its original composition if break-through occurred quickly. The region studied by

Ž .Wilson and Skiles 1988 has an extensive and well-mapped conduit network, and the dye was injectedvia wells directly into conduits. The injected dyereturned in several peaks over longer periods thanexpected for areas controlled by conduit flow indi-cating that the dye traveled through anastomosingconduits as well as dissolution-enhanced matrix

Žporosity referred to as Aspongy matrixB by WilsonŽ ..and Skiles 1988 . The high flow rates and long tails

Ž .observed by Wilson and Skiles 1988 could thus,reflect substantial flow through conduits, while theslow flow rates calculated for the Santa Fe SinkrRisesystem might reflect flow through smaller matrix andlower permeability rocks.

The apparent loss of water from conduits to thematrix has important implications for water manage-ment in the region, as well as in other areas with

Žporous and permeable carbonate rocks e.g., Recker.et al., 1988; Wilson and Skiles, 1988 . Contaminants

that are introduced into conduit systems duringflooding may not be rapidly flushed back to thesurface and could become entrained into the matrixflow, complicating remediation efforts. Analyses offlow through karst aquifers requires considering flowof water both through matrix and conduits and evalu-ations of the potential for exchange of water betweenthese two components of the aquifer. The loss of

surface water to the matrix alters the chemical com-position of the ground water and chemical equilib-rium between the water and surrounding rocks. Slowflow through the matrix allows for reestablishmentof chemical equilibrium, which would tend to cause

Ždissolution of the matrix rocks e.g., Upchurch and.Lawrence, 1984 .

5.3. Calcite dissolution

Although the Naq and Cly concentrations de-crease through time at the Rise well, the concentra-tions of other components remain constant. Somecomponents, such as SO2y and Mg2q would not be4

influenced by mixing with water that leaked from theconduits because their concentrations in the Octoberflood waters and presumably, also the winter floodwaters are similar to the original concentration in the

Ž . 2qRise well Fig. 7 . The Ca and alkalinity concen-trations are lower in the flood water by ;0.7 mMŽ .;75% than at all times in the Rise well water,however, and would thus, be expected to decreasealong with the Naq and Cly concentrations becauseof dilution in the Rise well. These concentrationsdecrease only slightly in the Rise well, however,

Ž .between the first and final sampling Table 3 . Al-though the Naq and Cly concentrations decrease

Ž .overall by ;40% over the study period Fig. 5 , theCa2q concentration increases slightly by 0.07 mMŽ .;8% from the first to the third sampling trip andthen, decreases to a value identical within error to

Ž .the initial sample Table 3 . Alkalinity decreasesapproximately steadily through time but by less thanthe decrease in the Naq and Cly concentrations. The

Ž .relative decrease in alkalinity is 0.5 mM ;20%between the first and final sampling trip but only by

Ž .0.2 mM ;6% between the second and final sam-pling trip.

Calcium and alkalinity differ geochemically fromNaq and Cly in carbonate aquifers because they canreact with the aquifer rocks, and these reactions maybuffer the concentrations of Ca2q. Saturation of thewater with respect to calcite has been calculated

Žusing the modeling program PHREEQC Parkhurst.and Appelo, 1999 using both alkalinity and indepen-

dently measured DIC concentrations for all of theŽ .samples Table 3 . Only the results that are based on


alkalinity are shown in Table 3 because there is littledifference in the saturation index if alkalinity or DICare used in the calculations. The largest differenceoccurs where there is very little dissolved carbon, forexample, at Rise 4. For this sample, the saturationindex is y3.87 using the measured DIC concentra-tion instead of y4.86 using alkalinity, indicatingthat the water is strongly undersaturated with respectto calcite. These calculations show that the riverwater is near saturation with respect to calcite atlow-flow conditions. In contrast, the flood water is

Žstrongly undersaturated with respect calcite Table.3 . The water at the Rise well shows a trend of slight

decrease in saturation state through time but thecharge balance errors suggest that this trend may bean artifact of the data. Nonetheless, the Rise wellwater remains near calcite saturation, regardless ofthe decreasing concentrations of Naq and Cly.

If undersaturated river water is lost to the matrixduring floods as suggested by the decreasing Naq

and Cly concentrations at the Rise well, dissolutionŽreactions are likely to occur e.g., Upchurch and

.Lawrence, 1984 . Dissolution is more likely as waterflows slowly through small matrix porosity ratherthan quickly through conduits because of the longerresidence time of the water in the subsurface as wellas from larger surface area to volume ratio of thesmall pore spaces than in the conduits. If dilution ofNaq and Cly is caused by mixing of ground waterwith greatly undersaturated flood waters, then thenear saturation with respect to calcite at the Risewell suggests that the flood water have regainedequilibrium with calcite through dissolution of thecarbonate matrix rocks. Such mixing between waterfrom a sinking stream and the matrix rocks, and theresulting chemical changes could provide an impor-tant setting to measure dissolution reaction rates insitu and the extent of regional karstification. Thesemeasurements, however, would require precise infor-mation about the volumes of water lost from theconduits, as well as detailed information aboutchanges in its chemical composition along the flowpath.

6. Conclusions

The temporal and spatial variations of the chemi-cal composition of water in the Santa Fe SinkrRise

system appears to reflect the exchange of waterbetween matrix and conduits in the Floridan aquifer.Available discharge measurements show that morewater discharges from the River Rise than enters theRiver Sink at low to intermediate discharge rates,indicating an additional source of water, probablyfrom the eastern conduits. The chemical compositionof water at the River Rise also suggests that atlow-flow conditions, much of the water that dis-charges from the River Rise is derived from theeastern conduit system. This conduit system does nothave a continuous source of surface water and is notlinked to a well-developed or large surface drainagearea, suggesting that the primary source of water tothe conduits is from matrix porosity.

Discharge measurements show that as dischargeincreases, the fraction of water at the River Rise thatoriginated at the River Sink increases. This observa-tion is supported by observations of little change inthe Cly and Naq concentrations from the River Sinkto the River Rise during flooding. Concentrations ofNaq and Cly also decrease through time in theground water at a single observation well located;2 km down the regional ground water gradientfrom the conduit system. These decreasing concen-trations suggest that some flood water has flowedfrom the conduits into the matrix and is movingdown the regional ground water gradient as a plumeof dilute water. Decreasing chemical concentrationsare observed only in the conservative ions, however,such as Naq and Cly, while reactive components,such as Ca2q and alkalinity remain constant. Thesechanges may reflect dissolution reactions of the car-bonate matrix rocks as flood water, which is greatlyundersaturated with respect to calcite, flows into thematrix. The loss of water from the conduits to thematrix, thus, has important implications for karstifi-cation in the region as well as water management.This mechanism would provide a path for contami-nants in the surface water to enter ground waterstored in the matrix, which in this region, as well asmany other karst areas, is the primary water supply.

Acknowledgements

We thank Mr. Dale Kendrick and his staff atO’Leno State Park for allowing us frequent access to


the park at odd hours. This work would not havebeen possible without the help and support of thepark staff. Mike Poucher kindly provided prelimi-nary versions of the cave maps. We also thank thetwo anonymous reviewers for Chemical Geologywho made numerous and very helpful comments.The work has been supported by NSF Grant EAR-9725295.

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