Using d18O and dD to Quantify Ground-Water/ Surface-Water Interactions
in Karst Systems of Florida

Brian G. Katz, Research Hydrologist

U.S. Geological Survey

227 N. Bronough St., Suite 3015, Tallahassee, FL 32301

 

Abstract

Stable isotopes of oxygen and hydrogen are naturally occurring tracers that can provide quantitative information about surface-water/ground-water interactions. d18O and dD were used to determine the amount of surface water mixing with ground water in three different karst systems of northern Florida. In one area, water from a sinkhole lake (Lake Barco) with an enriched isotopic signature moved downward and laterally beneath the lake and mixed with ground water. Fractions of lake water that mixed with ground water ranged from 0.23 to 0.65 and were related to the proximity of the sampling site to the lake and the presence of buried solution features that facilitate the connection between the lake and the Upper Floridan aquifer. In a second area, a recharge pulse from the Little River (a sinking stream) produced an enriched isotopic signature in water samples from some wells downgradient from the river capture zone and from a series of sinks that received Little River water during high-flow conditions. Fractions of Little River water that mixed with ground water and water in sinks ranged from 0.13 to 0.96 and were related to the proximity to the river capture zone and the degree of connectivity between the sinks and zones in the Upper Floridan aquifer. In a third area, water from wetlands and shallow sinkhole lakes with an enriched isotopic signature has mixed with deep ground water from the Upper Floridan aquifer in an area of northwestern Leon County. Fractions of evaporated surface water that mixed with ground water ranged from 0.10 to 0.34 and are related to heavy pumping of municipal supply wells and leakage from sinkhole lakes.

Introduction

Fundamental to understanding the factors controlling ground-water quality in karst systems is an accurate delineation of flow patterns and quantification of hydrochemical interactions between ground water and surface water. As a result of dissolution of the carbonate rocks that comprise the Upper Floridan aquifer in northern Florida, numerous karst features (sinkholes, swallow holes, solution lakes) have developed, resulting in water from the land surface moving directly into the aquifer. Very little is known about the quality of water that moves downward through karst features and naturally recharges the Upper Floridan aquifer, the principal source of water supply in this area. Natural recharge of water from sinking streams to this aquifer can result in water-quality contamination, such as high concentrations of iron, hydrogen sulfide, and organic material, and undesirable bacteria, protozoa, and fungi (Krause 1979; McConnell and Hacke 1993). Concentrated recharge of water through sinkholes and other solution features provides little opportunity for attenuation of contaminants prior to entering the aquifer system.

This paper focuses on the use of d18O and dD to quantify interactions between ground water and surface water. Due to the enrichment of d18O and dD in surface water that undergoes evaporation, the resulting isotopic signature is different than that of ground water and provides an ideal conservative tracer for evaluating the extent of mixing of surface water and ground water (Gonfiantini 1986). The signature of recharge from sinking streams in karst aquifers has been traced over distances of several kilometers using these stable isotopes (Greene 1997). Differences between the composition of the water isotopes (d18O and dD) in rainfall, ground water, stream runoff to a sinkhole, and lake water are used to quantify mixing of ground water and surface water. Results are presented for three study areas that represent various types of interactions between ground water and surface water: (1) leakage of water from a seepage lake, (2) a recharge pulse from a sinking stream during high-flow conditions, and (3) recharge from wetland areas and solution lakes. Other chemical and isotopic tracers were used in these studies; however d18O and dD were most effective in quantifying mixing between ground water and surface water (Katz et al 1995a, 1997). Results from this study provide a framework for a better understanding of the hydrochemical interactions between ground water and surface water in karst systems and for evaluating the susceptibility of ground water to surficial contamination.

Study Areas

A brief description of each study area is provided in this paper. More detailed information on the hydrogeology and physiography of each study area can be found in the references listed in each section. The climate of the three study areas is humid subtropical, with 30-year (1961-90) mean annual rainfall ranging from 131 and 134 cm at Lake Barco and Little River, respectively, to 167 cm at Leon County (Owenby and Ezell 1992).

Lake Barco

Lake Barco, located in north-central Florida, is an acidic seepage lake typical of other seepage lakes along ridges, uplands, and highlands in Florida that have no surface-water inflow or outflow. The Lake Barco catchment is in the Central Lake physiographic district (Brooks 1981), which is characterized by the formation of active sinkholes. Lake Barco is considered to be a cover-collapse sinkhole, which is typical of many of the lakes in the Central Lake District (Sinclair and Stewart 1985; Arrington and Lindquist 1987). Ground-penetrating radar (GPR) and seismic reflection surveys provided direct evidence of karst features and downwarping of beds within the surficial deposits in the Lake Barco basin (Sacks et al 1992).

The hydrogeologic framework consists of the Ocala Limestone (Upper Eocene age) unconformably overlain by the Hawthorn Group (Miocene age), which consists of a variable mixture of sand, gravel, clay, phosphate, and carbonate sediments (Scott 1988). The intermediate confining unit within the Hawthorn Group is probably breached in places (Sacks et al 1992). Undifferentiated surficial deposits (Holocene to Pliocene age) lie above the Hawthorn Group and consist of poor to well sorted sands with variable amounts of silt and clay. The location of the 1PNB well nest (upgradient from Lake Barco), the 2PNB well nest (downgradient from the lake), and other sampling sites are shown in figure 1. Samples of ground water, lake water, and rainfall were collected during May 1991 through September 1992.

Little River Sinks

The Little River, which drains a watershed of 88 km2, is a second order ephemeral stream in eastern Suwannee County (fig. 2). The Little River is typical of streams that originate in the Northern Highlands physiographic subdivision that disappear underground as they approach the toe of the Cody Scarp, which separates the Northern Highlands from the karst plain of the Gulf Coastal Lowlands. Direct localized recharge of river water is concentrated in sinkholes along the Cody Scarp and typically receives little filtration as it enters the Upper Floridan aquifer (Katz and Catches 1996). After a period of sustained rainfall the Little River flows into its first capture point, Mud Sink, which is typically laden with sediment from the stream channel. After its acceptance capacity is exceeded, water from Mud Sink overflows into a channel that leads into another sinkhole, named Stick Sink (fig. 2), approximately 120 m downstream. The channel continues downstream from Stick Sink, leading into another group of small sinkholes. These sinkholes contain dry caves that apparently captured flow from the Little River prior to the opening and enlargement of Mud and Stick Sinks.

Land use in the watershed is mainly agricultural where pasture is the principal use, as well as some row crops, poultry, and dairy farms. The locations of wells sampled, and sampling sites for Mud Sink, Stick Sink, and Little River are shown in figure 2. Water samples were collected during low-flow conditions (November-December 1995) and high-flow conditions (April 1996).

Leon County Study Area

In Leon County, Florida, more than 3,300 karst features have been identified based on areal photography, topographic maps, and satellite imagery (Benoit et al 1992). These karst features include sinkholes, closed depressions, swallow holes, springs, open basins suspected of originating from solution processes, and large lake basins with known sinkholes. Study sites in Leon County are located in the Northern Highlands (Brooks 1981), which contains large solution (sinkhole) lakes, such as Lake Jackson, (fig. 3) and smaller lakes.

In addition to recharge to the Upper Floridan aquifer through solution lakes, there are numerous locations in the northern part of the study area where stormwater runoff from small drainage basins flows directly into sinkholes and recharges the Upper Floridan aquifer (Benoit et al 1992). One of the largest of these systems is the FG sinkhole (fig. 3), which receives surface water from a drainage area of about 1,010 ha and flows directly into the sinkhole, approximately 24 m deep, 20 m wide, and 60 m long. About 0.1 to 0.2 m3/s of water typically enters the aquifer from this sinkhole during periods of normal rainfall (Katz et al 1997). The location of sampling sites, including municipal wells (96-130 m in depth below land surface) and FG sink, and potentiometric-surface contours for the Upper Floridan aquifer are shown in figure 3. Water samples were collected from wells and FG sink during November 1994 to May 1994.

Methods

In the three study areas, ground-water samples were collected from monitoring wells by using a positive displacement dual piston pump with a 0.95-cm Teflon discharge line at pumping rates of approximately 0.06 L/s (Katz et al 1995a, 1997). Wells were purged for at least three casing volumes with the pump intake positioned approximately 1 to 2 m above the screened interval. The pump was subsequently lowered into the top of the screened interval or open hole interval for sampling. Water samples from municipal supply wells that tap deep parts of the Upper Floridan aquifer in Leon County (denoted CW-xx and LVW-1, fig. 3) were collected prior to any treatment using in-line turbine pumps (pumping rates ranged from approximately 30 to 160 L/s). Surface-water samples (Lake Barco, Lake Bradford, FG sink, Mud and Stick Sinks) were collected at a depth of approximately 0.5 to 1 m below the water surface, using a peristaltic pump with silicone rubber and tygon tubing. Samples of rain water were collected using a wet/dry atmospheric deposition collector at or near each study area.

Stable isotopes of oxygen and hydrogen in water samples were analyzed at the U.S. Geological Survey Isotope Fractionation Laboratory in Reston, Virginia, using techniques described by Coplen et al. (1991 1994). Standard d (delta) notation (Gonfiantini 1981) is used for the stable isotopes, as defined by:

d (per mil) = [(Rsample/Rstandard) - 1] 1,000 (1)

where R = 18O/16O and D/H for d18O and dD, respectively. Oxygen- and hydrogen-isotope results are reported in per mil relative to VSMOW (Vienna Standard Mean Ocean Water) and are normalized on scales such that the oxygen and hydrogen isotopic values of SLAP (Standard Light Antarctic Precipitation) are -55.5 per mil and -428 per mil, respectively (Coplen 1994). The 2s precision of d18O and dD results is 0.2 and 1.5 per mil, respectively.

A two-component mixing model was used to estimate the fraction of surface water that mixes with ground water. For a two-component mixture, the fraction of surface water (fsw) in the mixture is defined as:

fsw = (Ym - Ygw)/(Ysw - Ygw) (2)

where Ym, Ygw, and Ysw denote the concentrations of d18O and dD in the mixture, ground-water, and surface-water end members, respectively.

Differences in the isotopic composition of surface water, rainfall, and ground water result in relatively high precision for detecting the mixing proportion of surface water in ground water. The sensitivity (S) of the method for detecting the proportion of lake water or stream water that mixes with ground water can be determined by the following expression (Payne 1983):

S =+/-X/Y; (3)

where +/-X is the variability (1s) of the isotopic composition of the lake or stream, in per mil; and Y is the difference between the isotopic composition of surface water and ground water, in per mil.

Results and Discussion

Differences in the content of D and 18O in ground water, rainfall, and surface water were used to determine mixing processes in the ground-water flow system in the three study areas. D and 18O data typically are plotted on a diagram showing dD versus d18O relative to VSMOW. Mean annual values of dD and d18O in precipitation collected at many locations around the world plot along a line with a slope of 8 and intercept of +10 (dD=8d18O+10), commonly referred to as the global meteoric water line (MWL) (Craig 1961). The variability in isotopic composition of rainfall from one site to another is a function of several factors, including storm-track origin, rainfall amount and intensity, atmospheric temperature, and the number of evaporation and condensation cycles (Dansgaard 1964). The stable-isotopic composition of waters relative to the MWL reveals important information on ground-water recharge patterns, the origin of waters in hydrologic systems, and mixing of ground water and surface water. For example, rainwater in the three study areas is slightly depleted in dD and d18O relative to the isotopic composition of seawater (dD and d18O = 0.0 per mil), because rain originates as evaporation from the Gulf of Mexico and most likely has been through only one evaporation-condensation cycle.

Interactions Between Lake Barco and Ground Water

The ground-water samples upgradient from Lake Barco (1PNB well nest) clearly show a meteoric origin, as their isotopic composition is similar to that of rainfall (3 month composite samples collected during April 1991 through September 1992; Katz et al 1995a). The similarity in isotopic composition between rainfall and upgradient ground water indicates a rapid recharge rate with evapotranspiration processes not affecting the stable isotopic composition of water. Water samples from two in-lake wells, MLW-2 and MLW-6 (fig. 1) have isotopic compositions similar to meteoric water and plot along the MWL (fig. 4), indicating that ground-water inflow occurs at these sites. These wells were originally installed beneath the bottom of Lake Barco (Sacks et al 1992); however, during 1989-90 lake volume had decreased by 45%, and when water samples were collected in 1992 these wells were outside of the lake perimeter.

The isotopic compositions of ground water downgradient from Lake Barco (MLW-4 and 2PNB well nest) have enriched values relative to meteoric water and plot along a mixing line described by the expression, dD= 4.6d18O - 1.3 (r2 = 0.987). The mixing line connects the isotopic composition of the surface-water and ground-water end members—evaporated Lake Barco water and ground water upgradient from the lake (fig. 4). The isotopic shift in the composition of ground water downgradient from Lake Barco provides evidence for lateral flow from the lake toward the downgradient sites, despite the strong downward head gradient at the nested wells at the 2PNB site (Sacks et al 1992). The isotopic composition of ground water downgradient from Lake Barco was nearly identical for samples collected during the three separate occasions, May 1991, November 1991, and August-September 1992, (Katz et al 1995a) and, for the sake of clarity, the isotopic compositions are shown in figure 4 for the most recent samples.

The relative position of the sites downgradient from Lake Barco along the mixing line indicates the relative proportion of lakewater outflow (leakage) that has mixed with meteoric water (ground water upgradient from the lake). Isotope mass-balance calculations (eqn. 2) using d18O indicates that the largest fraction of lake water leakage (0.65) occurred in water samples from well MLW-4 (located directly beneath the lake) and 2PNB-FL (0.64), which is hydraulically connected to the lake as a result of the breached confining unit (table 1). Progressively smaller fractions of lake-water leakage (0.56 - 0.23) have mixed with ground water from 2PNB-60 to 2PNB-20, respectively (table 1). Distinct differences in the isotopic composition of lake water and ground water provide a high precision estimate for the limit of detection of lake water in ground water (+/-0.043) using eqn. (3) and data for both d18O and dD. The high degree of precision also provided constraints when using mixing ratios of lake-water leakage and meteoric (recharge) water to model the chemical evolution of ground water downgradient from Lake Barco (Katz et al 1995b).

Interactions Between the Sinking Little River and Ground Water

Samples of ground water collected during low-flow conditions had similar d18O and dD values as were observed in rainfall (monthly composites collected during June 1995 through May 1996). The isotopic composition of these samples plot along the global meteoric water line (Craig 1961), indicating that they are probably little affected by evaporation (fig. 5). In contrast, during high-flow conditions, the d18O and dD composition of water from Little River had a more enriched isotopic composition than that of rainfall and most samples of ground water, indicating that evaporation has occurred (table 1). Also during high-flow conditions, water samples from four sites had higher d18O and dD values than samples collected during low-flow conditions: JOW-3D, JOW-5S, Stick Sink and Mud Sink (fig. 5). The isotopic composition of water from these four sites and the Little River plot along a mixing line described by the expression dD=5.3d18O+0.31 (r2=0.982), which connects the isotopic composition of the end members, Little River, and ground water collected during low-flow conditions. The enriched isotopic signature in water samples from the four sites indicates that river water is mixing with ground water in the following proportions (table 1): Mud Sink (0.85-0.96), Stick Sink (0.44-0.50), JOW-3D (0.59-0.66), and JOW-5S (0.13-0.16).

Interactions between river water and ground water in the study area can be highly variable and are dependent upon the distance from the capture zone for the Little River and the degree of interconnection between the aquifer and the Little River sinks. The aquifer is typical of many carbonate aquifer systems that contain both conduit networks and diffuse flow, where conduits and fractures have an important influence on the interactions between river water and ground water.

Interactions Between Surface Water and Deep Ground Water, Leon County

d18O and dD values were similar for samples of rainfall (monthly composited samples) collected during January through August 1995, samples of water from seven municipal supply wells, and surface runoff flowing into FG sink (all plot along the global meteoric water line). A most surprising finding was that water from five deep municipal supply wells located in the northwestern part of the study area (CW-19, CW-23, CW-15, CW-26, and LVW-1) was enriched in 18O and D (fig. 6). This isotopic enrichment indicates that surface water which has undergone evaporation, is mixing with ground water in deep parts of the Upper Floridan aquifer. The isotopic composition of water from these five sites plot along a mixing line described by the expression dD=4.9d18O-0.78 (r2=0.996), connecting the isotopic composition of an evaporated surface water (approximated by the average isotopic composition of water from Lake Barco) and deep ground water that plots along the global meteoric water line (fig. 6). Using d18O and dD, the following proportions of surface water that mixed with ground water ranged from 0.10 (LVW-1) to 0.34 (CW-26) (table 1). Mixing of surface water and ground water is supported by estimates of ground-water age. Recharge to ground water has occurred within the last 40 years, based on tritium analyses of ground water (Katz et al 1997).

Mixing of surface water with ground water at depths greater than 60 m is probably facilitated by water moving downward through sinkholes into the Upper Floridan aquifer and possibly by heavy pumping in certain areas of the aquifer. Contributing areas for selected water- supply wells were delineated by tracking particles backward toward areas of recharge by using a calibrated three-dimensional flow model (Davis 1996). A large, saturated wetland area and shallow sinkhole lakes lie within contributing areas for wells in the northwestern part of the study area. Water from deep wells in the Upper Floridan aquifer upgradient from these sources of enriched surface water had d18O and dD values that plot along the global meteoric water line (Sprinkle 1989), indicating that little or no evaporation occurred during recharge to the Upper Floridan aquifer north and upgradient of the study area.

Summary and Conclusions

Oxygen and hydrogen stable isotopes are highly effective naturally occurring tracers of mixing of different water sources because oxygen and hydrogen constitute and move with water molecules. Surface water, which is preferentially enriched in d18O and dD, has an isotopic signature that provides conservative tracers for evaluating the extent of mixing of river water and ground water. Stable isotope measurements can provide very useful quantitative information about recharge patterns and interactions between ground water and surface water.

The use of d18O and dD in quantifying mixing between surface water and ground water was demonstrated for three different hydrologic systems in the karst terrain of northern Florida. Water from a sinkhole lake (Lake Barco) with an enriched isotopic signature is moving downward and laterally beneath the lake and mixing with downgradient ground water. Fractions of lake water that mixed with ground water ranged from 0.23 to 0.65 and were related to the proximity of the site to the lake bottom and the presence of solution features that facilitate the connection between the lake and the Upper Floridan aquifer. A recharge pulse from the Little River, a sinking stream, produced an enriched isotopic signature in water from wells downgradient from the river capture zone and from a series of sinks that receive Little River water during high-flow conditions. Proportions of Little River water that mixed with ground water and water in sinks ranged from 0.13 to 0.96 and were related to the proximity to the river capture zone and the degree of connectivity between the sinks and zones in the Upper Floridan aquifer. Water from wetlands and shallow sinkhole lakes has mixed with deep ground water in an area of northwestern Leon County. Proportions of evaporated surface water that mixed with ground water ranged from 0.10 to 0.34. Recharge of enriched surface water into the Upper Floridan aquifer might result from heavy pumping of municipal supply wells.

Mixing of surface water and ground water from the Upper Floridan aquifer indicates that the aquifer is highly susceptible to contamination from activities at the land surface, particularly where karst features are present. The vulnerability of the aquifer in all three study areas to contamination has been demonstrated by ground water that receives recharge of relatively recent origin (within the last 40 years based on measurements of tritium; Katz et al 1995a, 1997) and the presence of perchloroethylene in deep ground water in the Upper Floridan aquifer in the Leon County area.

The collection and analysis of water samples for stable isotopes of water should be included as part of monitoring programs where interactions between ground water and surface water are likely to exist. The combination of isotopic and chemical data provides a powerful tool for identifying and quantifying the processes controlling ground-water quality. A better understanding of hydrochemical interactions between surface water and ground water, and of the processes controlling the chemical composition of ground water, will assist regulators in making more informed environmental decisions for protecting the valuable water resources of the Upper Floridan aquifer.

Acknowledgments

The studies described in this paper were funded jointly by the U.S. Geological Survey and the Florida Department of Environmental Protection. The author gratefully acknowledges L.A. Sacks and J.E. Landmeyer for their review comments and suggestions that significantly improved earlier versions of this paper.

References

Arrington, D.V., and Lindquist, R.C. 1987. Thickly mantled karst of the Interlachen area, in Beck, B.F., and W.L. Wilson, eds., Proceedings of second multidisciplinary conference on sinkholes and the environmental impacts of karst. Florida Sinkhole Research Institute, Orlando:31-39.

Benoit, A.T., Johnson, J.L., Rains, L., Songer, E.F., and O’Rourke, P.L. 1992. Characterization of karst development in Leon County, Florida, for the delineation of wellhead protection areas. Northwest Florida Water Management District, Havana. Special Report 92-8:83.

Brooks, H.K. 1981. Physiographic divisions of Florida: Center for Environmental and Natural Resources, University of Florida, Gainesville: 11.

Coplen, T.B. 1994. Reporting of stable hydrogen, carbon, and oxygen isotopic abundances. Pure and Applied Chemistry, 66:273-276.

Coplen, T.B., J.D. Wildman, and J. Chen. 1991. Improvements in the gaseous hydrogen-water equilibration technique for hydrogen isotope ratio analysis, Analyt. Chem., 63:910-912.

Craig, H. 1961. Isotopic variations in meteoric waters. Science 133:1702-1703.

Dansgaard, W. 1964. Stable isotopes in precipitation. Tellus, 16:436-468.

Davis, J.H. 1996, Hydrogeologic investigation and simulation of ground-water flow in the Upper Floridan aquifer of north-central Florida and southwestern Georgia and delineation of contributing areas for selected city of Tallahassee, Florida, water-supply wells. U.S. Geological Survey Water-Resources Investigations Report 95-4296:55.

Gonfiantini, R. 1981. The d-notation and the mass-spectrometric measurement techniques, in J.R. Gat, and R. Gonfiantini, eds., Stable isotope hydrology: Deuterium and oxygen-18 in the water cycle. International Atomic Energy Agency, Vienna, Austria. Ch. 4:35-84.

Gonfiantini, R. 1986. Environmental isotopes in lake studies, in Handbook of Environmental Isotope Geochemistry, P. Fritz and J. Ch. Fontes, eds. Elsevier, N.Y. 113-168.

Greene, E.A. 1997. Tracing recharge from sinking streams over spatial dimensions of kilometers in a karst aquifer. Ground Water, 35(5):898-904.

Katz, B.G., Lee, T.M., Plummer, L.N., and Busenberg, E. 1995a. Chemical evolution of groundwater near a sinkhole lake, northern Florida: 1. Flow patterns, age of groundwater, and influence of lakewater leakage: Water Resources Research, 31(6):1549-1564.

Katz, B.G., Plummer, L.N., and Busenberg, E., Revesz, K.M., Jones, B.F., and Lee, T.M. 1995b. Chemical evolution of groundwater near a sinkhole lake, northern Florida: 2. Chemical patterns, mass-transfer modeling, and rates of chemical reactions: Water Resources Research, 31(6):1565-1584.

Katz, B.G., and Catches, J.S. 1996, The Little River basin study area-- hydrochemical interactions between ground water and surface water: Southeastern Geological Society, Tallahassee, FL, Guidebook 36:22-28.

Katz, B.G., Coplen, T.B., Bullen, T.D., and Davis, J.H. 1997. Use of chemical and isotopic tracers and geochemical modeling to characterize the interactions between ground water and surface water in mantled karst, Ground Water, 35(6):1014-1028.

Krause, R.E. 1979. Geohydrology of Brooks, Lowndes, and western Echols Counties, Georgia. U.S. Geological Survey Water-Resources Investigations Report 78-117:48.

Lee, T.M. 1996. Hydrogeologic controls on the groundwater interactions with an acidic lake in karst terrain, Lake Barco, Florida. Water Resources Research, 32:831-844

McConnell, J.B., and Hacke, C.M. 1993. Hydrogeology, water quality, and water-resources development potential of the Upper Floridan aquifer in the Valdosta area, south-central Georgia. U.S. Geological Survey Water-Resources Investigations Report 93-4044:44.

Owenby, J.R., and Ezell, D.S. 1992. Monthly station normals of temperature, precipitation, and heating and cooling degree days 1961-90. U.S. Department of Commerce, Climatography of the United States 81:26.

Payne, B.R. 1983. Interaction of surface water with groundwater, in International Atomic Energy Agency, Guidebook on Nuclear Techniques in Hydrology. IAEA, Vienna, Technical report series 91:319-325.

Sacks, L.A., Lee, T.M., and Tihansky, A.B. 1992, Hydrogeologic setting and preliminary data analysis for the hydrologic-budget assessment of Lake Barco, an acidic seepage lake in Putnam County, Florida: U.S. Geological Survey Water-Resources Investigations Report 91-4180:28.

Scott, T.M. 1988. The lithostratigraphy of the Hawthorn Group (Miocene) of Florida, Florida Geological Survey, Tallahassee, Bulletin 59:148.

Sinclair, W.C., and Stewart, J.W. 1985. Sinkhole type, development, and distribution in Florida, Florida Bureau of Geology, Map Series110.

Sprinkle, C.L. 1989. Geochemistry of the Floridan aquifer system in Florida and in parts of Georgia, South Carolina, and Alabama. U.S. Geological Survey. Professional Paper 1403-I:105.

 

 

 

 

 

 

Figure 1. Map of Lake Barco study area showing location of sampling sites for ground water and lake water.

 

 

Figure 2. Map of Little River study area showing location of sampling sites for

ground water, river water and rainfall.

Figure 3. Map of Leon County study area showing location of sampling sites for

ground water and surface water.

 

 

Figure 4. Deuterium and oxygen-18 content of rainfall, ground water, and lake water from

the Lake Barco study area compared to the global meteoric water line.

 

Figure 5. Deuterium and oxygen-18 content of ground water, river water, and rainfall from

the Little River study area compared to the global meteoric water line.

 

 

 

 

Figure 6. Deuterium and oxygen-18 content of ground water, river water, and rainfall from

the Leon County study area compared to the global meteoric water line.

Table 1. Mixing Proportions of Surface Water and Ground Water in
Three Study Areas of Northern Florida

Concentrations of d18O and dD are in per mil; Fsw denotes fraction of surface water mixing with ground water, using equation (2); ESW denotes evaporated surface water (see text).

Site name

d18O

dD

Fsw
(
d18O)

Fsw
(
dD)


Lake Barco study area (August-September 1992)

MLW-4

0.95

2.50

0.65

0.62
2PNB-20

-2.25

-11.0

0.23

0.25
2PNB-40

-1.00

-5.55

0.39

0.40
2PNB-60

0.25

-2.50

0.56

0.48
2PNB-80

-0.45

-2.00

0.46

0.49
2PNB-FL

0.85

2.00

0.64

0.60
Lake Barco

3.60

16.5

1.0

1.0


Little River sinks study area (April 1996)

Mud Sink

-1.67

-9.2

0.96

0.85
Stick Sink

-2.75

-14.9

0.44

0.50
JOW-3D

-2.15

-10.9

0.66

0.59
JOW-5S

-3.17

-15.6

0.13

0.16
Little River

-1.6

-7.7

1.0

1.0


Leon County study area (February-March 1995)

CW-15

-1.99

-9.47

0.17

0.19
CW-19

-1.11

-6.60

0.28

0.26
CW-26

-0.77

-3.78

0.33

0.34
CS-23

-1.89

-11.2

0.18

0.14
LVW-1

-2.43

-12.7

0.11

0.10
ESW

4.20

19.8

1.0

1.0