Ground-water discharge

Ground-water recharge

Detection limit

10

1

0.1

0.01

M104 M106 M105 M103 M207 M203 M205 M206 M201

RECHARGE OR DISCHARGE, IN CENTIMETERS PER DAY

RESEARCH SITES ACROSS EVERGLADES NUTRIENT REMOVAL (ENR) PROJECT

U.S. Department of the Interior

U.S. Geological Survey

In cooperation with 

South Florida Water Management District

Interactions between Surface Water and Ground 

Water and Effects on Mercury Transport in the 

North-central Everglades

Water-Resources Investigations Report 02-4050

Prepared as part of the

U.S. Geological Survey Place-Based Studies Program
Front Cover: Ground-water discharge and recharge estimates are plotted on top of an aerial photo of the Everglades 

Nutrient Removal (ENR) project. The view is to the south and shows various plant communities, open water, 

and remnant canals from the time when this area of the Everglades was farmed. Beyond the canals on the far right is 

land that still is being farmed.
U.S. Department of the Interior

U.S. Geological Survey

Interactions between Surface Water and Ground 

Water and Effects on Mercury Transport in the 

North-central Everglades

By Judson W. Harvey1, Steven L. Krupa2, Cynthia Gefvert1, 2, Robert H. Mooney3, Jungyill Choi1, 

Susan A. King4, and Jefferson B. Giddings2

Water-Resources Investigations Report 02-4050

In cooperation with 

South Florida Water Management District

Reston, Virginia

2002

1U.S. Geological Survey, Reston, Virginia

2South Florida Water Management District, West Palm Beach, Florida

3U.S. Geological Survey, Miami, Florida

4U.S. Geological Survey, Middleton, Wisconsin
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of trade or product names in this report is for identification 

purposes only and does not constitute endorsement by the 

U.S. Government.

For additional information write to: Copies of this report can be purchased from:

Judson Harvey U.S. Geological Survey

U.S. Geological Survey Branch of Information Services

430 National Center Box 25286, Federal Center

Reston, VA 20192 Denver, CO 80225-0286

jwharvey@usgs.gov

This document is available on compact disc and on the Web at http://sofia.usgs.gov/index.html
Harvey and others—Interactions between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades—WRIR 02-4050
Contents III

CONTENTS

Abstract ................................................................................................................................................................................ 1

Introduction .......................................................................................................................................................................... 3

Purpose, scope, and source of funding ....................................................................................................................... 5

Hydrologic setting: Characteristics of pre- and post-drainage Everglades ................................................................ 6

Site information .......................................................................................................................................................... 7

Everglades Nutrient Removal Project .............................................................................................................. 7

Water Conservation Area 2A ............................................................................................................................ 7

 Acknowledgments ..................................................................................................................................................... 10

Hydrogeology of the north-central Everglades .................................................................................................................... 10

Geologic setting of study areas .................................................................................................................................. 11

Lake Flirt Marl Formation ................................................................................................................................ 11

Fort Thompson Formation................................................................................................................................. 11

Anastasia Formation ......................................................................................................................................... 11

Caloosahatchee (Marl) Formation .................................................................................................................... 12

Tamiami Formation .......................................................................................................................................... 12

Hawthorn group ................................................................................................................................................ 12

Subaerial exposure and weathering affecting aquifer hydraulic properties ............................................................... 12

Lithology of the Surficial aquifer ............................................................................................................................... 13

Lithologic comparison between WCA-2A and ENR ................................................................................................. 17

Geophysical logging results and correlations ............................................................................................................. 19

Caliper logging ................................................................................................................................................. 19

Natural gamma ray logging............................................................................................................................... 19

Neutron logging ................................................................................................................................................ 21

Electric logs ...................................................................................................................................................... 21

Hydraulic conductivity of the Surficial aquifer .......................................................................................................... 22

Comparison of hydraulic conductivity in ENR and WCA-2A ................................................................................... 23

Comparison of hydraulic conductivities to other studies ........................................................................................... 25

Evidence for layers restricting vertical flow .................................................................................................... 26

Quantifying recharge and discharge in the north-central Everglades and identifying the key controlling factors .............. 27

Approach .................................................................................................................................................................... 28

Land-surface topography, surface-water slope, and water-level fluctuations ............................................................ 30

WCA-2A .......................................................................................................................................................... 30

ENR .................................................................................................................................................................. 30

Horizontal hydraulic gradients ................................................................................................................................... 31

Horizontal ground-water flow velocities .................................................................................................................... 31

Vertical hydraulic gradients ........................................................................................................................................ 32

ENR .................................................................................................................................................................. 32

WCA-2A .......................................................................................................................................................... 32

Summary of vertical hydraulic gradients ......................................................................................................... 35

Peat hydraulic properties ............................................................................................................................................ 35

Measured vertical head gradients compared with hydrogeologic model simulations ................................................ 38

Recharge and discharge estimates .............................................................................................................................. 38

Seepage-meter results from ENR ..................................................................................................................... 38

Darcy-flux calculations from WCA-2A............................................................................................................ 41

Seepage-meter results from WCA-2B and WCA-3A ...................................................................................... 45

Water balance ............................................................................................................................................................. 48

Use of geochemical tracers to identify sources and pathways of water flow in the Everglades .......................................... 49

Approach .................................................................................................................................................................... 50
IV Contents

Source of ground-water recharge ............................................................................................................................... 51

Distinguishing sources of recharge using major-ion composition ............................................................................. 51

Background - Source and transitional waters .................................................................................................. 53

Fresh recharge water .............................................................................................................................. 53

Relict seawater ....................................................................................................................................... 55

Fresh formation water ............................................................................................................................ 55

Mixed waters and other transitional waters ............................................................................................ 55

Geochemical tracing of surface-water and ground-water interactions ....................................................................... 56

Effect of ground-water and surface-water interactions on mercury transport in the north-central Everglades ................... 59

Approach .................................................................................................................................................................... 60

Data set I ........................................................................................................................................................... 60

Data set II ......................................................................................................................................................... 60

Laboratory analyses .......................................................................................................................................... 61

Distribution of mercury in ground water, surface water, and peat porewater ............................................................ 61

Relation to ground-water geochemical type and source of recharge ......................................................................... 63

Relation to sulfate.............................................................................................................................................. 63

Source of recharge water .................................................................................................................................. 63

Ground-water component of the mercury budget at ENR ......................................................................................... 66

Computation of mercury fluxes ........................................................................................................................ 66

Mercury fluxes with recharging and discharging water in ENR ...................................................................... 68

Evidence for reactivity of mercury in peat and in the Surficial aquifer ..................................................................... 73

Summary and conclusions .................................................................................................................................................... 75

References cited ................................................................................................................................................................... 79

FIGURES

1-3. Maps showing:

1. Everglades and adjoining areas, south Florida ................................................................................................. 4

2. Location of study sites in the Everglades Nutrient Removal (ENR) project .................................................... 8

3. Location of study sites in Water Conservation Area 2 (WCA-2) ..................................................................... 9

4-5. Diagrams showing:

4. Lithostratigraphic cross section with formation names, ENR .......................................................................... 14

5. Lithostratigraphic cross section with formation names, Water Conservation Area 2A (WCA-2A) ................ 15

 6-14. Graphs showing:

6. Standard penetration test N values from the Surficial aquifer .......................................................................... 16

7. Uniformity coefficients, hydraulic conductivities from drawdown tests, and 

generalized lithology from the Surficial aquifer ............................................................................................... 18

8. Grain size from the Surficial aquifer at site S10C (WCA-2A) and site MOP2 (ENR) .................................... 19

9. North (left) to south (right) changes in borehole gamma counts ...................................................................... 20

10. Water levels and horizontal hydraulic gradients along research transects in 

ENR (A) and WCA-2A(B) ................................................................................................................................ 33

11. Distribution of vertical hydraulic gradient in ENR .......................................................................................... 34

12. Water levels (A) and vertical hydraulic gradients (B) at ENR .......................................................................... 36

13. Water levels (A) and vertical hydraulic gradients (B) at WCA-2A ................................................................... 37

14. Hydraulic conductivity of wetland peat in ENR (A) and wetland peat and underlying 

sand layer in WCA-2A (B) ............................................................................................................................... 39

15. Schematic showing hydrogeologic simulation of head change in aquifer ............................................................... 40

16-23. Graphs showing:

16. Time-averaged discharge and recharge fluxes across transect A-A’ in ENR .................................................... 42

17. Cumulative distributions of vertical fluxes measured with seepage meters at sites in the 

wetland interior in ENR (A), and at sites near levees in ENR and WCA-2A (B) ............................................. 44

18. Observed discharge and recharge and simulated fluxes in ENR ...................................................................... 45

19. Cumulative distributions of vertical fluxes calculated by Darcy’s law on Eastern transect (A), 

and Western transect (B) in WCA-2A .............................................................................................................. 46
Contents V

20. Vertical fluxes observed over a 2-year period on the Eastern transect (A), and 

Western transect (B) in WCA-2A ..................................................................................................................... 47

21. Spatial trends in discharge and recharge in north-central Everglades .............................................................. 50

22. Water-stable isotopic ratios δ2H and δ18O from precipitation, surface water, and 

ground water in ENR and WCA-2A ................................................................................................................. 52

23. Piper diagrams illustrating variability in major ion composition of surface and ground 

waters from ENR (A) and WCA-2A (B) ........................................................................................................... 54

24-25. Diagrams showing:

24. Ground-water flow paths delineated by specific conductivity (contours) and geochemical 

water type (color codes) at ENR ....................................................................................................................... 57

25. Ground-water flow paths delineated by specific conductivity (contours) and geochemical 

water type (color codes) at WCA-2A ............................................................................................................... 58

26-33. Graphs showing:

26. Distribution of dissolved total mercury (A), methylmercury (B), 

and ratio of methylmercury to total mercury (C) in surface water and ground water ..................................... 62

27. Dissolved methylmercury and sulfate concentrations in surface water and ground water ............................... 64

28. Dissolved mercury and oxygen isotopic ratio in surface water and ground water ........................................... 65

29. Dissolved methylmercury and total mercury .................................................................................................... 67

30. Dissolved total mercury (A) and methylmercury (B) in shallow wells at ENR ................................................ 69

31. Time course of total mercury (A) and methylmercury (B) concentrations at ENR .......................................... 70

32. Profiles of estimated concentrations of dissolved total mercury (A) and 

methylmercury (B) in peat porewater at ENR .................................................................................................. 72

33. Average ground-water fluxes and storage rates (grams per year) of total mercury (A)

and methylmercury (B) compared with surface-water and atmospheric fluxes at ENR .................................. 73

TABLES

1. Chronostratigraphy of the Quaternary sediments underlying the north-central Everglades, south Florida ............ 12

2. Summary of hydrogeologic, lithostratigraphic, and chronostratigraphic units in

Everglades Nutrient Removal (ENR) project and Water Conservation Area 2A (WCA-2A) .................................. 13

3. Number of standard penetration test blows at three sites ........................................................................................ 16

4. Mean uniformity and gradation coefficients at six boreholes .................................................................................. 17

5. Peaks in neutron geophysical logs and the corresponding lithology at ENR ........................................................... 21

6. Results of core analysis from boreholes in WCA-2A .............................................................................................. 23

7. Hydraulic conductivity determined from core samples ........................................................................................... 24

8. Hydraulic conductivity compared in two layers at several sites in ENR ................................................................. 24

9. Hydraulic conductivity of peat, peat-to-rock transition zone, and shallow underlying sand layer .......................... 24

10. Summary of quartz content of selected boreholes ................................................................................................... 25

11. Summary of horizontal hydraulic conductivity as determined by drawdown tests ................................................. 26

12. Summary of horizontal hydraulic conductivity values from the Surficial aquifer 

in Palm Beach County and eastern Broward County ............................................................................................... 27

13. Horizontal hydraulic gradients and solute velocities of ground-water flow ............................................................ 29

14. Time-averaged vertical fluxes at study sites in ENR ............................................................................................... 43

15. Time-averaged vertical fluxes at study sites in WCA-2A ........................................................................................ 43

16. Basin-scale estimates of recharge and discharge ..................................................................................................... 49

17. Mercury concentrations in shallow ground water and surface water at ENR .......................................................... 68

18. Annual budget for recharge, discharge, and subsurface storage of dissolved mercury in ENR ............................... 71
VI Contents

Multiply By To obtain

Length

foot (ft) 30.48 centimeter (cm)

mile (mi) 1.609 kilometer (km)

Area

acre (ac) 4,047 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2)

Volume

gallon (gal) 3.785 liter (L)

Flow rate

foot per day (ft/d) 30.48 centimeter per day (cm/d)

Hydraulic conductivity

foot per day (ft/d) 30.48 centimeter per day (cm/d)

CONVERSION FACTORS, VERTICAL DATUM, AND 

ABBREVIATED WATER-QUALITY UNITS

1929 NGVD: In this report, “1929 NGVD” refers to the National Geodetic Vertical Datum of 1929–a geodetic datum derived from a general 

adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.

Hydraulic Conductivity: The standard unit for hydraulic conductivity is volume per time per unit cross-sectional area of sediment, such as 

ft3/(ft2d). In this report, the mathematically reduced form, foot per day (ft/d), is used for convenience. 

Abbreviated water-quality units used in this report: Constituent concentrations, water temperature, and other water-quality measures are 

given in metric units. Constituent concentrations are given in milligrams per liter (mg/L), or nanograms per liter (ng/L).

Specific conductance (SC) of water is given in microsiemens per centimeter at 25 degrees Celsius (mS/cm at 25°C). The unit is equivalent to 

micromhos per centimeter at 25 degrees Celsius (mmho/cm), a unit formerly used by the U.S. Geological Survey.

Additional abbreviations:

inches (in)

millimeter (mm)

micron (μ)

milliliter (ml)

grams per year (g/yr)

inches per mile (in/mi)

ohm-meters (ohm-m)
Abstract 1

ABSTRACT

The hydrology of the north-central 

Everglades was altered substantially in the past 

century by canal dredging, land subsidence, 

ground-water pumping, and levee construction. 

Vast areas of seasonal and perennial wetlands were 

converted to uses for agriculture, light industry, 

and suburban development. As the catchment area 

for the Everglades decreased, so did the sources of 

water from local precipitation and runoff from 

surrounding uplands. Partly in response to those 

alterations, water-resources managers 

compartmentalized the remaining wetlands in the 

north-central Everglades into large retention 

basins, called Water Conservation Areas (WCAs). 

In spite of efforts to improve how water resources 

are managed, the result has been frequent periods 

of excessive drying out or flooding of the WCAs 

because the managed system does not have the 

same water-storage capacity as the pre-drainage 

Everglades. Linked to the hydrological 

modifications are ecological changes including 

large-scale invasions of cattail, loss of tree islands, 

and diminishing bird populations in the 

Everglades. Complex interactions among 

numerous physical, chemical, and biological 

factors are responsible for the long-term 

degradation of the ecological character of the 

Everglades.

Over the past 15 years, a new set of smaller 

wetland basins, called Stormwater Treatment 

Areas (STAs), have been designed and constructed 

by water-resources engineers on the former 

wetlands adjacent to WCAs. The purpose of STAs 

is to remove excess nutrients from agricultural 

drainage water prior to its input to WCAs. STAs 

tend to be about one-tenth the size of a WCA, and 

they are located on former wetlands on the 

northwestern side of WCAs on sites that were 

managed as farmland for much of the twentieth 

century in an area referred to as the Everglades 

Agricultural Area, or EAA.

The objective of the present investigation 

was to quantify interactions between surface water 

and ground water in the Everglades Nutrient 

Removal Project (ENR), a prototype project for 

the STAs that began operation in 1994. 

Determining the effect of ground water on the 

mercury balance of the ENR treatment wetland 

was an important additional objective. In order to 

broaden the relevance of conclusions to all parts of 

the north-central Everglades, interactions between 

surface water and ground water and mercury also 

were investigated in Water Conservation Area 2A 

(WCA-2A) and, to a lesser extent, in two other 

WCA basins, WCA-2B and WCA-3A.

An important conclusion of this study is that 

creation of the WCA basins, and accompanying 

water-resources management, have appreciably 

increased both recharge and discharge in the 

north-central Everglades compared with 

pre-drainage conditions. Recharge and discharge 

are highest near the northern and northwestern 

edges of the Everglades, in the relatively small 

basins such as ENR and the STAs that share 

borders with both WCA-1 and the EAA. All 

basins experienced greater increases in recharge 

relative to discharge, because of the effects that 

land subsidence and ground-water pumping 

outside the Everglades had on hydraulic gradients. 

The highest basin-wide estimate of recharge was 

measured in ENR, where recharge averaged 0.9 

centimeter per day (cm/d) over a 4-year study 

period. For perspective, that estimate of recharge 

is the equivalent of 30 percent of pumped 

Interactions between Surface Water and Ground Water 

and Effects on Mercury Transport in the North-central 

Everglades

By Judson W. Harvey, Steven L. Krupa, Cynthia Gefvert, Robert M. Mooney, Jungyill Choi, 

Susan A. King, and Jefferson B. Giddings
2 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

surface-water inflows and 230 percent of average 

daily precipitation in ENR. Ground-water 

discharge was 10 times smaller than recharge at 

ENR. The present study estimated a 

basin-averaged recharge for WCA-2A (0.2 cm/d) 

that was a factor of 4 smaller than ENR. Although 

preliminary, that estimate of recharge is 5 times 

higher than previous estimates (approximately 

0.04 cm/d), probably because the newer 

measurements were able to quantify recharge and 

discharge at finer spatial and temporal scales. 

Recharge at WCA-2A is smaller than ENR 

because WCA-2A has a smaller topographic 

gradient (3 x 10-5 and 2 x 10-4 in WCA-2A and 

ENR, respectively), as well as a smaller ratio of 

perimeter length to total wetland surface area 

(6 x 10-5 and 4 x 10-4 in WCA-2A and ENR, 

respectively), which decreases the importance of 

processes outside the wetlands such as land 

subsidence or ground-water pumping. At the 

present time, recharge and discharge are thought to 

be higher in the WCAs compared to the 

pre-drainage Everglades (perhaps by a factor of 4 

or 5), although that comparison is uncertain 

because of the difficulty of estimating 

pre-drainage hydrologic fluxes. The reason that 

recharge and discharge are thought to be higher 

now compared to pre-drainage conditions is that 

water-resources management has increased 

fluctuations in surface-water levels. The present 

study showed that the magnitude of recharge and 

discharge, as well as temporary reversals between 

recharge and discharge, are related to increased 

surface-water fluctuations caused by large water 

releases from WCA-1 into WCA-2A.

The most important geologic factor 

affecting interactions between surface water and 

ground water in the north-central Everglades is the 

hydraulic conductivity (K) of the Surficial aquifer. 

Estimates of K in the top 40 feet (ft) of the aquifer 

at both ENR and WCA-2A are higher (by more 

than an order of magnitude) than previously 

published estimates of K for the northern 

Everglades (typically, reported as 5 ft/day). 

Finding higher than anticipated hydraulic 

conductivities in the upper sand and limestone 

layers of the Surficial aquifer has important 

implications. In particular, it was found that the 

upper sand and limestone layers with high 

permeability are the main parts of the aquifer with 

appreciable freshwater. Sampling of major-ion 

chemistry in ground water showed that freshwater 

was usually located only at shallow depths, 

approximately the top 40 ft of the 200-ft deep 

Surficial aquifer. Hydraulic and chemical results, 

therefore, indicate that in many areas of the 

north-central Everglades, interactions between 

surface water and ground water primarily involve 

the top layers (layers 2 and 3) of the Surficial 

aquifer, causing appreciable recharge and 

discharge to a depth of approximately 40 ft.

Geochemical measurements provided 

further information about the source of the thin 

layer of fresh ground water beneath the 

north-central Everglades. Water-stable isotopic 

ratios of hydrogen and oxygen showed that the 

source of fresh ground water was recharge of 

Everglades surface waters, specifically, recharge 

of surface waters from wetland sloughs that had 

been present long enough in the surface flow 

system to be substantially evaporated. An 

exception was the portion of the aquifer beneath 

the interior of ENR, where stable isotopes 

indicated that recharge occurred quickly and 

without appreciable evaporation. This result, along 

with the distinct ionic signature of the water, is 

consistent with an interpretation that the source of 

recharging water beneath ENR was precipitation 

onto ENR during the time period it was managed 

for agricultural purposes. The "light" stable 

isotopic composition of that water indicates that 

precipitation infiltrated quickly through the 

unsaturated zone without appreciable evaporation. 

Another exception is apparent in ground water 

near levees. The amount and type of salts in 

ground water in the vicinity of levees indicate that 

ground-water seepage beneath the levee causes 

deep mixing in the Surficial aquifer that results in 

upward movement of relict seawater from the 

bottom two-thirds of the aquifer (below 60 ft) to 

shallow ground water and to wetland surface 

water.
Introduction 3

Part of the motivation for the present study 

was a concern that arose among the 

water-resources managers that designed the ENR 

treatment wetland. The concern was whether 

mercury methylation, and, thus, mercury 

bioavailability, might increase when agricultural 

soils were re-flooded and managed once again as 

wetlands. The present study complements the 

work of many other mercury investigators in the 

Everglades by specifically addressing the effect of 

interactions between surface water and ground 

water on mercury cycling.

Total dissolved mercury (HgT) was detectable in all monitoring wells in ENR and WCA-2A 

at an average concentration of 0.7 nanogram per 

liter (ng/L), which is slightly below the average 

concentration in surface water (1 ng/L). An important exception was shallow wells on the western 

side of ENR, where the average concentration was 

40 percent higher (1.4 ng/L) than surface water. 

Higher concentrations of HgT in ground water on 

the western side of ENR was the result of recharge 

from ENR surface water combined with release of 

HgT from solid phases in peat to recharging water. 

Dissolved methylmercury (MeHg) in ground water 

was undetectable in all deep wells (greater than 

40 ft deep) and most shallow wells (less than 

0.02 ng/L compared with 0.1 ng/L in ENR surface 

water). Shallow wells beneath the interior of ENR 

were the exception, with detectable MeHg concentrations as high as 0.2 ng/L. Wells with detectable 

MeHg are of interest because they are the same 

wells classified by water-stable isotopes and major 

ion chemistry as "agricultural recharge water." In 

general, HgT and MeHg concentrations were not 

positively correlated with sulfate concentrations at 

either ENR or WCA-2A.

A budget was developed for ground-water 

fluxes of mercury at ENR, which made possible a 

comparison with the surface and atmospheric 

components of the mercury budget for ENR 

developed by other researchers. Recharge of HgT

from surface water to ground water was a major 

pathway for transport of total dissolved mercury 

but not MeHg. Recharge of HgT accounted for a 

loss from ENR surface water equivalent to 10 

percent of the total inputs of HgT to surface water. 

In comparison, recharge of MeHg was not 

detectable and accounted, therefore, for none of 

the losses of MeHg from surface water in ENR. 

Chemical data and water-stable isotopic 

ratios indicate that most surface water recharged in 

ENR is discharged to a seepage canal on the 

western and northern side of ENR. Transport of 

recharged water through the Surficial aquifer to 

the seepage canal appears to take place in a matter 

of weeks to months, with only relatively minor 

mixing with deeper ground water. Measurements 

of HgT in the seepage canal suggested that HgT had 

not yet discharged to the canal at the end of the 

4-year study period. Because the flow path 

between points of recharge in ENR and discharge 

in the seepage canal was short, it was concluded 

that mercury was retained or delayed in its 

transport through the aquifer by interaction with 

aquifer sand or limestone or fine organic 

sediments at the base of the seepage canal.

INTRODUCTION

Surface-water resources in the Florida Everglades are managed to accommodate a rapidly growing 

urban area to the east and an agricultural industry operating in former wetlands to the northwest. Management 

of the north-central Everglades for flood control and 

water supply has changed the character of flow in the 

wetlands. Since the 1960s, the wetlands have been 

divided into large artificial basins called Water Conservation Areas (WCAs) that are fed by drainage from 

Lake Okeechobee, runoff from the Everglades Agricultural Area (EAA), rainfall, and ground-water discharge 

directly into wetlands or canals that overflow into the 

wetlands. Surface water flows from one conservation 

area to the next, moving southward through the wetlands, canals, culverts, and spillways, eventually into 

Florida Bay or the Gulf of Mexico (fig. 1). Along the 

way, surface flow may be depleted by evapotranspiration or by recharge to ground water. A portion of the 

surface water in the Everglades replenishes ground 

water that will be withdrawn later for domestic use 

from well fields to the east of the Everglades. In other 

areas, recharge contributes to high water tables and 

seepage problems for housing developments located 

just east of the Everglades. During wet periods in south 
4 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

0 5 10 MILES

0 5 10 KILOMETERS

80o15'

25o

30'

26o

00'

26o

30'

Miami

Lake

Okeechobee

Miami-Dade County

Everglades

National

Park

Everglades

agricultural

area WCA-1

3A-15

WCA3B

WCA-3A

WCA2B

WCA2A

Hendry County Collier County Monroe County

Biscayne Bay

Florida Bay

A'

B'

B

80o45'

Broward County

South Florida

ecosystem 

boundary

Atlantic Ocean

Palm Beach County

A

EVERGLADES NUTRIENT 

REMOVAL (ENR) PROJECT

STORMWATER TREATMENT 

AREA (STA)

WATER CONSERVATION 

AREA (WCA)

HYDROGEOLOGIC SECTION

RESEARCH SITES—Not shown

in figures 2 and 3

PRIMARY CANALS

A A'

3A-15

PB-00019

EXPLANATION

PB-00019

ATLANTIC

RIDGE

COASTAL

Figure 1. Central Everglades and adjoining areas, south Florida, showing locations of Water 

Conservation Areas (WCA), Everglades Nutrient Removal (ENR) project, and Stormwater Treatment 

Areas (STA). Southwestern coastal areas of Everglades in Monroe County are not shown.
Introduction 5

Florida, a large amount of surface water moves eastward through canals and discharges directly to the 

Atlantic Ocean.

Concern has been growing for many years in 

south Florida over the long-term decreases in surface 

water flowing through the Everglades, and the effects 

of these decreased flows on wildlife within Everglades 

National Park (ENP). Simultaneous to the decreasing 

surface flow, there has been increasing awareness of 

the deteriorating surface-water quality in WCAs and 

accompanying changes in the ecology of the wetlands. 

Ecological changes include disappearance of tree 

islands, proliferation of cattails, and loss of wading 

bird populations. In the past 20 years, these concerns 

have fueled wide-ranging discussions on how to 

improve water management in the Everglades in a way 

that would restore proper ecosystem function. In 2000, 

Congress approved a plan for restoration of the Everglades, referred to as the Comprehensive Everglades 

Restoration Plan or CERP. The overall goal of CERP is 

to restore some of the pre-drainage conditions, including the overall volume of surface flow, and characteristics of the depth, duration, and flow patterns of surface 

water. Restoration objectives also include reducing 

excessive inputs of dissolved nutrients and other constituents that could have deleterious effects on biogeochemical processes and ecological characteristics in 

the Everglades (McPherson and Halley, 1996; South 

Florida Water Management District, 1995; Stober and 

others, 1996; Gerould and Higer, 1995). 

Evaluating the success of ongoing management 

and restoration efforts depends on reliable hydrologic 

information, including a better understanding of interactions between surface and ground water. One of the 

major initiatives already underway is the extensive 

rerouting of surface-water inflows to WCAs so that 

water first passes through Stormwater Treatment Areas 

(STAs) to remove excess nutrients before water enters 

the WCAs. A concern regarding the function of STAs 

is their potential effect in mobilizing toxic forms of 

mercury. Addressing that concern required considerable effort on the part of the South Florida Water Management District and cooperating agencies (Florida 

Department of Environmental Protection, U.S. Geological Survey, and others). One difficulty in developing a 

reliable mercury mass balance for STAs is uncertainty 

about how mercury is affected by interactions between 

surface water and ground water. 

In an effort to learn more about interactions 

between ground water and surface water in the Everglades, the U.S. Geological Survey (USGS) and the 

South Florida Water Management District (SFWMD) 

developed an agreement to undertake a detailed study 

of interactions between surface water and ground water 

in selected areas of the north-central Everglades. The 

study would provide insight about the effect of interactions between ground water and surface water and 

chemical balances of STAs and WCAs. Investigations 

of surface and ground-water interactions were focused 

in two principal areas. The first area was the 3,815-acre 

Everglades Nutrient Removal Project (ENR), a prototype STA. The second area was Water Conservation 

Area 2A (WCA-2A), a 105,000-acre basin with a long 

history of ecological research on the changing character of the Everglades.

Purpose, Scope, and Source of Funding

The purpose of this report is to interpret results 

and develop final conclusions of an investigation of 

surface-water and ground-water interactions and their 

effect on the mercury budget in the north-central Everglades. The overall goals of the project were to: 

1.) quantify ground-water recharge and discharge 

in the ENR and WCA-2A, 

2.) determine relative importance of geologic, 

climatic, and water-resources management in 

affecting ground-water recharge and discharge, 

and 

3.) use improved estimates of ground-water 

recharge and discharge to develop accurate 

hydrologic budgets and chemical mass balances for mercury in the two areas.

A previous report (Harvey and others, 2000) contains 

detailed study methods and complete data sets covering 

all topics, including borehole drilling, geophysical 

measurements, sampling of ground-water geochemistry, and design and operation of shallow piezometers 

and seepage meters. The present report and the report 

by Harvey and others (2000) constitute the final deliverables for Cooperative agreement C-6661 between 

USGS and the South Florida Water Management District. Funding for the investigation also came from the 

USGS Place-Based Studies Program.
6 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

Hydrologic Setting: Characteristics of Preand Post-drainage Everglades

A thorough investigation of Everglades hydrology requires an understanding of the pre-drainage 

hydrologic system. The pre-drainage Everglades 

received water primarily from direct rainfall, periodic 

overflow from Lake Okeechobee, and runoff from surrounding pine flatwoods and other upland systems 

(Gleason and Stone, 1994). In addition, the slough systems of the Everglades probably received ground-water 

discharge and shallow subsurface runoff from the adjacent low-lying pinelands. 

The driving force for water flow in the Everglades is the water-surface slope, which is controlled 

by the regional topographic gradient. In the 

pre-drainage Everglades, the topographic gradient was 

a relatively consistent 2 inches per mile (in/mi), with 

only minor undulations of the natural landscape affecting water flow. Topography varied across the full 

(pre-drainage) width of the north-central Everglades 

(approximately 50 mi). On the western side of the 

Everglades, a major slough system was present that 

graded into a broad sawgrass plain in the central area, 

back into another major slough system on the east side. 

Microtopography in the sloughs consisted of alternating ridge and slough systems with typical spacing of 

approximately 0.5 mi.

Changes in topography (for example, because of 

subsidence or construction of levees) or water levels 

(because of canal drainage) easily perturb the direction 

of water flow in the Everglades. Canal construction and 

drainage began to modify water levels and topography 

substantially in the northern and north-central parts of 

the Everglades beginning about 1912. The initial effort 

was to construct four major north-south canals to drain 

water to the Atlantic Ocean. Early canal drainage in the 

Everglades Agricultural Area (EAA) led to excessive 

oxidation of the peat in the vast sawgrass plain and 

swamp forest directly south of Lake Okeechobee. 

Drainage and oxidation eventually caused between 3 

and 10 ft of subsidence in the agricultural area over the 

past century. Subsidence and continual pumping in the 

EAA to keep agricultural fields dry have had the effect 

of reversing the horizontal direction of ground-water 

flow in some areas of the Everglades. Where ground 

water once flowed toward the southeast, it presently 

flows toward the northwest (Miller, 1988). 

Drainage canals continue to be the primary 

water-management effort in the north-central Everglades. During excessively wet conditions, the major 

drainage canals shunt excess water from Lake 

Okeechobee or the EAA to the Atlantic Ocean (fig. 1). 

Under more typical wet-season conditions, drainage 

canals deliver and store water in the WCAs. East of the 

Everglades the canals have various functions, including 

drainage of the low-lying pinelands and aquifer 

recharge to balance losses by ground-water pumping. 

Interactions between surface flow in canals and ground 

water have been frequently investigated in south Florida (Miller, 1978: Chin, 1990; Genereux and Slater, 

1999; Nemeth and others, 2000, Bolster and others, 

2001). 

The conversion of wetlands to agriculture compressed the northern part of the Everglades to approximately one-third its pre-drainage width. By the 1950s, 

it was apparent that the canals were too effective in 

draining wetlands that were becoming increasingly 

important for sustaining water supply to the newly 

formed Everglades National Park, and to the growing 

population along the Florida Atlantic Coast. Construction of levees during the 1950s and early 1960s began 

to enclose the large basins now known as the WCAs. 

These areas are large levee-enclosed basins that 

encompass only the easternmost part of the 

pre-drainage system. WCAs were designed for multiple purposes, including storage for later delivery to 

Everglades National Park, and protection from flooding 

for the drained areas just outside the wetlands. The 

WCAs are all that remain of the north-central Everglades and their construction and management have 

had substantial effects on surface- and ground-water 

flow in Palm Beach and Broward Counties (fig. 1). 

Under pre-drainage conditions, surface flow in 

the Everglades was augmented by substantial shallow 

runoff from the surrounding uplands. Under water 

management, the water levels outside the WCAs normally are maintained at lower levels than inside the 

WCAs, which causes net recharge from surface water 

to ground water in the WCAs (Miller, 1988). Seepage 

losses resulting from flow of recharged water beneath 

levees represent an important component of water loss 

from the WCAs. Seepage appears to be greatest along 

the eastern and northwestern borders of the WCAs, 

where land is now being managed for a variety of uses, 

including agriculture, light industry, or suburban development (Miller, 1988). Water losses from the Everglades by seepage were large enough that they became 

obvious almost as soon as the levee-construction 

method was tested in the 1950s (U.S. Army Corps of 
Introduction 7

Engineers, 1952). Seepage at many of the Everglades 

levees began to be widely investigated beginning in the 

1960s (Klein and Sherwood, 1961; Swayze, 1988; 

Genereux and Guardiario, 1998; Nemeth and others, 

2000, Sonenshein, 2001). Although often less studied, 

seepage flow also occurs beneath the levees that separate WCAs (Harvey, 1996; Harvey and others, 2000). 

Because recharge now occurs at locations where formerly the Everglades gained water from shallow runoff 

and ground-water discharge, seepage losses have 

become one of the most important unintended side 

effects of water management in the north-central Everglades. 

Another factor associated with water management that may have affected surface-water and 

ground-water interactions is the increasing fluctuations 

of surface-water-levels in the WCAs compared with 

pre-drainage conditions. From the 30-year comparative 

simulations of the South Florida Water Management 

Model (SFWMM) and Natural System Model (NSM), 

surface-water-level fluctuations under pre-drainage 

conditions appear to be from 50 to 75 percent of 

present day fluctuations. As shown in the present study, 

the increased water-level fluctuations in WCAs drive 

recharge and discharge in interior areas of the wetlands 

far from levees.

Site Information

The study described within this report was conducted in the following area: Everglades Nutrient 

Removal Project (ENR), Water Conservation Area 2A 

or 2B (WCA-2A or 2B), or in Water Conservation Area 

3A (WCA-3A). Measurements were made at 17 sites in 

ENR, 7 sites in WCA-2A, and 1 site each in WCA-2B 

and WCA-3A (figs. 1, 2, and 3). Specific locations for 

study were chosen both to satisfy the need for broad 

spatial coverage, as well as to co-locate study activities 

with previous or ongoing ecological investigations. The 

most comprehensive sets of measurements were conducted in the ENR and in WCA-2A. Two or more seepage meters were used to measure vertical water fluxes 

across the sediment surface at all the interior wetland 

sites. Most of the sites (10 sites at the ENR and 7 sites 

at WCA-2A) also had one or more research wells 

emplaced in the Surficial aquifer underlying the Everglades, and a surface-water recorder. 

Everglades Nutrient Removal Project

Although the ENR is a large, constructed wetland (3,815 acres), it is relatively small compared to the 

WCAs that generally are more than 100,000 acres. The 

land encompassed by the ENR was formerly part of the 

historical Everglades. It was drained and farmed beginning in the mid-1900s up until construction of the ENR 

beginning in 1989. The purpose of the ENR was to test 

the capacity of a constructed wetland (with controlled 

hydrology, and managed aquatic and emergent wetland 

plants) to remove nutrients from agricultural drainage 

waters (fig. 2). The ENR project area is located on the 

western border of WCA-1, where water levels are 

maintained at high elevations. Located to the west of 

the ENR is the Everglades Agricultural Area (EAA), 

where land is drained to maintain a low water table for 

agriculture. Drainage is accomplished by a system of 

canals that transports the water southward into the 

WCAs. Guardo and Tomasello (1995) and Guardo 

(1999) modeled surface-water hydrodynamics and calculated water-balance fluxes and hydrologic residence 

times for the ENR project. Preliminary work showed 

that both ground-water recharge and discharge are 

important components of ground-water interactions at 

ENR (Harvey, 1996). This is important because of the 

overall steep gradient and step-changes in hydraulic 

head over a distance of 5–10 mi from WCA-1 (where 

water levels are approximately 14 ft above sea level) to 

the EAA (where water levels are approximately 8 ft 

above sea level) (fig. 2). Abtew and Mullen (1997) 

developed initial estimates of net seepage in ENR as 

part of determining a project-wide water balance. 

Hydrogeologic investigations within the ENR project 

included geotechnical investigations for levee and 

pump station footings (Burns and McDonnell, 1991; 

Hutcheon Engineers, 1996) and two studies of seepage 

under the levees (Hutcheon Engineers, 1996; Rohrer, 

1999). 

Water Conservation Area 2A 

WCA-2A is located to the south-southeast of 

ENR (fig. 1). Similar to ENR, WCA-2A shares a 

boundary with WCA-1 and is affected by the high 

water levels that are maintained in WCA-1. WCA-2A 

is 25 times larger in area (105,000 acres) than ENR, 

and therefore, is less likely to be affected by 

ground-water interactions that result from levee underflow. The construction of levees that eventually surrounded WCA-2A began in about 1920. By about 
8 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

P01

P13

P12

P10

P09 P09

P14

P08

G259

G259

ENR004

ENR012-

INLET

ENR0011-

CANAL 

RETURN

ENR002-

OUTLET

P07

P06

P05

P04

P03

P02

M408

M408

M204

M203

M205

M201(N + S)

M206

M207

MP2

P11

MP1

M401

M303 M303

M103

M102

M104(N + S)

M105 M106

MOP1

MOP2

MP3

MOP3

SEEPAGE METER AND WELL LOCATION 

AND IDENTIFIER

EXPLANATION

SEEPAGE METER LOCATION AND IDENTIFIER

WELL LOCATION AND IDENTIFIER

SURFACE-WATER SAMPLING LOCATION

AND IDENTIFIER

HYDROGEOLOGIC SECTION AND

RESEARCH TRANSECT

CANAL

LEVEES

EAA

EAA

CELL 1

CELL 

4

CELL 2

Seepage Canal

Seepage Canal

Seepage Canal

CELL 3

WCA-1

N

0

0

3,000 FEET

500 1,000 METERS

A A'

A'

A

Miami

3A-15

WCA3B

WCA-3A

WCA2B

WCA2A

WCA-1

Figure 2. Location of study sites in Everglades Nutrient Removal (ENR) project, north-central Everglades, south Florida. ENR 

is bordered by Water Conservation Area 1 (WCA-1) to the east and the Everglades Agricultural Area (EAA) to the west and 

north. A seepage canal that collects ground-water discharge separates ENR from the EAA (thin arrows show direction of flow). 

Note that true locations for sites MOP1, MOP2, and MOP3 are further from ENR than indicated, as shown by thick arrows.
Introduction 9

27

S10C

S10C

Hillsboro Canal

F1

F4

E4

U3

U3

U1

WCA-2A

WCA-2B

2B-S

2B-S

REMNANT TREE ISLANDS

AREA OF CATTAIL INFESTATION

EAA WCA-1

Light industry and suburban development

WCA-3

0

0

1 2 3 4

5

5 10 MILES

10 KILOMETERS

27

EXPLANATION

SEEPAGE METER AND WELL LOCATION 

AND IDENTIFIER

SEEPAGE METER LOCATION AND 

IDENTIFIER

WELL AND IDENTIFIER AND 

WATER-MANAGEMENT SPILLWAY 

LOCATION

E1

N

B

B'

EAST TRANSECT

WEST TRANSECT

HYDROGEOLOGIC SECTION

RESEARCH TRANSECT

LEVEES AND ROADS

CANALS OR AIRBOAT TRAILS

B B'

3A-15

WCA3B

WCA-3A

WCA

2B

WCA2A

WCA-1

Miami

WCA2B

Figure 3. Location of study sites in Water Conservation Area 2 (WCA-2), north-central Everglades, south Florida. WCA-2 

shares borders with Water Conservation Area 1 (WCA-1) to the northeast, the Everglades Agricultural Area (EAA) to the 

northwest, WCA-3A to the southwest, and areas of light industry and suburban development to the east.

Approximate locations of airboat trails are shown.
10 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

1963, WCA-2A was completely compartmentalized by 

levees and canals, including the split between 

WCA-2A and 2B. Since about 1975, researchers have 

investigated the ecology of WCA-2A, documenting, 

for example, the loss of tree islands and a transition 

from a sawgrass wetland to one affected by extensive 

cattail growth in some areas. Excess nutrients from 

agricultural runoff, and multi-year droughts and flooding have been investigated previously to better understand those ecological changes. The effect of 

interactions between ground water and surface water 

has not been investigated in WCA-2A. Therefore, 

studying the hydrology of WCA-2A is an excellent 

complement to similar studies in ENR, because of the 

much larger area and much longer history at WCA-2A 

of nutrient pollution (Urban and others, 1993; Jensen 

and others, 1995). 

The wetlands in the vicinity of ENR were converted to agriculture early in the 1900s, and then reconstructed as a wetland in 1993. In contrast, WCA-2A 

remained more in a natural state, without the direct 

effects of drainage, ditching, grading, or farming. 

WCA-2A also has not been affected by blasting and 

excavation of extensive irrigation canals, nor by extensive peat shrinkage and oxidation, as is the case at 

ENR. The engineered land slope of ENR is approximately three times as steep in the direction of flow 

compared with the natural slope in WCA-2A (1 x 10-4

at ENR compared with 3 x 10-5 at WCA-2A). Average 

water-level differences when compared with WCA-1 

were similar at the two study areas, 2.4 and 2.8 ft at 

ENR and WCA-2A, respectively. However, variability 

in water-level differences was greater at WCA-2A 

compared with ENR. Expressed as a coefficient of variation, the temporal variability of water-level differences about the mean was 50 percent at WCA-2A and 

18 percent at ENR.

Unlike ENR, where an attempt is made to maintain relatively stable operating water levels, WCA-2A 

has large (up to 4 ft) fluctuations in water levels lasting 

from weeks to months. These water-level fluctuations 

result, in part, from the operation of control structures 

that release water from WCA-1, and partly from natural processes such as rainfall and evapotranspiration. 

Another important feature affecting water flow at 

WCA-2A is a berm on the tailwater side of the Hillsboro canal (fig. 3). Water in the canal flows to the east 

at times of low water before water enters the wetland. 

However, when large releases from the S10C spillway 

occur, the high water in the tailwater canal overtops the 

berm and moves directly into the wetland in a southwestern direction.

 Acknowledgments

The authors are indebted especially to the following individuals whose help was critical to project 

success: Gene Shinn, Chris Reich, Don Hickey, David 

Krabbenhoft, Bill Orem, Jonah Jackson, Eric Nemeth, 

Jessica Thomas, Brent Banks and Aaron Higer of the 

USGS; and Larry Fink, Carl Miles, Pete Rawlik, Darren Rumbold, Wossenu Abtew, Kevin Rohrer, Jim Vincent, Doug Ellington, Pete Dauenhauer, Bruce Webb, 

Stan Jones, Paul McGinnes, Mark Hummel, Ben Harkenson, Sharon Niemczyk, Anne Shoffner, Jennifer 

Cornwell, Emily Hopkins, Heidi Bazell, Angela Chong 

and John Lukasiewicz of SFWMD.

HYDROGEOLOGY OF THE 

NORTH-CENTRAL EVERGLADES

The Surficial aquifer is a principal source of 

fresh drinking water in south Florida. Sediments within 

the aquifer were deposited during the middle Pleistocene epoch and range in age from 1.8 million years 

before present (BP) to 13,000 years BP (Perkins, 

1977). These Surficial aquifer sediments are composed 

mainly of shallow water marine facies, including coral 

limestones, beach and offshore sandbar complexes, 

lagoonal limestones, and an oolitic ridge along the 

coast of Miami (Perkins, 1977; Hoffmeister, 1974). 

The Surficial aquifer includes the highly transmissive 

Biscayne aquifer, which underlies Miami-Dade, Broward and eastern Palm Beach Counties. The Biscayne 

aquifer is thickest beneath the Atlantic coastal ridge to 

the east of the Everglades, and it thins from east to 

west, disappearing beneath the north-central Everglades. Aquifers to the west of the Biscayne and 

beneath the Everglades generally have been ignored as 

potential sources of ground water, both because of the 

lower transmissivities (Fish, 1988) and because of the 

higher total dissolved solids in ground water beneath 

the Everglades (Howie, 1987; Miller, 1988).

Except for a few studies mentioned above, there 

is little comprehensive information available about the 

hydrogeology beneath the north-central Everglades. 

Previous site-specific investigations included various 
Hydrogeology of the North-central Everglades 11

studies at ENR. A study by Hutcheon Engineers at 

ENR was associated closely with engineering design 

projects for STAs, but provided relatively little in the 

way of lithologic or hydrogeologic data. Rohrer (1999) 

broadly characterized the hydrogeology of the ENR 

from boreholes placed on ENR levees and focused 

attention on the near-surface layer of the Surficial aquifer. A goal of the present investigation was to characterize in detail the geology and hydraulic properties of 

the Surficial aquifer at new drilling sites in ENR and 

WCA-2A. Of particular importance was identifying 

layers of relatively high or low hydraulic conductivity 

and their relation to physical properties and geologic 

classification of aquifer layers. 

Geologic Setting of Study Areas

In Palm Beach County, the Surficial aquifer 

extends from near ground surface to depths in excess of 

165 ft below land surface (bls). Deposition of the shallow water marine units in south Florida was regulated 

by eustatic sea-level fluctuations associated with glacial and interglacial stages of the Pleistocene (Perkins, 

1977). Climatic instability, and glacial retreat and 

advance during the Pleistocene caused sea level to 

repeatedly recede and advance over large areas of south 

Florida. Many of the sediments appear lithologically 

similar but often represent different depositional events 

(Perkins, 1977). Sediment deposition during this period 

resulted from dune building, near-shore progradation of 

the coastline, and soil development. 

The Quaternary and Tertiary deposits of south 

Florida are dominated by shallow water marine carbonates and siliciclastic materials deposited as part of reef 

systems, tidal flats, and coastal barrier/bar complexes. 

Interbedded within these units are indications of subaerial exposure, including paleosols and freshwater 

limestones (Perkins, 1977). The stratigraphy of these 

Quaternary sediments is described briefly below following from Parker and others (1955), Brooks (1968), 

and Perkins (1977). 

Lake Flirt Marl Formation

The Lake Flirt Marl is Pleistocene in age. 

According to Schroeder and others (1958), it is up to 6 

ft thick, relatively impermeable, and composed primarily of calcareous mud with some areas of dense limestone. The Lake Flirt Marl underlies the organic (peat) 

soils throughout much of the Everglades and coastal 

marshes (Parker and others, 1955). Reese and Cunningham (2000) found the Lake Flirt Marl in southwestern 

Palm Beach County to be composed of silty marl or 

quartz sand with a marl matrix. The areal distribution 

of the Lake Flirt Marl and lithographic textures are 

consistent with deposition in freshwater lakes (Reese 

and Cunningham, 2000).

Fort Thompson Formation

The Fort Thompson Formation consists of alternating beds of marine, brackish and freshwater limestones similar to those found at the type locale along 

the Caloosahatchee River. The formation overlies the 

Caloosahatchee Marl and is Pleistocene in age (Parker 

and Cooke, 1944; Mitterer, 1975). The thickness of the 

formation is about 40 ft in eastern Miami-Dade, Broward and Palm Beach Counties (fig. 1) where it makes 

up the highly productive zone of the Biscayne aquifer 

(Fish and Stewart, 1991). This formation covers the 

greatest geographical expanse of all Quaternary formations in southern Florida. The depositional environment 

of this formation can be linked to late Quaternary sea 

level fluctuations. The discontinuity surfaces of the 

Fort Thompson Formation can include dense, 

well-indurated laminated crusts (Giddings, 1999). Core 

samples collected during this study are primarily from 

the Fort Thompson Formation. 

Anastasia Formation

The Anastasia Formation consists of alternating 

offshore bar, beach ridge, and dune system deposits 

and may be at least 39 ft thick along the coast (Perkins, 

1977). The age of the formation is estimated to be late 

Pleistocene and is considered to be contemporaneous 

with the Fort Thompson Formation (Parker and others, 

1955). This formation can be divided into two distinct 

facies: a coquina facies and a shell rock facies (Lovejoy, 1983). The coquina facies represents a high-energy 

environment typical of an offshore bar complex and 

generally is aligned with the present coastline. The 

shell rock facies is found behind the coquina facies, 

between the offshore bar complex and the shoreline. 

The shell rock facies is characterized by a diverse molluscan fauna with minimal damage to the fossils in a 

fine-grained quartz matrix, suggesting a shallow bay 

origin (Lovejoy, 1983). The Anastasia Formation is 

seen in areas east of the study sites.
12 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

Caloosahatchee (Marl) Formation 

The transition between the Tertiary Pliocene and 

the Quaternary Pleistocene occurs in the upper members of the Caloosahatchee Formation (Enos, 1977). 

The Caloosahatchee Marl is composed of sandy marl, 

clay, and silt-size particles interbedded with shell beds 

(Land and others, 1973). Parker and others (1955) 

describe the Caloosahatchee Marl as sandy marl, clay, 

and silt with interbedded layers of sand and shell beds. 

Hydraulic conductivities range from 1 ft/d to 10 ft/d 

(Scott, 1977). Parker and others (1955) state that in 

many places the Caloosahatchee Marl is thinner 

because of erosion and solutioning, whereas in some 

places it is absent or appears only in isolated patches. 

Tamiami Formation

The Tamiami Formation underlies the Caloosahatchee Formation and generally is described as cream, 

white, and greenish-gray clayey marl, silty and shelly 

sands, and shell marl that may be hardened locally into 

limestone (Schroeder and others, 1958). In Broward 

County, the formation grades from hard, sandy limestone interbedded with calcareous sandstone to green 

marly silt (Parker and others, 1955). Fish (1988) 

describes the Tamiami Formation in Broward County 

as greenish clay marl, silty and shelly marl with calcareous marl, locally hardened to impure limestone. The 

Tamiami Formation is estimated to be about 6 million 

years before present (Hoffmeister, 1974). Parts of the 

upper Tamiami are cavity-riddled and hydraulically 

similar to the Anastasia Formation (Russell and Wexler, 1993). In this study, the formation was found only 

at ENR site MP3-A.

Hawthorn Group 

The Hawthorn Group underlies the Tamiami Formation and is described by Scott (1988) as highly complex. In south Florida, the upper layer of the Hawthorn 

Group consists of phosphatic siliciclastic sediments of 

fine- to coarse-grained quartz sand, quartz silt, and clay 

minerals (Scott, 1992). According to Miller (1987), 

impermeable and semi-permeable marls (calcareous 

clays) of the Hawthorn Group form the base of the 

Surficial aquifer. Because of the phosphatic sediments, 

the Hawthorn Group in south Florida generally has a 

higher gamma-ray signature than underlying or overlying layers (Scott, 1988). In this study, these sediments 

were seen only at the bottom of the borehole at site 

MP3-A. 

A summary of the chronostratigraphy of the 

Surficial aquifer is shown in table 1. The age of each 

formation, its average thickness, primary lithology, and 

Q unit designations are shown in table 2.

Subaerial Exposure and Weathering 

Affecting Aquifer Hydraulic Properties

Subaerial exposure and weathering is one of the 

most important factors affecting the hydraulic properties of Surficial aquifer sediments (Perkins, 1977). 

Exposure to the atmosphere and chemical weathering 

of the exposed rock by dissolution resulted in formation of discontinuous bands of dense caliche-type 

crusts, paleosols, freshwater limestone, and laminated 

crusts (Perkins, 1977; Beach, 1982). Identifying subaerial exposure surfaces is an accepted means of identifying the boundaries between different stratigraphic 

subdivisions of the Quaternary units. In practice, these 

Age (years before present)

Q Unit Mitterer, 1974 Giddings, 1999 Formations in Study Areas

Q5 125,000 115,000 Anastasia and Fort Thompson

Q4 180,000 200,000 Anastasia and Fort Thompson

Q3 236,000 350,000 Anastasia and Fort Thompson

Q2 324,000 Estimated 400,000 Anastasia and Fort Thompson

Q1 – Estimated 800,000 Anastasia and Fort Thompson

Table 1. Chronostratigraphy of the Quaternary sediments underlying the 

north-central Everglades, south Florida

[–, not applicable]
Hydrogeology of the North-central Everglades 13

subaerial exposure surfaces are difficult to identify 

because their thicknesses may be appreciably less than 

3 ft (Perkins, 1977). Detailed continuous geologic logs 

are, therefore, the only reliable means to identify these 

surfaces. 

Perkins (1977) differentiated the depositional 

framework of south Florida into five distinct marine 

units punctuated by episodes of subaerial exposure. 

The estimated age of the Q5 unit was assigned based 

upon radiometric dating of the Key Largo Limestone 

and Miami Limestone (Osmond and others, 1965; 

Broecker and Thurber, 1965). Dates for the remaining 

four Quaternary sediments were estimated from amino 

acid racemization results from the Fort Thompson Formation (Mitterer, 1974, 1975; Giddings, 1999). Drilling at ENR site MP3 fully penetrated this complete 

Quaternary sequence. The chronostratigraphic details 

of these units are shown in table 1.

Another potentially important type of weathering 

surface was described by Krupa (1999). Krupa (1999) 

established the presence of radioactive crusts in the 

sediments that he referred to as Secondary Depositional Crusts (SDC). These SDCs are less distinctive 

than the Q unit indicators and they usually are not visible. They typically are associated with increases in the 

natural gamma measurements at a depth just below a 

subaerial exposure surface. Evidence indicates the 

SDCs are a function of the paleo ground-water interface location (Krupa, 1999). Krupa proposed that the 

crusts are formed by residual leachate deposited at the 

ground-water interface as a result of downward percolation of rainfall through the vadose zone. The SDCs 

were found to exhibit levels of elements such as Ca, 

Mg, Al, Sr, Fe, U, Th, and K above the natural background level of the area. Krupa (1999) suggested those 

elements could be important in the hydraulic properties 

of the rock because of the effects of secondary cementation within the pore network.

Lithology of the Surficial Aquifer

To characterize the lithology, the deepest borehole at each levee site was sampled continuously from 

top to bottom. Split-spoon sampling was used for 

unconsolidated sediments, and conventional or wireline 

coring was used for consolidated sediments. Detailed 

sampling methods are described in Harvey and others 

(2000). Visual observations of split-spoon and core 

samples indicate that the aquifer generally is composed 

of sand overlying limestone with interbedded sand 

stringers in the top third. The bottom two-thirds of the 

aquifer is composed of sands of varying grain size. A 

detailed lithologic description of these core and sand 

samples and the grain-size analyses are given in Harvey 

and others (2000). Lithostratigraphic interpretations for 

two sections (locations shown in figure 1) are provided 

in figures 4 and 5. 

The relative percentage of sampling by split 

spoon or coring provides an indication of site-to-site 

lithologic variation. Sites MP1, MP3, and MOP1 in the 

northern part of ENR (fig. 2) were sampled by coring 

for more than 80 percent of their lengths. The high percentage of coring indicates that sediments are mostly 

consolidated in the upper 100 ft at those sites. In contrast, two boreholes south of ENR, sites MOP2 and 

Layer 

Average 

thickness 

(ft)

Primary 

lithology

Common

formation/group name Geologic time scale (epochs)

Perkins 

Q1 – Q5

Series

Estimated age of

formation

(years BP)

1a 2.3 Peat Recent Holocene – <5000

1b 1.0 Marly sand/sand Lake Flirt Marl Transition to Pleistocene – 20,000

2 2.0 Sand Upper Fort Thompson Pleistocene Q5 – Q4 125,000 to 210,000

3 27.0 Limestone with 

sand stringers Fort Thompson Pleistocene Q3 – Q2 270,000 to 500,000

4 91.0 Sand

Fort Thompson

Caloosahatchee Marl

Tamiami

Lower Pleistocene

Upper Pliocene

Q1

–

–

500,000 to 1,600,000

1,600,00 to 2,800,000

5 >93.0 Fine sand Tamiami/Hawthorn Pliocene to Miocene – 2,800,000 to 23,700,000

Table 2. Summary of hydrogeologic, lithostratigraphic, and chronostratigraphic units in Everglades Nutrient Removal (ENR) 

project and Water Conservation Area 2A (WCA-2A), north-central Everglades, south Florida

[BP, before present; <, less than; >, greater than; –, not applicable]
14 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

DCB

A

PZP

C

BA

PZP

C DBA

LAYER 5-TAMIAMI FORMATION/HAWTHORN GROUP

LAYER 4-LOWER FORT THOMPSON/CALOOSAHATCHEE MARL/UPPER TAMIAMI

LAYER 3-FORT THOMPSON

LAYER 2-UPPER FORT THOMPSON

MP-3

CLUSTER

MP-203 MP-103

MP-2

CLUSTER

MP-1 

CLUSTER

Seepage

Canal

ENR

A'

EAST

WCA-1

A

WEST

EAA

D WELL SCREEN LOCATION AND DESIGNATION—Well number 

is above cluster; A is deepest screen and D is the shallowest. See figures 1 and 2 for location map of cross section

EXPLANATION

30

20

10

0

80

60

0

40

20

1,500 4,500 FEET 3,000

500 1,500 METERS 1,000

LEVEE

LAYER 1 A/B - RECENT/LAKE FLIRT MARL

L-7

Canal

Figure 4. Lithostratigraphic cross section with formation names, Everglades Nutrient Removal (ENR) project, north-central Everglades, south Florida.
Hydrogeology of the North-central Everglades 15

15

10

5

0

30

0

20

10

2 MILES 1 0

1 0 4 KILOMETERS 2 3

METERS

FEET

GW-4GW-3

GW-4GW-3

GW-4GW-3

CBA

LAYER 4-TAMIAMI

LAYER 3-FORT THOMPSON

GW-3 WELL SCREEN LOCATION AND DESIGNATION—Well number 

is above cluster; A is deepest screen and C is the shallowest; GW-3 deep, GW-4 shallow. See figures 1 and 3 for location of cross section

EXPLANATION

LAYER 2-UPPER FORT THOMPSON

LAYER 1 A/B-RECENT/LAKE FLIRT MARL

U-3 

WELL 

CLUSTER

F-4 

WELL 

CLUSTER

F-1

WELL 

CLUSTER

Seepage

Canal

WCA-2A WCA-1

B'

NORTHEAST

WCA-1

B

SOUTHWEST

WCA-3 LEVEE AND S10C

WELL CLUSTER

Hillsboro Canal

Figure 5. Lithostratigraphic cross section with formation names, Water Conservation Area 2A (WCA-2A), north-central Everglades, south Florida. Note that this section shows only the top half (approximately 100 feet) of the Surficial aquifer in this part of the study area.
16 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

-85

-75

-65

-55

-45

-35

-25

-15

-5

5

15

25

0 120 20 40 60 80 100

BLOW COUNTS, IN UNCORRECTED N VALUES 

ELEVATION, IN FEET RELATIVE TO 1929 NGVD

S10C

MOP2

S10C, were sampled by standard penetration tests 

(SPT) for at least 75 percent of their lengths, indicating 

mostly unconsolidated sediments (figs. 2 and 3). Site 

MP2 in central ENR was sampled with roughly an 

equal combination of SPT and coring. 

Additional information about sediment density is 

found in "blow count" summaries from the SPT sampling. Blow count refers to the number of times per 

foot that the split-spoon sampler was hammered by the 

drilling rig. Harder formations require more "blows" 

than softer formations to penetrate 1 ft. Blow counts 

are informative regarding relative sediment packing 

and density, as well as the presence or absence of caverns or solution features. Blow counts, therefore, are 

best used to compare boreholes between sites of similar 

lithology. Holtz and Kovacs (1981) classify sediment 

density based on N, the total of the uncorrected blow 

counts for the second and third 6-in segments of a standard 24-in sample. The number of samples at three 

sites (MP2, MOP2, S10C) separated into ranges of N is 

shown in table 3.

The results in table 3 indicate that the unconsolidated sediments at WCA-2A are less dense than those 

at ENR. Blow counts for sites S10C (in WCA-2A) and 

MOP2 (in ENR) are plotted with sample elevation in 

figure 6. Gaps indicate elevations where SPT sampling 

was not possible. Whereas the overall range of N values 

at the two boreholes is similar, variations are apparent. 

Near the top of the aquifer (5 ft NGVD) N is equal to 3 

at S10C compared with 50 at MOP2. At –46 to –56 ft 

NGVD, the N values at MOP2 also are higher than 

those of S10C. The denser sands at ENR could be a 

result of differences in grain-size sorting or greater sediment compaction. 

Grain-size analysis is a tool that is used for interpretation of depositional environments (Boggs, 1995) 

and for determination of the sediment hydraulic conductivity. To quantify variations in sample grain size, 

sorting, and sediment classification, samples were 

sieved according to ASTM Standard No. D-422 (American Society of Testing and Materials, 1991). The 

results were analyzed to determine the d10, d17, d20 and 

d60; that is, the grain size for which the indicated fraction of sample (10, 17, 20, 60) is smaller. This information then was used to estimate hydraulic conductivity 

(Vukovic and Soro, 1992) and to determine the degree 

Blows (N values) 0 - 4

Very loose

5 - 10 

Loose

11 - 30 

Medium

31 - 50 

Dense

51 or more

Very dense

Number of samples - MP2 0 0 1 7 11

Number of samples - MOP2 2 1 12 11 15

Number of samples - S10C 5 5 15 7 5

Table 3. Number of standard penetration test blows at three sites in the Everglades Nutrient 

Removal (ENR) project and Water Conservation Area 2A (WCA-2A), north-central 

Everglades, south Florida 

Figure 6. Standard penetration test N values from the 

Surficial aquifer at sites S10C (WCA-2A) and MOP2 (ENR), 

north-central Everglades, south Florida.
Hydrogeology of the North-central Everglades 17

of sorting of the sand grains. The uniformity coefficient 

(d60/d10) and the gradation coefficient [(d30)2/(d10*d60)] 

were the selected measures of sorting (Kasenow, 1994). 

Whereas only selected data are given as part of this discussion, all sieve analysis data from this investigation 

are available in Harvey and others (2000). 

The sorting of sediment grains may reflect depositional conditions and processes (Boggs, 1995). Sediments underlying the ENR generally were well sorted 

with some poorly sorted layers near the top of the Surficial aquifer (Harvey and others, 2000). Two ENR 

sites, MP1 and MP2, demonstrate well-sorted sediments throughout the depth of the boreholes (fig. 7). 

Several boreholes at the northern edges of the ENR 

study area had more poorly sorted sands in the upper 

portions of the Surficial aquifer. For example, poorly 

sorted sediments were found in the upper layers at MP3 

(+10 to +2 ft NGVD), grading to mostly well sorted 

sediments below a limestone layer. MOP1 had poorly 

sorted sands at 6 to –4 ft NGVD. Well sorted sediments 

with a poorly sorted layer found at –18 to –22 ft NGVD 

were the primary sediments found at MOP2, located 

just south of ENR.

Sediment analyses of WCA-2A sediments are 

limited to the S10C borehole. The other sites were 

drilled using a modified wireline core barrel system 

that does not collect unconsolidated sediments (MacIntyre, 1975). Sands at S10C primarily were poorly 

sorted and coarser in grain size than the sands at ENR 

(table 4). The exception to poor sorting of sediments at 

S10C sediments is the interval between –50 to –60 ft 

NGVD, where the sediments are well sorted (fig. 7). 

These variations are reflective of the changing depositional processes in a marginal-marine environment.

Lithologic Comparison Between WCA-2A 

and ENR

At S10C, sediments generally are poorly sorted 

and the d20 in the complete borehole (Fort Thompson 

and Tamiami Formations) ranges from 0.12 mm (very 

fine) to 0.46 mm (medium). Various cycles of coarsening upwards and fining upwards appear in the S10C 

profile, which indicates periods of sea-level transgression and regression (fig. 8). In contrast, sediments from 

ENR site MOP2 have a d20 that ranges from 0.11 mm 

(very fine) to 0.25 mm (fine/medium) and primarily are 

well sorted. The ENR profiles show much smaller 

changes with elevations (fig. 7). The geometric mean of 

d10, d17, d20, and d60 values and the uniformity coefficient (Cu) are all larger for WCA-2A compared with 

the ENR boreholes. Possible sub-areal caps and caliche 

crusts are present in the limestone core samples at varying depths and are described in Harvey and others 

(2000, p. 94 and 98).

These sedimentary differences support the theory 

that the depositional environments at WCA-2A were 

different than those at the ENR (Enos, 1977). This difference probably resulted because the ENR is located 

about 20 mi north of and slightly west of WCA-2A. 

Whereas both sites have similar land-surface elevations, WCA-2A is closer to the current and 

paleo-shorelines of peninsular Florida and would have 

been flooded earlier, longer, and possibly more frequently by sea-level transgressions and regressions. 

Because of the changing sea levels, the chemical composition of the water within the aquifer and the depositional processes varied. Future work could involve 

correlating the sub-aereal caps and caliche crusts and 

the effect of these sedimentary structures on 

ground-water quality.

Site

MP1 MP2 MP3 MOP1 MOP2 S10C

Geometric Mean

Gradation coefficient (Cc) 0.93 0.09 0.80 0.76 0.85 0.52

Uniformity coefficient (Cu) 2.1 3.7 4.5 3.6 3.2 10.9

Table 4. Mean uniformity and gradation coefficients at six boreholes in Everglades Nutrient 

Removal (ENR) project and Water Conservation Area 2A (WCA-2A), north-central Everglades, 

south Florida
18 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

0 6 12 18 24 30 36 42 48 54

Cu UNIFORMITY COEFFICIENT 

ELEVATION, IN FEET RELATIVE TO 1929 NGVD

Layer 3

47 ft/d

208 ft/d

130 ft/d 

Layer 1

Layer 2

Layer 4

Well

sorted 

Poorly 

sorted

47 ft/d DRAWDOWN TEST RESULTS

EXPLANATION

Sand

Limestone-Sand

Sand

Levee fill-sand

(B)

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

0 15 20 25 30 35 40 45 50 55 5 10

 

68 ft/d

50 ft/d

11 ft/d

ELEVATION, IN FEET RELATIVE TO 1929 NGVD

Well

sorted 

Poorly 

sorted

Cu UNIFORMITY COEFFICIENT 

Layer 3

Layer 4

Limestone-Sand

Sand

(A)

Figure 7. Uniformity coefficients, hydraulic conductivities from drawdown tests, and generalized lithology from the 

Surficial aquifer at sites MP2 (ENR) (A) and S10C (WCA-2A) (B), north-central Everglades, south Florida.
Hydrogeology of the North-central Everglades 19

-85

-75

-65

-55

-45

-35

-25

-15

-5

5

15

25

0 0.2 0.4 0.6 0.8 1

d20, IN MILLIMETERS

ELEVATION, IN FEET RELATIVE TO 1929 NGVD

S10C

MOP2

Geophysical Logging Results and 

Correlations

The deepest boreholes at the levee sites were 

logged with geophysical probes prior to casing installation. Whereas the underlying lithology cannot be determined solely by geophysical logs, the general 

characteristics of lithology and water content can be 

summarized (Boggs, 1995). The log traces of each 

borehole and all geophysical data are summarized 

graphically on north-south and east-west cross-sections 

and shown by Harvey and others (2000). The standard 

suite of geophysical logs collected during this investigation included caliper, natural gamma ray, neutron, 

and lateral logs. This report mainly discusses the caliper, gamma ray, and neutron logs because these tools 

were the most responsive during the logging operations 

and indicate intervals of lithologic change. 

Caliper Logging

Caliper traces of ENR wells consistently showed 

evidence of washouts throughout the length of the 

boreholes. Washouts often occur where there are heaving sands because of excessive hydraulic pressures, 

changes in lithology, solution holes, or a lack of sediment compaction. Washouts were most extensive at 

sites MP2 and MOP2, and were present in both limestone layers and well-sorted sand layers. These washouts probably indicate layers with varying degrees of 

cementation or compaction. The largest washout was 

seen at the MP3-A borehole at an elevation of –13 ft 

NGVD and extended 16 in from the center of the borehole. This elevation coincides with a lithologic change 

from sandy limestone to well-sorted sand. The caliper 

log from borehole S10C showed minimal evidence of 

washouts. 

Natural Gamma Ray Logging

Natural gamma ray logs traditionally have been 

used in Florida to correlate formations regionally and 

to identify clays or formations with natural radioactivity, such as the Hawthorn Group (Scott, 1988). Recent 

work in Florida by Krupa (1999) and Cunningham 

(1998) indicates that natural gamma ray responses 

from sediments in the Surficial aquifer do not always 

coincide with the phosphatic sediments. 

In the ENR, the natural gamma ray logs have 

peaks in layers from 0 to –25 ft NGVD at each site. 

However, only sites MP1-A, MOP1, MOP2, and 

MOP3 show natural gamma ray peaks in the interval 

between –25 to –40 ft NGVD. At greater depths, the 

logs generally are featureless, with a few smaller peaks. 

Further south at S10C, the natural gamma ray peaks 

occur below a depth of –35 ft NGVD, as seen in Harvey and others (2000, p. 118 and 121).

There is limited geophysical information surrounding WCA-2A and data are not available within 

WCA-2A; therefore, an available gamma ray log from 

well PB-00019 (Swayze, 1988) was used. This site is 

located 6 mi southeast of the S10C site. 

Two correlations of natural gamma ray logs were 

prepared. The correlation of the west – east ENR 

transect is shown in Harvey and others (2000, 

p. 120-129). The gamma ray correlations along the 

north-south ENR/WCA-2A transect are shown in figure 9. Whereas the natural gamma ray logs at S10C and 

PB-00019 show only minor inflections to a depth of 

–30 ft NGVD, the ENR traces show appreciable peaks 

on the gamma logs to a depth of about –35 ft NGVD. 

The natural gamma ray peaks below –35 ft 

NGVD are similar in both the ENR and WCA-2A 

(fig. 9). The sediment analysis did not indicate a lithologic explanation for the peaks, such as clays or phosFigure 8. Grain size from the Surficial aquifer at site 

S10C (WCA-2A) and site MOP2 (ENR), north-central 

Everglades, south Florida.
20 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

MILES

0 5 10 15 20 25 30 35 40

-75

-50

-25

0

0 220

MOP1-A

0 220

MP1-A

0 220

MOP2-A

0 320

S10C

0 150

PB00019

30 35 40 45 50 55

KILOMETERS

0 5 10 15 20 25 60

COUNTS PER SECOND COUNTS PER SECOND

DEPTH , IN FEET RELATIVE TO 1929 NGVD

INFERRED LOCATIONS 

OF SDC CRUSTS

EXACT LOCATION 

OF INTERFACE 

UNKNOWN

ORIGINAL SEDIMENTS 

WERE ERODED AWAY, 

NEWER SEDIMENTS

WERE THEN 

DEPOSITED 

(POORLY SORTED

SANDS MEDIUM 

GRAINS)

Figure 9. North (left) to south (right) changes in borehole gamma counts, showing inferred layers of secondary depositional 

crusts (SDC) and southern zone of erosion and deposition of poorly sorted sands with medium grain size, north-central 

Everglades, south Florida.
Hydrogeology of the North-central Everglades 21

phatic deposits. Therefore, they may be SDCs or 

erosional surfaces, which potentially can restrict vertical and lateral ground-water flow within the Surficial 

aquifer. Additional research would be needed to evaluate the effect of SDCs on water quality and flow.

 It is likely that SDCs were formed over thousands of years during the normal fluctuations of the 

ground water within the vadose zone in the sediments. 

Because S10C is closer to the Atlantic Ocean, it is 

more likely affected by tidal and nearshore processes. 

Thus, SDCs probably formed within both sites but subsequently were eroded away at the S10C site. After the 

erosion, poorly sorted sediments then were deposited 

over the area at S01C. 

Neutron Logging

The neutron log is a hydrogen detector and measures amount of hydrogen within the sediment pore 

space; therefore, it is an indicator of porosity. Elevated 

hydrogen content above the natural background can be 

attributed to lower porosity within the sediments. The 

neutron tool utilized in this investigation had not been 

calibrated recently. Therefore, the neutron log traces 

only are used to describe general characteristics and 

trends within the aquifer. 

The major peaks in the neutron logs occur at 

shallow (less than –30 ft NGVD) depths in the ENR, 

whereas peaks occur throughout the entire borehole at 

S10C (+18 to –77 ft NGVD). The neutron log activity 

at S10C indicates zones of reduced porosity. The neutron logs at sites MP1, MP2, MP3, MOP1, MOP2, and 

MOP3 all show elevated hydrogen levels at shallow 

depths (less than 20 ft) than at deeper depths, indicating 

lower porosity (table 5). At S10C, the neutron log 

showed a distinct deflection to the left (low hydrogen 

content) at an elevation of 5 to 11 ft NGVD. This 

deflection occurred in a layer of well-sorted sand and 

gravel and could indicate a layer of high porosity. The 

remainder of this log traces displayed elevated hydrogen levels at elevations of +2, –2.5, and –42.5 ft 

NGVD. These elevated levels also are in layers of 

well-sorted sand with gravel and indicate layers of 

lower porosity. These deflections correlate to deflections on the gamma ray log and also could indicate 

SDCs. 

Electric Logs

Electric or resistivity logs measure the electrical 

properties of the formation such as the resistivity of 

fluid in interconnected pores or the effective porosity 

(Keys, 1990). The resistivity of the formation is 

affected by lithology, porosity, and water quality. The 

long-normal (LN) resistivity profiles sometimes can 

provide a view of the preferential flow paths and associated water quality (Keys, 1990). Deflections or peaks 

occur when formation water of different resistivity is 

encountered by the probe. These logs are composed of 

“shallow” and “deep” penetrating sondes with each 

sonde investigating at a different distance into the formation, from the center of the borehole. In this study, 

the LN and short normal (SN) were used. The LN measures 64 in. into the formation and the SN measures 16 

in. into the formation. 

Typical resistivity traces of the shallow aquifers 

in Florida show higher resistivity at or near the surface, 

which generally declines with depth. In the Surficial 

aquifer, the shallow, more resistive water is recharged 

from the surface. Relict seawater in the deeper portion 

of the Surficial aquifer has lower resistivity. Resistivity 

traces that indicate underflow or preferential flow have 

Site

Elevation at 

neutron peak 

(1929 NGVD)

Elevation of core 

sample

(1929 NGVD)

Laboratory

Lithology Vertical K

(ft/d)

Porosity 

(percent)

MP1 -1.0 to -5.0 -0.9 to -5.9 0.002 11 Sandy limestone

MP2 -2.0 to -5.0 -4.4 to -9.4 1.1 12 Hard sandy limestone

MP2 -13.3 to -16.0 -15.4 to -19.4 0.28 22 Very sandy limestone

MOP1 -5.5 to -7.0 -6.5 to -7.5 0.12 16 Sandy limestone

MOP2 2.0 to -2.0 5.9 to -0.9 0.04 16 Sandy limestone

Table 5. Peaks in neutron geophysical logs and the corresponding lithology at Everglades 

Nutrient Removal (ENR) project, north-central Everglades, south Florida

[1929 NGVD, National Geodetic Vertical Datum of 1929 in feet; K, hydraulic conductivity; ft/d, feet per day]
22 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

the same gentle slope with periodic areas of reduced 

resistivity shown by a deflection, usually to the left, in 

the resistivity trace. This trace generally indicates a 

more dormant area (higher specific conductance) of the 

ground water. 

When the natural layering of water within an 

aquifer is disturbed from anthropogenic activities, mixing occurs. The resistivity logs showed that anthropogenic activities had affected the sites. The upper 20 ft 

of the Surficial aquifer showed freshwater on the resistivity logs, indicating that relict seawater had been 

replaced by freshwater recharge. This recharge is likely 

from surface water in the western part of the ENR. The 

SN and LN traces in this study are generally smooth, 

with resistivity steadily decreasing with depth. This 

result is shown at MOP3 by the three electric traces 

exhibiting higher resistivity in the upper sediments and 

smooth traces of steadily decreasing resistivity in the 

bottom of the borehole. In contrast, site MP3 shows 

deflections to the left and right at elevations between 

–60 and –80 ft NGVD. This well is on a levee at the 

northwestern corner of the ENR. The resistivity traces 

at ENR sites MP1 and MOP2A show the highest resistivity levels with measurements of 400 ohm-m in the 

top 10 ft of the borehole.

The water underlying S10C shows relatively 

high resistivity at depth. This high resistivity indicates 

deeper recharge of freshwater, likely as a result of 

anthropogenic activity. Results of the lateral logging 

suggest some preferential flow paths, indicated by the 

deflections in the trace seen throughout the depth of the 

borehole. The SN resistivity log had an increase in 

resistivity to the right at –50 ft NGVD indicating a possible preferential flow path or freshening of the ground 

water. These differences may indicate water from different recharge sources.

Hydraulic Conductivity of the Surficial 

Aquifer

Horizontal hydraulic conductivity (Kh) of the 

Surficial aquifer at the different sites was estimated by 

three methods: calculations based on sieve analysis 

results of unconsolidated samples, steady-state air permeability tests on limestone cores, and slug tests. The 

permeability tests also directly estimated vertical 

hydraulic conductivity (Kv). Kv also was mathematically inferred from sieve and slug test results (Walton, 

1987). Hydraulic conductivity (K) of the peat was 

determined using bail tests from drive points, seepage 

meter data, and head data from drive points and surface 

water (Harvey and others, 2000).

Hydraulic conductivity was calculated for the 

unconsolidated samples obtained from six boreholes, 

using a software program by Vukovic and Soro (1992). 

This approach determines Kh by averaging the results 

of 10 different calculations of conductivity. Resulting 

Kh estimates for unconsolidated samples ranged from 

5.9 to 510 ft/d. Detailed results for each borehole are 

listed in Harvey and others (2000, p. 132–145). 

Permeability tests were conducted on 16 core 

samples from the ENR and 11 samples from WCA-2A 

(Core Laboratories, Inc., 1999). Porosity and vertical 

and horizontal hydraulic conductivity were determined. 

The minimum, maximum, and average horizontal 

hydraulic conductivity values for all sites ranged from 

0.02 to 510 ft/d. Detailed results from ENR core samples are provided in Harvey and others (2000) while 

data from all WCA-2A sites are presented in table 6. 

Core samples from the ENR boreholes generally 

were limestone with varying amounts of sand. One 

sample at MOP1 was cemented limy sand. The majority of samples were moldic, had pinpoint porosity, and 

contained a variety of fossils. The total porosity of 

these ENR samples ranged from a low of 9.4 percent in 

a shallow layer at MP3 to a high of 32 percent in a deep 

layer in the same borehole. The geometric mean of all 

porosities in the ENR samples was 17 percent. 

The Kh of the core samples from ENR ranged 

from less than 0.02 ft/d to about 510 ft/d, with a geometric mean of 31 ft/d. This mean value is much higher 

than the average reported Kh of limestone of 5.9 ft/d 

(Kasenow, 1994). The Kv of the sediments ranged 

from 0.002 to 3400 ft/d with a geometric mean of 

7.5 ft/d. This value is slightly higher than the reported 

average Kv of 3.1 ft/d (Kasenow, 1994). The ratios of 

vertical to horizontal hydraulic conductivity (Kv/Kh) 

ranged from a minimum of 0.002 to a maximum of 8, 

with a geometric mean of 0.2. The mean ratio of 0.2 is 

slightly lower than the average value reported for limestone (0.5) by Kasenow (1994). Because of this lower 

Kv/Kh ratio, the preferential flow is more likely to be 

horizontal where the consolidated material is present. 

Cores from WCA-2A primarily were limestone 

with varying amounts of sand. One sample at U1GW3 

contained very fine limy sand. Limonite, formed by 

oxidation of iron-bearing minerals or by precipitation 

in bogs, was seen in shallow samples at E4GW3 and 

F4GW3. Fossils were found in most samples and pin-
Hydrogeology of the North-central Everglades 23

point porosity was seen at sites F4GW3 and S10C. 

About half the samples were moldic or showed rootlets. The porosity of WCA-2A samples ranged from 

10.9 percent between –3 to –8 ft NGVD (E4GW3) to 

31.3 percent at –20 to –25 ft NGVD (U3GW3). The 

geometric mean of all porosities in the WCA-2A core 

samples was 19.6 percent. 

The Kh of the cores ranged from 4.1 ft/d to nearly 

502 ft/d, with a geometric mean of 63 ft/d. This geometric mean is an order of magnitude larger than the 

mean published value of 5.9 ft/d (Kasenow, 1994). 

Likewise, Kv of the cores ranged from 6.1 to 430 ft/d 

with a geometric mean of 26.4 ft/d, an order of magnitude larger than published averaged values. The ratios 

of vertical to horizontal hydraulic conductivity (Kv/Kh) 

ranged from 0.04 to 3.5 with a geometric mean of 0.4. 

The mean ratio of 0.4 is similar to the mean values calculated for limestone (0.52) using data from Kasenow 

(1994). 

Comparison of Hydraulic Conductivity in 

ENR and WCA-2A 

ENR cores had the largest variations in hydraulic 

conductivity at shallow elevations (10.6 to –12.6 ft 

NGVD) as shown in table 7. The low vertical and horizontal K measurements at shallow elevations in the 

ENR indicate a restrictive layer or caprock, as suggested by Rohrer (1999). Although the restricting layer 

is common in limestone above 0 ft NGVD, it is not spatially continuous, as evidenced by many cores with Kv

values higher than 1 ft/d. This discontinuous restrictive 

layer was observed during drilling operations at site 

M301. Here, the drilling fluids did not return to the surface for subsequent re-circulation, but emerged 18 ft 

away from the drill rig. 

Core analyses found large variations in Kv in the 

consolidated sediments of the ENR boreholes. Less 

variation was seen in WCA-2A cores. A thin layer with 

very low Kv (0.001 - 0.1 ft/d) was found to overlie a 

layer of high Kh and Kv at sites MP1, MOP1, and 

MOP2. The elevation of these restricted flow/high flow 

layers varies at each site; the restrictive layer was 

between 5.9 and –7.5 ft NGVD and the high flow layer 

was between –4.6 and –15.9 ft NGVD. The layers of 

high hydraulic conductivity act as preferential flow 

paths encouraging horizontal flow because of the overlying restrictive layer. When this restricting layer is 

eroded or fractured, a preferential pathway for vertical 

transport of water and its constituents is created 

because the Kv is much higher than the Kh. The vertical 

and horizontal hydraulic conductivity and porosity for 

two shallow layers in ENR sites MOP1, MOP2, and 

MP1 are shown in table 8. The extreme differences in 

Kv at the different elevations at these sites can be seen 

clearly in table 8. 

A variety of methods was used to estimate the 

average hydraulic conductivity of the peat. The methodology is addressed by Harvey and others (2000). In 

the ENR, the K of the peat is two orders of magnitude 

less than the K of layer 2 (table 9). In WCA-2A, the K

of the peat also is appreciably less than the K of layer 2. 

Properties of the core samples, such as the silica/calcium carbonate content, color, and fossil content, 

were used to identify the formation and Q series of the 

sediments. Quartz content of these four boreholes varWell

name

Sample 

depth

(feet bls)

Elevation

in feet

(1929 NGVD)

Vertical 

hydraulic 

conductivity

(ft/d)

Horizontal 

hydraulic 

conductivity 

(mean in ft/d)

Porosity 

(percent) Lithographic description 

S10C-A 36 - 41 -13.6 to -18.6 8.3 4.1 25 LS, chalky, slightly sandy, pp

S10C-A 30 - 35 -7.6 to -12.6 16 85 24 LS, fossils, very sandy, pp

U1-GW3 5 - 10 5.9 to 0.9 26 61 17 LS, fossils, quartz sand, moldic

U1GW3 15 - 20 -4.1 to -9.1 6.1 45 19 Tan, fine-grained sand, v limy, sl rootlets

E4-GW3 0 - 5 12.0 to 7.0 40 29 17 LS, fossils, adundant quartz sand, limonite

E4-GW3 10 - 15 2.0 to -3.0 430 450 25 LS, fossils, quartz sand, slightly moldic

E4-GW3 15 - 20 -3.0 to -8.0 16 4.6 11 LS, slight quartz sand, slightly fractured 

F4-GW3 0 - 5 11.5 to 6.5 7.5 180 22 LS, fossils, quartz sand, limonite, sl moldic

F4-GW3 10 - 15 1.5 to -3.5 90 500 19 LS, fossils, qtz sand, sl pinpoint porosity

U3-GW3 10 - 15 1.2 to -3.8 8.5 24 14 LS, fossils, slight quartz sand, sl rootlets

U3-GW3 20 - 25 -8.8 to -13.8 130 500 31 LS, fossils, slight quartz sand

Table 6. Results of core analysis from boreholes in Water Conservation Area 2A (WCA-2A), north-central Everglades, 

south Florida

[bls, below land surface; 1929 NGVD, National Geodetic Vertical Datum of 1929 in feet; ft/d, feet per day; mean refers to an arithmetic mean; LS, 

limestone; pp, pinpoint porosity; v, very; sl, slight; qtz, quartz]
24 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

Site 

name

Sample 

elevation

(1929 NGVD)

Vertical K

(ft/d)

Maximum

horizontal K

(ft/d)

Minimum 

horizontal K

(ft/d)

Porosity 

(percent)

MP1 -0.9 to -5.9 0.002 0.03 0.01 11

MP2 10.6 to 5.6 0.68 20 13 11

MP3 2.2 to -2.8 6.5 3.4 3.0 9

MOP1 -6.5 to -7.5 0.12 77 0.31 16

MOP2 0.4 to -4.6 3,100 690 98 22

M203 0.63 to -4.4 100 130 110 22

S10C -7.6 to -12.6 16 140 29 24

Table 7. Hydraulic conductivity determined from core samples beneath Everglades 

Nutrient Removal (ENR) project and Water Conservation Area 2A (WCA-2A), 

north-central Everglades, south Florida 

[1929 NGVD, National Geodetic Vertical Datum of 1929 in feet; K, hydraulic conductivity; ft/d, feet per day]

Site

Name

Sample elevation

(1929 NGVD)

Vertical K 

(ft/d)

Maximum

horizontal K

(ft/d)

Minimum 

horizontal K 

(ft/d)

Porosity 

(percent)

MOP1 -6.5 to -7.5 0.12 77 0.31 16

MOP1 -7.5 to -12.5 3,400 – – 29

MOP2 5.9 to 0.9 0.04 20 17 16

MOP2 -4.6 to -9.6 1,000 550 470 24

MP1 -0.9 to -5.9 0.002 0.03 0.01 11

MP1 -10.9 to -15.9 400 260 250 26

Table 8. Hydraulic conductivity compared in two layers at several sites in Everglades 

Nutrient Removal (ENR) project, north-central Everglades, south Florida 

[1929 NGVD, National Geodetic Vertical Datum of 1929 in feet; K, hydraulic conductivity; 

ft/d, feet per day; –, not applicable]

Area Sediment

Layer

Approximate 

sediment depth 

(cm)

Approximate 

sediment depth

(ft)

Mean K

(cm/d)

Mean K

(ft/d)

ENR Peat and transition layer 105 3.4 8.1 0.3

WCA-2A Peat and transition layer 131 4.3 43 1.4

Underlying sand – – 270 8.9

Table 9. Hydraulic conductivity of peat, peat-to-rock transition zone, and shallow underlying sand layer at 

Everglades Nutrient Removal (ENR) project and Water Conservation Area 2A (WCA-2A), north-central 

Everglades, south Florida 

[K, hydraulic conductivity; ft/d, feet per day; ft, feet; cm/d, centimeter per day; –, not applicable]
Hydrogeology of the North-central Everglades 25

ied greatly by both site and elevation as shown in table 

10. These differences indicate changing depositional 

environments. As stated previously, shallow water 

marine carbonates and siliciclastic material dominate 

the Quaternary and portions of the Pleistocene deposits 

in south Florida. The lithology of the core samples 

from WCA-2A sites E-1, F-1, F-4 and ENR site M-204 

were described in detail in by Harvey and others (2000, 

p. 96–98).

South Florida Water Management District staff 

sorted though the sieved samples for whole and partial 

fossils to assist in stratigraphic correlation. Fossils 

were identified and logged. Because of the large number of fossil species identified, each species was 

assigned a numerical identification number. Preliminary descriptions are provided in Harvey and others 

(2000). A stratigraphic correlation was not in the scope 

of this study.

Comparison of Hydraulic Conductivities to 

Other Studies

The field methodology for drawdown (slug) tests 

in wells is described in Harvey and others (2000). The 

formulations of Bouwer (1989) and Bouwer and Rice 

(1976) were used to determine the hydraulic conductivity for the partially penetrating wells in an unconfined 

aquifer. The results of the drawdown tests show great 

variation and are shown in table 11. The lowest hydraulic conductivity was 1.0 ft/d at MP3-A. At this well, the 

bottom of the screen is at –174 ft NGVD and is in silty 

sediments. The highest hydraulic conductivity measured at ENR, 242 ft/d, was at MP3-C in fine- to 

medium-grained sand at an elevation of –47 ft NGVD. 

The mean hydraulic conductivity of all ENR wells, 

determined by drawdown tests, was 32 ft/d.

The variation of Kh among wells was greatest in 

WCA-2A. Site U1 had the lowest and highest hydraulic 

conductivities in WCA-2A. U1GW4 had a Kh of 32 ft/d 

at an elevation of –5.8 ft NGVD, and U1GW3 had a Kh

of 1261 ft/d at –13.4 ft NGVD. The mean hydraulic 

conductivity of all the wells in WCA-2A, determined 

by drawdown tests, was 116 ft/d. The variation in Kh

within the same lithologic layers in the study area as 

well as the minimum, maximum, and mean Kh in each 

layer are shown in table 11. Three of the layers have a 

maximum Kh greater by an order of magnitude than the 

minimum Kh.

The results from the drawdown tests were compared to results from earlier tests in eastern Broward 

and Palm Beach Counties by Dames and Moore (1988) 

and Fish (1988), along with Rohrer’s (1999) results 

from ENR. The hydraulic conductivity values from this 

study were higher than results from these earlier Surficial aquifer studies. Results from these investigations 

and this study are shown in table 12. 

Hydraulic conductivity values in comparable 

lithologic units (for example, sand) vary significantly 

with each study. Dames and Moore (1988) completed 

slug tests at 11 wells in coastal south Palm Beach 

County at elevations of –15 ft to –85 ft (1929 NGVD). 

These wells, completed in poorly sorted sands, have 

hydraulic conductivity values that range from 1 ft/d to 

7 ft/d with a mean of 2.8 ft/d. Slug tests done in 

Rohrer’s (1999) ENR seepage investigation have 

hydraulic conductivity values ranging from 0.04 ft/d to 

150 ft/d with a mean of approximately 5 ft/d. It should 

be noted that the wells in the Rohrer study generally 

were constructed on or near the levees surrounding the 

ENR. Some of these wells have screens in the unconsolidated levee material, and may not reflect the 

hydraulic conductivity of the undisturbed aquifer material. This screen placement could affect the average 

hydraulic conductivity value determined from that 

study.

This study found the mean horizontal hydraulic 

conductivity to be approximately 31 ft/d in ENR and 

120 ft/d in northern WCA-2A. Sand layers were found 

to have hydraulic conductivities of 15 ft/d in ENR and 

30 ft/d in WCA-2A. Dames and Moore (1988) reported 

a value of about 3 ft/d. This difference in K further 

demonstrates the large variation of hydraulic conductivity found in the study areas.

Borehole Depth below land 

surface (ft)

Quartz content

Minimum 

(percent)

Maximum 

(percent)

E1 6.5 to 15 30 40

F1 4.5 to 29 5 40

F4 5 to 26 10 37

M204 4.5 to 20 5 45

Table 10. Summary of quartz content of selected 

boreholes in Everglades Nutrient Removal (ENR) 

project and Water Conservation Area 2A (WCA-2A), 

north-central Everglades, south Florida

[ft, feet]
26 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

Evidence for Layers Restricting Vertical Flow

Sediment deposition, sedimentary processes, and 

sediment weathering are important in creating layers in 

the Surficial aquifer where preferential horizontal flow 

occurs, or where vertical flow of ground water is 

restricted. Anthropogenic practices, such as peat 

removal and blasting for canals, also are locally important. The lithology of the Surficial aquifer in the two 

study areas is primarily sand with limestone layers concentrated between 10 and –30 ft NGVD. Whereas 

sands generally are fine grained and well sorted in the 

ENR, they are coarser and more poorly sorted at S10C 

in WCA-2A. The presence of the gravel-size fraction 

changes from not present in the northern part of ENR 

to gravel in every sample from the southern part of the 

study area at S10C. The upper sand layer (layer 2) is 

relatively thin compared to other sand layers or absent, 

and the limestone layer (layer 3) is thicker in ENR 

compared with the limestone layer at S10C in 

WCA-2A. 

Core samples include sandy limestone and limestone with varying porosities (11 to 31 percent) and up 

to 45 percent quartz composition. The sandy limestone 

primarily was found at approximately –18 ft NGVD. 

Generally, porosity was controlled by secondary processes. The depositional sequences at the ENR and 

WCA-2A initially were similar, but subsequent erosion 

of the upper 60 ft of south Palm Beach County sediments, including WCA-2A, occurred during the transgressive period of the Upper Miocene/Pleistocene 

epochs. Periods of sea-level regression followed, which 

deposited new sediments of larger grain size than originally deposited. These new sediments have higher 

hydraulic conductivity than the older sediments. The 

limestone formed in and near the WCA-2A is not as 

hardened by post-depositional processes and is not as 

vertically restrictive to ground-water flow as the limestone in ENR.

Preferential flow paths in sand generally are in 

the shallower sand at S10C (layer 2) but distributed at 

varying depths in the ENR. ENR preferential paths 

were seen in layers 2, 3, and 4 with the majority in the 

deeper sands of layer 4 (fig. 4). Results from slug tests 

show that K values are higher in WCA-2A by approximately an order of magnitude. This difference is greatest in the shallow layer of WCA-2A.

Evidence for restricted vertical flow comes 

mainly from the ENR site. Dense, low-porosity limestone with low vertical hydraulic conductivity was 

found at elevations ranging between 3 and –7 ft NGVD 

in that area. With vertical hydraulic conductivities as 

low as 0.002 ft/d, those layers locally act as an aquitard. Rohrer (1999) also observed a vertically restrictive layer for flow that he referred to as "caprock." 

Site/Layer Type of measurement

Elevation at bottom of 

well screen 

(1929 NGVD)

KH

(ft/day)

KHA

(ft/day) 

ENR 

Layer 3

Minimum (at M204-P) -19.1 5 –

Maximum (at M303-P) -18.2 170 –

Mean – 28 45

ENR 

Layer 4

Minimum (at MOP1-B) -52.5 16 –

Maximum (at MP3-C) -46.3 240 –

Mean – 48 64

WCA-2A 

Layer 2

Minimum (at U1-GW4) -5.8 32 –

Maximum (at E1-GW4) 0.7 130 –

Mean – 65 83

WCA-2A 

Layer 3

Minimum (at F4-GW3) -13.5 58 –

Maximum (at U1-GW3) -13.4 1260 –

Mean – 270 660

Table 11. Summary of horizontal hydraulic conductivity as determined by drawdown tests in 

Everglades Nutrient Removal (ENR) project and Water Conservation Area 2A (WCA-2A),

north-central Everglades, south Florida

[1929 NGVD, National Geodetic Vertical Datum of 1929 in feet; KH, mean hydraulic conductivity calculated using 

geometric mean; KHA, mean hydraulic conductivity calculated using arithmetic mean; ft/d, feet per day; –, not 

applicable]
Quantifying Recharge and Discharge in the North-central Everglades and Identifying the Key Controlling Factors 27

Interpretations made here differ from Rohrer’s interpretations in that this study found the vertical restricting 

layer is not horizontally continuous enough over the 

area to serve as the primary layer restricting vertical 

flow. It also is likely that blasting to create canals and 

borrow pits produced important avenues of vertical 

exchange and may have affected hydraulic properties 

throughout the ENR. The data from interior areas of the 

ENR show core samples with very high vertical values 

of K. Also, the return of drilling fluids once was 

observed to occur 18 ft away from a drill site, at a location far removed from canals or blasting. These observations further support the conclusion that a vertical 

restricting layer in limestone beneath ENR is not as laterally continuous as the peat layer (with K less than 

1 ft/d) that overlies the aquifer. Additional support for 

this conclusion comes from the Danish Hydraulic Institute (1999), which had to increase vertical K in its simulations of ground-water flow in ENR above what 

would be expected for a horizontally continuous layer 

that restricts ground-water flow.

The peat layer, with relatively low vertical 

hydraulic conductivity on the order of 1 ft/d, most 

likely functions as the restrictive unit to vertical 

ground-water flow. Whereas certain limestone layers 

have vertical K values less than peat by an order of 

magnitude or more, the horizontal continuity of the 

peat layer proves its primary importance as the layer 

that restricts vertical flow. 

QUANTIFYING RECHARGE AND 

DISCHARGE IN THE NORTH-CENTRAL 

EVERGLADES AND IDENTIFYING THE 

KEY CONTROLLING FACTORS

A better understanding of how water-resources 

management has altered natural hydrologic patterns in 

the Everglades is required for successful restoration. 

Inseparable from natural patterns of surface-water flow 

and ground-water flow in the Everglades are the interactions between them. Those interactions affect water 

flow in both the wetlands and aquifer, and are, therefore, important to the overall water balance of the Everglades as well as the movement and fate of chemicals, 

including contaminants.

Interactions between surface water and ground 

water are well studied in certain areas of the Everglades. Most investigations were conducted in the 

immediate vicinity of levees and/ or canals (Klein and 

Sherwood, 1961; Miller, 1978; Chin, 1990; Sonenshein, 2001). Very few investigations have considered 

interactions between surface water and ground water in 

the expansive interior areas of the Everglades. Merritt 

(1996) combined surface- and ground-water flow models in Everglades National Park, showing the importance of wetland-ground water interactions. A 

drawback of Merritt’s approach was that the model was 

calibrated mostly on hydraulic heads, an approach 

known to produce non-unique solutions that could 

result in uncertainty in model fluxes. The best way to 

improve the accuracy of modeling is to acquire more 

Publication Aquifer Material 

Type

Number of 

Samples

Minimum K

(ft/d)

Mean K1

(ft/d)

Maximum K

(ft/d)

Fish, 1988, Eastern Broward County2 Fine to medium sand 3 16. 29 44

Rohrer, 1999, ENR2 Sand/ limestone 20 0.04 5 150

Dames & Moore, 1988, Eastern Palm Beach County3 Sand 11 1.0 3 7

Harvey and others, 2000, ENR3 Sand/ limestone 23 1.0 32 240

Harvey and others, 2000 WCA-2A3 Sand/ limestone 15 32. 120 1300

Harvey and others, 2000, ENR4 Sand 121 6.0 15 180

Harvey and others, 2000, WCA-2A4 Sand 37 8.0 30 510

1 Mean hydraulic conductivity is the geometric mean.

2 Slug test. 

3 Drawdown test.

4 Calculated from grain-size sieve analysis. 

Table 12. Summary of horizontal hydraulic conductivity values from the Surficial aquifer in Palm Beach County and 

eastern Broward County, north-central Everglades, south Florida. Investigations with less than three samples were 

not included 

[K, hydraulic conductivity; ft/d, feet per day]
28 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

detailed data sets or complementary data of a different 

type.

At a regional scale, the Hydrologic Systems 

Modeling Group of the South Florida Water Management District (SFWMD) has conducted the most comprehensive hydrologic modeling in the Everglades. An 

important accomplishment of that group was developing the South Florida Water Management Model 

(SFWMM), a coupled model of surface-water and 

ground-water flow for the present-day Everglades. A 

companion model, the Natural Systems Model (NSM), 

simulates hydrologic patterns across the pre-drainage 

Everglades topography using the same algorithms and 

input data. Both the SFWMM and NSM operate by 

computing water flow and storage in a network of 2-mi 

by 2-mi grid cells covering much of the eastern 

two-thirds of south Florida. The SFWMM and NSM 

models are most appropriate for regional-scale simulations of surface-water flow and often have not been 

tested against detailed measurements or models.

The purpose of the present project was to collect 

and analyze detailed hydrologic and geochemical data 

at indicator sites in the north-central Everglades in 

order to develop reliable estimates of recharge and discharge. Therefore, this study project complements the 

regional-based SFWMM and NSM models by providing site-specific data sets for comparison with those 

models. The ultimate goal is to improve the accuracy of 

local and regional hydrologic models that will be used 

for testing hypotheses about Everglades hydrology, or 

for testing future water-resources management scenarios. 

The specific questions addressed in this section 

are:

1.) What are the dominant pathways of 

ground-water flow in the north-central Everglades? How have they changed from 

pre-drainage conditions? Are present-day flow 

directions persistent over seasons or years?

2.) What are the major factors controlling recharge 

and discharge in the wetlands and the spatial 

and temporal variability of those fluxes?

3.) Are recharge and discharge substantial in comparison to other water-balance fluxes? What is 

the relative importance of recharge and discharge in interior areas of wetlands? Are the 

important factors that control recharge and discharge near levees the same in the interior 

areas of the wetlands?

Approach

Ground-water recharge and discharge in the 

Everglades usually has been estimated by surface-water 

mass balance modeling (MacVicar and others, 1994; 

Choi and Harvey, 2000). The precision of those estimates is linked closely to the density and quality of surface-water flow and stage measurements (Choi and 

Harvey, 2000). This investigation builds on former 

studies through analysis of ground-water hydraulic 

data, peat hydraulic properties, and through direct measurements of vertical fluxes between ground water and 

surface water using seepage meters. Those additional 

measurements allowed characterization of spatial and 

temporal variability of recharge and discharge at the 

study sites. 

The purpose of this approach was broader than 

just improving the accuracy of previous estimates of 

recharge and discharge. As mentioned previously, 

hydraulic data from ground water have the advantage 

of pinpointing locations of recharge and discharge. By 

determining the spatial distribution of recharge and discharge, it was possible to test hypotheses about the factors that are most important in controlling recharge and 

discharge in the Everglades. In particular, most previous work assumed that ground-water flow beneath 

levees dominates discharge and recharge in the Everglades. The present study quantified those processes in 

interior areas of the wetlands to test that assumption. 

Results were expected to be used in guiding future 

improvements of regional models such as SFWMM.

Detailed methods were published previously in 

Harvey and others (2000). The only information given 

here is that which is immediately relevant to understanding the results presented in this section. Harvey 

and others (2000) should be consulted for all further 

information concerning survey accuracies, well and 

seepage-meter design and emplacement, slug tests in 

peat, and measurement of surface-water and 

ground-water levels. 

Regional horizontal hydraulic gradients were 

computed for ENR or WCA-2A ground water in a 

given aquifer layer by calculating the inclination and 

direction of a plane that matched most closely the measured heads. By convention, a negative gradient indicated flow in a direction from areas of higher to lower 

hydraulic head. The average computed directions of 

horizontal flow are reported in table 13. Those flow 

directions were compared with the orientation of three 

research transects (one in ENR and two in WCA-2A) 

that had been established previously (fig. 1, also see 

figs. 2 and 3). Research transects in ENR and WCA-2A 
Quantifying Recharge and Discharge in the North-central Everglades and Identifying the Key Controlling Factors 29

were chosen because the orientation of transects was 

thought to be similar to average regional directions of 

ground-water flow. Horizontal hydraulic gradients 

along the research transects were computed by dividing 

the difference in head in two wells screened in the same 

layer by the distance between the wells. Because the 

general orientation of transects did compare well with 

the computed regional gradients in table 13, it was possible to make further computations of horizontal gradients using only the transect data. The advantage of 

computing gradients from transect data, instead of relying on regional horizontal gradients alone, was the ability to compare results between specific areas of 

interest, such as areas close to and far from levees. 

Vertical hydraulic gradients also were computed 

using data from all of the study sites in ENR and 

WCA-2A where two or more wells were screened at 

different depths. The computation used the difference 

in head between surface water and shallow wells 

divided by the peat thickness. The sign convention 

resulted in a positive vertical gradient when recharge is 

occurring (downward flow from surface water to 

ground water) and a negative gradient when discharge 

occurs (upward flow from ground water to surface 

water). 

Hydraulic conductivity (K) of peat was estimated 

in the peat by slug tests in piezometers. Slug-test 

results were compared with an independent estimate 

based on seepage-meter measurements that were combined with measured vertical hydraulic gradients to calculate K using Darcy’s law. Detailed methods and 

complete results are provided in Harvey and others 

(2000). 

Recharge and discharge fluxes through peat were 

estimated using seepage meters or, alternatively, 

Darcy-flux computations. For reasons explained later, 

only seepage-meter results are reported for ENR sites 

and only Darcy-flux calculations are reported for 

ENR - Intermediate Layer (K=28 ft/d; depth = +5.76 to -22.0 ft NGVD)

WET–8/19/1997 DRY–6/15/1998

Flow direction (degrees) 296 297

Solute velocity1 (ft/d) 0.07 0.06

Gradient (ft/ft) -0.002 -0.001

ENR - Deep Layer (K=48 ft/d; depth = -40.0 to -90.0 ft NGVD)

WET–8/19/1997 DRY–6/15/1998

Flow direction (degrees) 310 311

Solute velocity1 (ft/d) 0.10 0.09

Gradient (ft/ft) -0.002 -0.001

WCA-2A - Shallow Layer (K=73 ft/d; depth = +5.0 to -5.8 ft NGVD)

WET–8/19/1997 DRY–6/15/1998

Flow direction (degrees) 224 173

Solute velocity1 (ft/d) 0.01 0.01

Gradient (ft/ft) -0.0002 -0.0001

WCA-2A - Intermediate Layer (K=156 ft/d; depth = -6.0 to -19.0 ft NGVD)

WET–8/19/1997 DRY–6/15/1998

Flow direction (degrees) 219 190

Solute velocity1 (ft/d) 0.03 0.01

Gradient (ft/ft) -0.0002 -0.0001

Table 13. Horizontal hydraulic gradients and solute velocities of 

ground-water flow beneath Everglades Nutrient Removal (ENR) 

project and Water Conservation Area 2A (WCA-2A), in the 

north-central Everglades, south Florida

[NGVD, National Geodetic Vertical Datum of 1929; K, horizontal hydraulic 

conductivity; ft, feet; ft/d, feet per day]

1 Calculated from ground-water flow divided by porosity (0.3).
30 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

WCA-2A sites. The seepage-meter approach, meter 

design, procedures for installation and use, major 

assumptions, and precision and limit of detection, all 

are described in Harvey and others (2000).

In contrast to results from ENR, seepage-meter 

measurements at WCA-2A were judged as unreliable. 

The major evidence for rejecting seepage-meter results 

at WCA-2A was that the direction of fluxes measured 

on particular sampling dates often did not agree with 

the direction indicated by hydraulic gradient data. For 

example, on the westernmost transect in WCA-2A 

(F1-F4-U3), seepage-meter measurements often indicated recharge at the same time that hydraulic gradient 

data suggested discharge (Harvey and others, 2000). 

The seepage-meter measurements at WCA-2A 

may have been compromised by problems that were 

specific to WCA-2A. For example, errors were caused 

in part by the higher hydraulic conductivity of peat at 

WCA-2A (table 9), which may have been high enough 

to cause measurement errors due to stretching or compression of seepage bags because of wind or moving 

vegetation. It should be noted that seepage-meter data 

collected at single sites in WCA-2B and WCA-3A 

were judged as reliable. This reliability was because 

seepage measurements from triplicate meters at 

WCA-2BS and WCA-3B15 tended to be similar and 

close to or slightly above detection limits. 

The alternative to seepage meters was calculation of vertical fluxes through peat using Darcy’s law. 

Briefly, the Darcy-flux approach used a measured vertical hydraulic gradient across the peat and a vertical 

hydraulic conductivity for peat to compute a vertical 

water flux between ground water and surface water. 

The vertical gradient was estimated as the difference 

between surface-water stage and water elevation in the 

shallowest monitoring well (generally the "GW4" well 

located approximately 10 ft below the peat surface), 

divided by the thickness of the peat at that location. 

Land-Surface Topography, Surface-Water 

Slope, and Water-Level Fluctuations

WCA-2A 

The present slope of the land surface in 

WCA-2A is similar to pre-drainage conditions, 3.3 x 

10-5, towards the south-southeast. The water surface in 

WCA-2A generally slopes toward the southwest on a 

grade similar to the average land-surface slope. Sometimes, the water surface slopes toward the southeast, 

depending on where the water releases through structures are occurring (Romanowicz and Richardson, 

2000). During the wet season, particularly when water 

is released from WCA-1 to WCA-2A, the water-surface slope can increase by as much as a factor of 2.

Long-term data collected by USGS and simulations by SFWMD suggest that water management 

imparts the dominant control on water-level fluctuations in WCA-2A. The largest and most rapid fluctuations in surface-water level in WCA-2A are caused by 

water releases from WCA-1, and not by precipitation 

and evapotranspiration. Comparisons between the Natural Systems Model and South Florida Water Management Model showed that the amplitude of water-level 

fluctuations for the pre-drainage Everglades in the 

vicinity of WCA-2A typically were 25 to 50 percent of 

present-day fluctuations.

A constructed berm on the extreme south side of 

the Hillsboro canal/levee complex (fig. 3), adjacent to 

WCA-2A, has an important effect on surface-water 

releases from WCA-1, and on water flow in WCA-2A. 

The purpose of the berm is to direct flow in the canal 

toward the east side of WCA-2A before the water 

enters the wetland and flows southward. When the 

water level rises rapidly in the tailwater canal because 

of a water release from WCA-1, overtopping of the 

berm eventually will result (at approximately 13.5 ft 

NGVD). After overtopping occurs, sheet flow across 

the berm supplies surface water directly to the wetland 

at all points along the Hillsboro levee. 

ENR

The slope of the land surface in ENR is much 

steeper than in WCA-2A, because of the engineering 

involved. Because of subsidence, the land surface 

slopes in the direction opposite (northwest) of the estimated pre-drainage topographic gradient (southeast). 

The area of steepest slope (0.001) is immediately to the 

west of the L-7 levee and continues across the eastern 

third of ENR. Continuing to the west the ground slope 

is 2 x 10-4 across the western two-thirds of ENR. Those 

slopes are approximately 30 times and 7 times greater, 

respectively, than the average land slope in WCA-2A. 

The ENR was designed so that surface water would 

flow toward the southwest. The engineered slope in 

that direction also is relatively steep (1 x 10-4). Even 

though the land-surface slope is much greater than 
Quantifying Recharge and Discharge in the North-central Everglades and Identifying the Key Controlling Factors 31

WCA-2A, the average water-surface slopes are similar 

(approximately 3 x 10-5).

Average differences in water level across the 

WCA-1 levee are similar at ENR and WCA-2A, 2.4 

and 2.8 ft, respectively. Variability of those water-level 

differences was greater at WCA-2A. Expressed as a 

coefficient of variation, the temporal variability of 

water-level differences across levees was 50 percent at 

WCA-2A compared with 18 percent at ENR. At ENR, 

surface-water levels are more widely monitored and 

tightly controlled by water-resources managers in an 

attempt to maintain relatively stable water levels in 

ENR. Greater variability at WCA-2A results because 

of water releases that can substantially decrease 

WCA-1 levels and increase WCA-2A levels in a matter 

of hours.

Horizontal Hydraulic Gradients

Average directions of ground-water flow were 

toward the northwest in the vicinity of the ENR, and 

toward the southwest in north-central WCA-2A (table 

13). At ENR, there was relatively little change between 

wet and dry seasons in the direction of ground-water 

flow. Ground-water flow at ENR was toward the northwest during both wet and dry periods, toward 296 and 

297 degrees (clockwise from north) at intermediate 

depths in the Surficial aquifer (table 13). The direction 

of ground-water flow in the deeper part of the aquifer 

also was stable over time but was more northerly in 

direction (310 degrees). It is possible that the estimated 

differences in flow direction at different levels in the 

aquifer are erroneous, the result of having fewer deep 

wells to use in the calculation of flow direction.

The magnitude of horizontal hydraulic gradients 

at ENR increased by 15 to 20 percent during wet conditions compared with dry conditions (table 13). 

Increased hydraulic gradients during wet conditions 

reflect the greater wet-season differences in water level 

between WCA-1 and the agricultural area.

At the WCA-2A study site, ground-water flow 

was toward the south-southwest (toward WCA-3A). 

During the wet season hydraulic gradients varied only a 

few degrees between intermediate and deeper wells 

(flow directions of 224 degrees and 219 degrees, 

respectively). Between wet and dry seasons, WCA-2A 

horizontal hydraulic gradients shifted considerably. 

The direction of flow in the shallow layer was toward 

224 degrees during the wet season compared with 173 

degrees during the dry season. The more easterly component of flow in dry conditions reflects the effect of 

seasonally low water levels to the east where 

ground-water pumping is from a well field between the 

Everglades and the Atlantic Coastal Ridge (Miller, 

1988). 

The magnitude of horizontal hydraulic gradients 

at WCA-2A increased by approximately a factor of 2 

during wet conditions. Increased ground-water hydraulic gradients during wet conditions likely are the result 

of opening spillways between WCA-1 and WCA-2A, 

and between WCA-2A and WCA-3A, which reduces 

ponding in WCA-2A and increases the surface-water 

slope.

The importance of land and water-surface slopes 

in affecting ground-water flow is apparent on plots of 

water-surface slope and hydraulic gradient along each 

research transect (fig. 10). The largest horizontal 

hydraulic gradients in ground water are near levees in 

ENR and WCA-2A. Hydraulic gradients near levees 

were approximately 4 times greater at ENR compared 

with WCA-2A. That difference largely is the result of 

the geographic position of the ENR on the boundary 

between WCA-1 and the EAA, where the land-surface 

and water-surface slopes are relatively steep because of 

subsidence in the EAA. 

Greater seasonal variability in horizontal hydraulic gradients beneath WCA-2A, compared with ENR, 

results in large part from greater variation in the difference between surface-water levels across the levee that 

borders WCA-1 (Harvey and others, 2000). As 

reported earlier, temporal variability in water-level differences across levees was greater at WCA-2A (coefficient of variation of 50 percent compared with 18 

percent for ENR). More variable horizontal gradients at 

WCA-2A also result from larger fluctuations in 

surface-water level in WCA-2A compared with ENR. 

For example, over a period of approximately a year and 

a half (June 1997 to October 1998), water levels in the 

interior wetlands of WCA-2A varied over 5 ft compared with only 2 ft at ENR (Harvey and others, 2000). 

Horizontal Ground-Water Flow Velocities

Horizontal ground-water flow velocities were 

computed based on hydraulic gradients computed from 

all of the available well data. Calculations of 

ground-water flow velocities for both wet and dry conditions are compared in table 13. An average 
32 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

depth-weighted hydraulic conductivity for the Surficial 

aquifer was used in the calculation (Harvey and others, 

2000, p. 175). Results are reported in terms of a solute 

velocity (Darcy-flow velocity divided by porosity). A 

porosity of 0.30 was used for all calculations. 

Ground-water flow velocities were higher at 

ENR than WCA-2A by approximately a factor of 6 and 

4 in intermediate and deeper layers during the wet season, respectively (table 13). During the dry season, 

flow rates were approximately an order of magnitude 

higher in ENR compared with WCA-2A (table 13). If 

vertical mixing in the Surficial aquifer is ignored (as a 

first-order assumption), and if all recharge is assumed 

to occur at the headwater canal adjacent to the levee, 

the velocities presented in table 13 are consistent with 

horizontal travel times on the order of hundreds of 

years beneath ENR and thousands of years beneath 

WCA-2A. Those estimates of travel time are useful as 

general indicators of flow, although they are unlikely to 

accurately reflect the actual ground-water ages because 

recharge in the wetland interior and vertical mixing are 

not considered. Although realistic calculations of the 

ground-water travel time are desirable, those are difficult to compute with confidence at the present time 

because the spatial distribution of recharge across the 

wetland and the extent of vertical mixing in the aquifer 

is still not well enough known.

Vertical Hydraulic Gradients

ENR 

The spatial distribution of recharge and discharge at ENR is shown by a contour map of vertical 

hydraulic gradients for a representative day in the wet 

season (fig. 11). Recharge occurs over all of the western two-thirds of ENR, and, based on the direction and 

high magnitude of the gradients, recharge appears to be 

the dominant interaction with ground water at ENR. 

Concluding that recharge is the dominant interaction 

with ground water from vertical gradients is consistent 

with the results of Choi and Harvey (2000), who found 

that recharge was an order of magnitude greater than 

discharge in ENR on the basis of a water and chloride 

budget. 

Both the steep land-surface slope across the ENR 

and the abrupt drop in water elevation across the western boundary levee contribute to the substantial driving 

force for recharge. Water level in the eastern part of 

ENR ranges between 14 and 15 ft and declines to 

between 11 and 12.5 ft on the western side of ENR. 

Water level then declines another 3.75 ft across the 

western boundary levee to the seepage canal located 

just outside the ENR (fig. 2). The water level in the 

seepage canal nearly always was held constant at just 

below 8 ft (relative to 1929 NGVD). 

The magnitude of vertical hydraulic gradients 

correlated positively with surface-water level in ENR 

(Choi and Harvey, 2000). Choi and Harvey (2000) 

showed that recharge in ENR was controlled by the rate 

of surface-water pumping into ENR, because that 

source of water strongly affects surface-water level, 

which in turn affects the driving force for recharge. 

Surface-water pumping generally is greatest in ENR 

during the wet season, which causes increases in 

ground-water recharge by as much as a factor of 3 during that season (Choi and Harvey, 2000). 

Ground-water discharge occurred along a relatively narrow band on the eastern side of ENR (fig. 11). 

The driving force for discharge was the water level 

across the L-7 levee in WCA-1, which was 2.4 ft higher 

on average (between June 1996 and October 1998) 

compared with ENR. Discharge was correlated positively with water-level differences across the L-7 levee, 

with highest water levels in WCA-1 and greatest 

water-level differences usually occurring in the wet 

season (Choi and Harvey, 2000). As pointed out by 

Choi and Harvey (2000), ground-water discharge in 

ENR was small (approximately one-tenth) compared to 

recharge. Although the largest vertical hydraulic gradients were near levees, vertical hydraulic gradients also 

were discernable in interior areas of ENR. An example 

from site MP102, a site that consistently showed downward hydraulic gradients with a magnitude of approximately 0.005, is shown in figure 12. Most of the 

interior areas of the western two-thirds of ENR had 

similarly small downward hydraulic gradients, which, 

in total, accounted for approximately 6 percent of all 

recharge in ENR (Harvey and others, 2000).

WCA-2A 

Highest vertical hydraulic gradients in WCA-2A 

were observed near the levee at site S10-C (Harvey and 

others, 2000). Similar to ENR, vertical hydraulic gradients at S10-C varied positively with the water-level difference across the levee. Normally, the water-level 

difference across the Hillsboro levee is about 2.8 ft, 

which is similar to the condition on the east side of 
Quantifying Recharge and Discharge in the North-central Everglades and Identifying the Key Controlling Factors 33

-0.0020

-0.0010

0.0000

0.0010

0.0020

0.0030

0.0040

0.0050

0 2,000 4,000 6,000 8,000 10,000 12,000

DISTANCE FROM MP3, IN FEET

DISTANCE FROM S10C, IN FEET

HORIZONTAL HYDRAULIC GRADIENT HORIZONTAL HYDRAULIC GRADIENT

8

9

10

11

12

13

14

15

WATER LEVEL, IN FEET 

RELATIVE TO 1929 NGVD

WATER LEVEL, IN FEET 

RELATIVE TO 1929 NGVD

MP3-D

M203-PZ

MP1-C

MP2-C

M103-PZ

-0.0004

-0.0003

-0.0002

-0.0001

0.0000

0.0001

0.0002

0.0003

0.0004

0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000

0

2

4

6

8

10

12

14

16

S10C-C

F1GW3

F4GW3

U3GW3

Gradient 8/19/97

Gradient 6/15/98

Water level 8/19/97

Water level 6/15/98

Gradient 8/19/97

Gradient 6/15/98

Water level 8/19/97

Water level 6/15/98

(A)

(B)

Figure 10. Water levels and horizontal hydraulic gradients along research transects in Everglades Nutrient Removal 

(ENR) project (A) and Water Conservation Area 2A (WCA-2A) (B), north-central Everglades, south Florida. Wet 

conditions (August 1997) and dry conditions (June 1998) are compared. A negative gradient indicates flow towards 

the northwest (ENR) or southwest (WCA-2A). See figures 2 and 3 for transect locations.
34 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

CELL 3

CELL 2

CELL 4

CELL 1

0.20

0.30

0.15

0.10

0.05

0.0

-0.05

-0.10

Discharge

Recharge

 Vertical Hydraulic 

Gradient (dimensionless)

WELL

EXPLANATION

LEVEES

CANALS

BUFFER

EAA

EAA

WCA-1

0

0

3,000 FEET

500 1,000 METERS

N

Miami

3A-15

WCA3B

WCA-3A

WCA2B

WCA2A

WCA-1

Figure 11. Distribution of vertical hydraulic gradients in Everglades Nutrient Removal Area (ENR), north-central 

Everglades, south Florida
Quantifying Recharge and Discharge in the North-central Everglades and Identifying the Key Controlling Factors 35

ENR. Unlike the L-7 levee, the Hillsboro levee contains spillways, called the S10 structures, that are 

opened periodically to release water from WCA-1 into 

WCA-2A. During the wet season, it is not uncommon 

for the S10 spillways to be opened, allowing drainage 

from WCA-1 into WCA-2A. When spillways open, 

water levels in the S10C headwater canal drop rapidly 

while water levels increase rapidly in the tailwater 

canal. Vertical hydraulic gradients at S10C temporarily 

decline by approximately a factor of 3 when the spillway is open (Harvey and others, 2000, p. 222).

Sites in the interior of WCA-2A had lower vertical hydraulic gradients compared with the S10C site 

(Harvey and others, 2000). Representative water levels 

and vertical gradients with time at site F1 are shown in 

figure 13. Surface water-level measurements and 

ground-water head measurements at interior sites typically tracked each other very closely, often differing by 

only hundreds of a foot. Many of the vertical gradient 

calculations are below 0.002, corresponding to approximately a 0.03-ft head difference. Three-hundredths of 

an inch is near the limit of maximum achievable accuracy in measuring vertical head differences in the Everglades, because of the fundamental limits of the 

equipment used. Because of this limitation, determining with confidence the direction of the vertical 

hydraulic gradient in the interior of WCA-2A sometimes was challenging. 

Despite the limitations on estimates of vertical 

gradients, patterns were apparent in vertical hydraulic 

gradients in interior areas of WCA-2A. The following 

interpretations are based on gradients between the shallowest instrumented layer in the aquifer (layer 2 instrumented with the GW-4 well) and surface water at all 

interior study sites. During the dry season, upward flow 

(discharge) was indicated on the western transect (F1 – 

F4 – U3) and neutral or possibly downward flow 

(recharge) was indicated on the E transect (E1 – E4 – 

U1) (fig. 3). During the wet season, there was greater 

variability in vertical hydraulic gradients. 

Surface-water and ground-water interactions in interior 

wetlands of WCA-2A appear to be affected most by 

occasional large water releases from WCA-1 after 

heavy rains near the end of the dry season. If those 

water releases are large enough to bring the water level 

in the tailwater canal above 13.5 ft (relative to 1929 

NGVD), surface water from the tailwater canal will 

overflow a berm and flow to the south through the 

WCA-2A wetland. Flow over the berm usually occurs 

at least once and often occurs two or more times per 

year (Harvey and others, 2000). Over a period of weeks 

to months following a water release, a wave of water 

initially1 to 4 ft high propagates southward through the 

wetland. As the wave peak passes a site on the transect, 

the direction of vertical ground-water flux reverses, 

from discharge to recharge (fig. 13).

Summary of Vertical Hydraulic Gradients

Vertical hydraulic gradients from both ENR and 

WCA-2A are greatest near levees within approximately 

0.5 km of levees. The direction of vertical gradients 

near levees generally was constant with time, although 

magnitudes varied by a factor of 2 or 3 depending on 

the surface-water level difference across the levee. Vertical hydraulic gradients near levees generally were 

smaller in WCA-2A than ENR (figs. 12 and 13). 

Because the water-level differences that drive flow 

beneath levees are similar at both sites, smaller vertical 

gradients probably are associated with the higher 

hydraulic conductivity (K) of the Surficial aquifer at 

WCA-2A (Harvey and others, 2000)

Vertical gradients were approximately 50 to 100 

times smaller in interior areas of the wetlands compared to sites near levees. Nevertheless, those fluxes 

are important to the basin-wide water balance because 

of the much larger areas of wetland involved. Recharge 

and discharge in interior areas of the wetlands are not 

primarily the result of ground-water flow beneath 

levees. In ENR, the greater than normal topographic 

and water-level slope toward the northwest (because of 

land subsidence and human engineering of the ENR 

wetland), exerts a major control over recharge in interior areas of ENR. The release of water from WCA-1 

into WCA-2A, a necessary part of water management 

in WCAs, also appeared to have an important effect on 

the direction of vertical flux in WCA-2A, causing relatively rapid reversals between recharge and discharge at 

research sites. 

Peat Hydraulic Properties

The overall mean estimate of K determined for 

Everglades peat was 26 cm/d or 0.84 ft/d (Harvey and 

others, 2000). That estimate of K is similar to that of 

very fine sand, but lower by approximately 2 orders of 

magnitude than the limestone and sand aquifer underlying the peat. That finding, and because the peat is laterally extensive, is the basis for concluding that peat is 
36 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

10.5

11.0

11.5

12.0

12.5

13.0

13.5

6/26/977/27/978/27/979/27/97

10/28/9711/28/9712/29/97

1/29/98

3/1/984/1/985/2/986/2/987/3/988/3/989/3/98

10/4/98

DATE

M102-SW

M102-P

M102-PZ

WATER LEVEL, IN FEET RELATIVE TO 1929 NGVD

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

6/26/977/27/978/27/979/27/97

10/28/9711/28/9712/29/97

1/29/98

3/1/984/1/985/2/986/2/987/3/988/3/989/3/98

10/4/98

DATE

VERTICAL GRADIENT, (DIMENSIONLESS)

M102-P to M102-Surface water

M102-PZ to M102-Surface water

M102-P to M102-PZ

Downflow

Upflow

(B)

(A)

Figure 12. Water levels (A) and vertical hydraulic gradients (B) at Everglades Nutrient Removal (ENR) project site 

M102, north-central Everglades, south Florida. Vertical gradients are calculated relative to each well and to surface 

water. A positive gradient indicates the potential for downward flow, also known as recharge.
Quantifying Recharge and Discharge in the North-central Everglades and Identifying the Key Controlling Factors 37

10.5

11.0

11.5

12.0

12.5

13.0

13.5

14.0

14.5

15.0

15.5

16.0

02/19/9704/20/9706/19/9708/18/9710/17/9712/16/9702/14/9804/15/9806/14/9808/13/9810/12/98

DATE

F1-GW3

F1-GW4

F1-Surface water

MISSING DATA

WATER LEVEL, IN FEET RELATIVE TO 1929 NGVD

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

0.020

02/19/9704/20/9706/19/9708/18/9710/17/9712/16/9702/14/9804/15/9806/14/9808/13/9810/12/98

DATE

F1-GW3 to F1-Surface water

F1-GW4 to F1-Surface water

F1-GW3 to F1-GW4

Downflow

Upflow

VERTICAL GRADIENT, (DIMENSIONLESS)

MISSING DATA

(B)

(A)

Figure 13. Water levels (A) and vertical hydraulic gradients (B) at Water Conservation Area 2A (WCA-2A) site F1, 

north-central Everglades, south Florida. Vertical gradients are calculated relative to each well and to surface water. A 

positive gradient indicates the potential for downward flow, also known as recharge.
38 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

the most important aquifer layer restricting vertical 

movement of water between ground water and surface 

water. The mean estimate of K in peat was higher for 

WCA-2A compared with ENR (43 and 8.1 cm/d or 1.4 

and 0.27 ft/d, respectively). Estimates of K in peat are 

plotted with depth at both sites in figure 14. Both sites 

indicated that K was higher at the bottom of the peat 

near the transition to sand or limestone. The average K

from the upper part of the peat at WCA-2A (30 cm/d or 

1 ft/d) was used in Darcy-flux calculations at 

WCA-2A, because that layer was more likely to control 

the vertical flux.

Measured Vertical Head Gradients 

Compared with Hydrogeologic Model 

Simulations

Factors affecting recharge and discharge were 

examined using a simple hydrogeologic model of 

ground-water flow beneath levees. An analytical solution from Barlow and Moench (1998) was used to compute the hydraulic head distribution in a "leaky aquifer" 

with a hydraulic restricting layer at the top (representing peat). One-dimensional (horizontal) flow through 

an aquifer with uniform hydrogeologic properties is 

assumed in the model, and allows for vertical leakage 

across the peat. The stress applied to the model was a 

sudden 1-m change in head at the left boundary (representing an increase in the water level of a canal that is 

separated from the wetland by a levee). Because of the 

one-dimensional flow assumption, the head at the left 

boundary of the aquifer (in contact with the canal) is 

equal to the canal water level and is constant with 

depth. That boundary condition is common to many 

models of interactions between surface water and 

ground water, and, in the present case, represents the 

hypothetical situation where the canal fully penetrates 

the aquifer (fig. 15a). A constant surface-water level in 

the wetland was simulated using the “source bed” 

option, which holds the hydraulic head constant at the 

top of the restricting layer. This option is the best way 

to simulate how surface water that discharged from the 

aquifer would quickly flow away from the levee vicinity toward the wetland interior, leaving surface-water 

levels in the vicinity of the levee approximately constant for the period of the simulation. The hydrogeologic parameters used in model simulations were as 

follows: aquifer depth –60 m, peat depth –1 m, K in 

aquifer –30 m/d, K in peat –0.3 m/d, and specific storage in both aquifer and peat –0.001 m.

The primary result of the hydrogeologic simulation was determining that the effect of ground-water 

flow beneath the levee extended only 0.5 km into the 

wetland (fig. 15b). Even in a simulation with a K value 

for peat that was two orders of magnitude smaller than 

measured values (approximately 0.3 m/d), the distance 

of interaction increased only by approximately a factor 

of 2. Beyond 0.5 km, the simulated vertical head gradient across the peat was 3 x 10-6 or less, which is much 

smaller than typical vertical hydraulic gradients actually measured in interior areas of WCA-2A (typically 

0.003 or higher, fig. 13). Even an improved 

two-dimensional model of ground-water flow beneath 

the levee probably would not change the observed mismatch between model and observations in the interior 

part of WCA-2A. Therefore, it was concluded that 

ground-water flow beneath levees is not an adequate 

explanation for a substantial part of the recharge and 

discharge that occurs in interior areas of WCA-2A. 

If water-level differences across levees and 

ground-water flow beneath the levee do not control 

recharge and discharge in interior areas of the wetlands, 

then what factors do? Other factors possibly controlling 

recharge and discharge may include fluctuations in surface-water levels in the wetlands. A constant 

surface-water level with no spatial or temporal changes 

in water-surface slope was assumed in the hydrogeologic model. Field observations discussed earlier indicated that surface-water levels and slopes change 

rapidly at times, because of water releases between 

WCAs. Surface-water slope in the wetland varies from 

essentially flat at certain times to a factor of 2 greater 

than the land-surface slope at other times. The effect of 

surface-water dynamics on interactions between surface water and ground water is discussed later as part of 

the interpretation of measured recharge and discharge 

fluxes.

Recharge and Discharge Estimates

Seepage-Meter Results from ENR

Seepage-meter measurements were made at 

ENR between August 1996 and April 1998. The absolute values of fluxes ranged from less than 0.04 cm/d 

(0.001 ft/d) to as much as 20 cm/d (0.66 ft/d). The typical uncertainty of a flux measurement was ±50 percent, 
Quantifying Recharge and Discharge in the North-central Everglades and Identifying the Key Controlling Factors 39

-250

-200

-150

-100

-50

0

0.01 0.1 1 10 100 1,000 10,000

HYDRAULIC CONDUCTIVITY, IN CENTIMETERS PER DAY

ENR (bail test)

ENR (Darcy flux)

Mean ENR 

(Darcy flux)

Mean ENR 

(bail test)

DEPTH BELOW PEAT SURFACE, IN CENTIMETERS

-250

-200

-150

-100

-50

0

0.01 0.1 1 10 100 1,000 10,000

HYDRAULIC CONDUCTIVITY, IN CENTIMETERS PER DAY

WCA-2A (bail test)

WCA-2A (Darcy flux)

Mean WCA-2A 

(Darcy flux)

Mean WCA-2A 

 (bail test) Mean WCA-2A-sand layer

(bail test) 

DEPTH BELOW PEAT SURFACE, IN CENTIMETERS

(B)

(A)

Figure 14. Hydraulic conductivity of wetland peat in Everglades Nutrient Removal (ENR) project (A) and wetland peat 

and underlying sand layer in Water Conservation Area 2A (WCA-2A) (B), north-central Everglades, south Florida.
40 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

AQUIFER

AQUITARD

SOURCE BED

IMPERMEABLE (NO-FLOW)

BOUNDARY

ho AND hc

,INITIAL AND FINAL 

CANAL STAGES

POTENTIOMETRIC HEAD IN

AQUIFER 

Z

X

b’

hc ho

0 x0

0.0

0.2

0.4

0.6

0.8

1.0

1.0 10.0 100.0 1000.0

DISTANCE FROM LEVEE, IN METERS

HEAD RESPONSE, IN METERS

KH = 30 meters per day

h(x,t)

b

Levee

KZ = 30 meters per day

KZ

 = 0.3 meter per day

KZ

 = 0.003 meter per day

KZ

 = 30, 0.3, 0.003 meters per day

EXPLANATION

(A)

(B)

xL

Figure 15. Hydrogeologic simulation of head change in aquifer capped with wetland peat responding to a 

change in water level at the left boundary (canal/levee complex). Schematic of simulated aquifer system (A), 

equilibrium head distribution after 1-meter rise in water level in canal at left boundary (B).
Quantifying Recharge and Discharge in the North-central Everglades and Identifying the Key Controlling Factors 41

based on replicate measurements from side-by-side 

seepage meters (Harvey and others, 2000). The lower 

end of that measurement range is the estimated detection limit of seepage meters at the study sites. The 

higher end of the measurement range is two orders of 

magnitude higher than average daily precipitation and 

evapotranspiration in the Everglades (Harvey and others, 2000). 

Seepage-meter measurements indicated that 

recharge occurred in most areas of ENR, which is consistent with the distribution of vertical hydraulic gradients (fig. 11). Time-averaged discharge and recharge 

fluxes on a transect across ENR are shown in figure 16. 

Fluxes were above detection at eight out of nine sites, 

and, for comparison, exceeded average annual precipitation (0.4 cm/d) at four out of nine sites.

All of the seepage-meter data at ENR over 

approximately a 2-year period are summarized and displayed graphically in table 14 and figure 17, respectively. The direction of vertical exchange at a site 

usually was constant over time, with recharge occurring at sites on the western side and in the interior and 

discharge occurring on the eastern side. The greatest 

contrast in fluxes was between sites near levees and 

sites in the interior of ENR. Fluxes near levees typically were two orders of magnitude higher than fluxes 

in the wetland interior. The observed pattern of 

decreasing fluxes with distance from levees is consistent with results from the hydrogeologic model used in 

this study, suggesting an exponential decrease in flux 

with distance from the levee (fig. 18). 

Seepage-meter measurements and hydrogeologic 

modeling agree that proximity to levees was the most 

important factor affecting the magnitude of recharge 

and discharge at ENR (fig. 18). Temporal variability in 

vertical fluxes generally was much smaller than spatial 

variability at ENR. Vertical fluxes at a location typically varied over time by less than a factor of 3, 

whereas fluxes varied spatially across ENR by over an 

order of magnitude. 

Darcy-flux calculations from WCA-2A

At WCA-2A, recharge and discharge fluxes were 

computed using Darcy’s law. Daily calculations were 

made for all sites in WCA-2A for the period January 

1997–December 1998. 

Discharge occurred on the tailwater side of the 

S10C levee site at WCA-2A. The median flux was 

approximately 2.5 cm/d, which is similar to the sites 

nearest levees in the ENR (table 15). However, fluxes 

were more variable at the S10C site compared with 

ENR. Greater variability in fluxes at the S10C site 

compared with levee sites at ENR is consistent with the 

greater variability in surface-water levels and vertical 

hydraulic gradients discussed earlier. The important 

difference with ENR is that, at WCA-2A, the major 

water supply is through a spillway. When opened, the 

S10C spillway lowers the local water level in WCA-1 

and raises water levels in WCA-2A in a matter of 

hours. In contrast, water is supplied to ENR by rapidly 

transporting water through a supply canal. Consequently, changes in the supply rate have less effect on 

the water-level difference between WCA-1 and ENR. 

Recharge in the interior areas of WCA-2A was 

greater by approximately a factor of 4 compared with 

ENR (0.2 and 0.05 cm/d, respectively). Larger vertical 

fluxes in the interior of WCA-2A are caused, at least in 

part, by the larger hydraulic conductivity of the peat at 

WCA-2A. On average, hydraulic conductivity of peat 

is approximately a factor of 6 greater in WCA-2A compared with ENR. Also important in explaining larger 

vertical fluxes at WCA-2A are the larger surface-water 

level fluctuations compared with ENR. Larger fluctuations in surface-water level increase the driving forces 

for recharge and discharge. Although greater than 

ENR, recharge and discharge in the interior of 

WCA-2A still are relatively small, even when compared with average annual precipitation or evapotranspiration (approximately 0.40 and 0.35 cm/d). 

Temporal variation in the interior fluxes at 

WCA-2A generally exceeded spatial variation, which 

is opposite of the result in ENR. Discharge occurred 

approximately 75 percent of the time at sites F1, F4, 

U3, and E4. Recharge occurred at site E1 approximately 75 percent of the time (table 15). Discharge and 

recharge each occurred approximately half the time at 

site U1. Fluctuations in recharge and discharge 

included reversals in flow direction at all sites except 

S10C (figs. 18 and 19). It was important, therefore, to 

characterize temporal variability of vertical fluxes at 

interior sites in WCA-2A. Flux percentiles for each site 

are shown in figure 19 and table 15. Over the period of 

the study, the vertical fluxes at most sites in the 

WCA-2A interior changed direction at least once. In 

general, recharge was more common at sites in the 

WCA-2A interior at the beginning of the 2-year period, 

and discharge was more common at the end (fig. 20). 

For sites E1, F1, and U3, the flux direction changed 

only once during the 2-year period. Flux direction 
42 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

0.01

0.1

1

10

M201 M206 M205 M203 M207 M103 M105 M106 M104

SITE NAME

Ground-water discharge

Ground-water recharge

West-side of ENR East-side of ENR

Detection limit

DISCHARGE OR RECHARGE, IN CENTIMETERS PER DAY

changed various times at sites E4 and F4. The flux 

direction changed frequently at site U1 (every 20 to 40 

days) on cycles that appeared to correlate with surface-water levels (fig. 20). 

There is no single method that is advisable for 

averaging vertical fluxes to account for spatial and temporal variability. For the purpose of a water balance 

(discussed in the next section), the approach used here 

was to average the median fluxes for "discharge" sites 

to characterize average discharge, and to do the same 

for "recharge" sites. The quotations mean that the definition of a discharge or recharge site depended on an 

ability to determine the dominant flux for a site, that is, 

where either discharge or recharge occurred at least 75 

percent of the time. Neither discharge nor recharge 

dominated at site U1. That site was, therefore, not considered in the average calculations. The result of averaging across remaining sites was a discharge flux of 

0.11 cm/d and a recharge flux of 0.18 cm/d. These 

results must be considered preliminary because data 

only were available for a relatively small number of 

sites. It is worth noting that other possible approaches 

to averaging the data set gave similar results, which 

provides some confidence in these preliminary estimates of discharge and recharge in WCA-2A.

Because water levels are always higher in 

WCA-1 compared to WCA-2A, it is clear that reversals 

between recharge and discharge are not consistent with 

the hypothesis that ground-water flow beneath levees is 

the main driving force. Instead, the results support the 

previous conclusions from hydrogeologic modeling, 

that ground-water flow beneath levees is effective 

mainly at distances less than 0.5 km from levees, and 

that vertical fluxes in the wetland interior are controlled 

by other processes. For example, regional gradients in 

water-table slope and surface-water level fluctuations 

are factors that could affect recharge and discharge in 

the wetland interior. The combination of those factors 

drives reversals between recharge and discharge at various timescales. For example, interannual rainfall variability, and its effects on regional water tables and 

water-resources management, drive multi-year fluctuations that explain the longest term trends in recharge 

and discharge. With only 2 years of data, however, 

Figure 16. Time-averaged discharge and recharge fluxes across transect A-A’ in Everglades Nutrient Removal 

(ENR) project, north-central Everglades, south Florida.
Quantifying Recharge and Discharge in the North-central Everglades and Identifying the Key Controlling Factors 43

Vertical Flux (cm/day)

Percentile M104 M106 M105 M103

10% 0.18 0.16 0.09 -0.21

25% 0.30 0.23 0.15 -0.12

50% 0.73 0.29 0.39 -0.03

75% 1.1 0.78 2.27 0.01

90% 1.3 4.7 4.7 0.02

Mean 0.760 1.2 1.5 -0.06

Max 3.145 8.4 8.4 0.13

Min 0.065 0.09 0.04 -0.32

n 110 43 40 37

Percentile M203 M205 M206 M201

10% -0.15 -0.36 -0.66 -12

25% -0.06 -0.17 -0.11 -9.6

50% -0.03 -0.07 -0.07 -3.3

75% -0.009 -0.05 -0.06 -2.3

90% 0.008 -0.02 -0.04 -1.1

Mean -0.05 -0.13 -0.14 -5.6

Max 0.22 0.19 -0.04 -0.20

Min -0.63 -0.66 -0.99 -22

n 53 43 14 68

Table 14. Time-averaged vertical fluxes at study sites in Everglades Nutrient 

Removal (ENR) project, north-central Everglades, south Florida, 1997-98. 

Discharge fluxes are positive and recharge fluxes are negative

[cm/d, centimeters per day; n, sample number]

Vertical Flux (cm/d)

Percentile S10C E1 E4 U1

10% 1.3 -0.27 -0.28 -0.73

25% 2.0 -0.18 0.00 -0.30

50% 2.8 -0.18 0.09 0.00

75% 3.3 0.00 0.28 0.38

90% 4.3 0.27 0.37 0.85

Mean 2.7 -0.10 0.07 0.01

Max 4.8 0.59 1.9 1.2

Min 0.18 -0.99 -1.7 -2.6

n 262 526 545 461

Percentile S10C F1 F4 U3

10% 1.3 -0.12 -0.14 -0.05

25% 2.0 0.00 0.00 0.00

50% 2.8 0.18 0.08 0.06

75% 3.3 0.35 0.23 0.12

90% 4.3 0.47 0.38 0.24

Mean 2.7 0.17 0.11 0.06

Max 4.8 0.70 0.84 0.56

Min 0.18 -0.35 -0.69 -0.46

n 262 533 516 451

Table 15. Time-averaged vertical fluxes at study sites in Water Conservation 

Area 2A (WCA-2A), north-central Everglades, south Florida, 1997-98. 

Discharge fluxes are positive and recharge fluxes are negative

[cm/d, centimeters per day; n, sample number]
44 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

0

10

20

30

40

50

60

70

80

90

100

-10.0 -8.0 -6.0 -4.0 -2.0 0.0 2.0 4.0 6.0 8.0 10.0

VERTICAL FLUX, IN CENTIMETERS PER DAY

CUMULATIVE PERCENT

M104

M105

M106

M201

S10C

Recharge Discharge

(B)

0

10

20

30

40

50

60

70

80

90

100

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5

VERTICAL FLUX, IN CENTIMETERS PER DAY

CUMULATIVE PERCENT

M103

M203

M205

Recharge Discharge M206

(A)

Figure 17. Cumulative distributions of vertical fluxes measured with seepage meters at sites in the wetland interior in 

ENR (A), and at sites near levees in ENR and WCA-2A (B). Note scale differences on x-axis (vertical flux).
Quantifying Recharge and Discharge in the North-central Everglades and Identifying the Key Controlling Factors 45

0.01

0.1

1

10

-100 100 300 500 700 900 1,100

DISTANCE FROM LEVEE, IN METERS

Westside recharge (mean)

Eastside discharge (mean)

Westside recharge (median)

Eastside discharge (median)

0.04 cm/d - Approximate seepage-meter detection limit

Vertical flux simulated by hydrogeologic

model 

DISCHARGE OR RECHARGE, IN CENTIMETERS PER DAY

those longer term trends are difficult to characterize 

except to indicate that long-term net recharge is correlated with multi-year periods of relatively wet conditions, whereas long-term discharge is correlated with 

multi-year periods of relatively dry conditions.

The present study showed that shorter timescale 

water-level fluctuations of weeks to months can control 

variations in recharge and discharge, including reversals in direction of fluxes. Water releases through spillways are important because those disturbances 

propagate as waves into the basin interior. Wave movement through the wetlands creates areas of high and 

low pressure over relatively short distances (hundreds 

of meters to kilometers). As discussed previously, 

recharge tends to occur beneath wave peaks whereas 

discharge tends to occur away from wave peaks. 

Another factor involved that was not previously discussed is the movement of pressure waves horizontally 

through the Surficial aquifer, in addition to wave propagation in surface water. Pressure waves travel at different speeds through different layers of the aquifer, 

causing simultaneous upward discharge and downward 

recharge from the sand layer beneath the peat. That 

process was most clear at the E4 site, where vertical 

hydraulic gradients indicate that the sand layer acts as a 

source of water that simultaneously discharges to surface water and recharges to the underlying limestone 

aquifer at times of low water in WCA-2A (Harvey and 

others, 2000, p. 225). 

Seepage-meter Results from WCA-2B and WCA-3A

Seepage meters at the one study site in WCA-2B 

indicated persistent recharge over time. At site 

WCA-3A15, neutral conditions or slight recharge was 

indicated. Only seepage-meter data were available 

from those sites; therefore, interpretations are made 

without the benefit of vertical head gradients to compare for consistency at other sites. Seepage-meter measurements were spaced far apart in space and time in 

those areas because of the remoteness of these study 

sites. Consequently, evidence for short-term reversals 

in vertical fluxes at those sites could not be thoroughly 

evaluated. Nevertheless, the results from those two 

Figure 18. Observed discharge and recharge and simulated fluxes in Everglades Nutrient Removal (ENR) project, 

north-central Everglades, south Florida. Both mean and median fluxes are plotted for each study site.
46 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

0

10

20

30

40

50

60

70

80

90

100

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

VERTICAL FLUX, IN CENTIMETERS PER DAY

CUMULATIVE PERCENT

F1

F4

U3

Recharge Discharge

(B)

0

10

20

30

40

50

60

70

80

90

100

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

VERTICAL FLUX, IN CENTIMETERS PER DAY

CUMULATIVE PERCENT

E1

E4

U1

Recharge Discharge

(A)

Figure 19. Cumulative distributions of vertical fluxes calculated by Darcy’s law on Eastern transect (A), and Western 

transect (B), in Water Conservation Area 2A (WCA-2A), north-central Everglades, south Florida.
Quantifying Recharge and Discharge in the North-central Everglades and Identifying the Key Controlling Factors 47

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

01/31/9704/01/9705/31/9707/30/9709/28/9711/27/9701/26/9803/27/9805/26/9807/25/9809/23/9811/22/98

DATE

F1

F4

U3

Recharge

Discharge

VERTICAL FLUX, IN CENTIMETERS PER DAY

(B)

-1.0

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1.0

01/31/9704/01/9705/31/9707/30/9709/28/9711/27/9701/26/9803/27/9805/26/9807/25/9809/23/9811/22/98

DATE

Recharge

Discharge

VERTICAL FLUX, IN CENTIMETERS PER DAY

E1

E4

U1

(A)

Figure 20. Vertical fluxes observed over a 2-year period on the Eastern transect (A), and Western transect (B), in 

WCA-2A, north-central Everglades, south Florida.
48 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

sites are valuable because they suggest a time-averaged 

trend of recharge at a site in WCA-2B referred to as 

2BS, and fluxes that are below detection or slightly on 

the side of recharge at a site in WCA-3A referred to as 

3A-15 (fig. 1). 

Water Balance

An important extension of this study to estimate 

recharge and discharge was to average those measurements over space and time for inclusion in basin-wide 

water balances for ENR and WCA-2A. At ENR, it was 

relatively straightforward to develop those estimates 

(Choi and Harvey, 2000), in part because of the high 

density of measurements and partly because multiple 

independent approaches proved useful and could be 

compared. Developing basin-scale estimates of 

recharge and discharge at WCA-2A was much more 

difficult, mostly because the density and frequency of 

water-level and chemical measurements were much 

less than at ENR. To develop these estimates at 

WCA-2A, the results of a 30-year hydrologic budget 

for WCA-2A developed by South Florida Water Management District using the SFWMM were consulted 

(SFWMD, C&SF Restudy, http:\\www.sfwmd.gov). 

The present study uses new data as well as previous 

information to develop recharge and discharge estimates in ENR and WCA-2A (table 16). Discrepancies 

are noted between new information and previous work, 

such as new estimates of vertical fluxes in the interior 

of WCA-2A that exceed previous estimates of total 

recharge and discharge in WCA-2A. These discrepancies were expected because differing methods and different periods of data record were used in each study. 

Those discrepancies are discussed and, where possible, 

resolved, in the remainder of this section. 

Recharge was the dominant interaction between 

the wetland and ground water at ENR when averaged 

over the entire basin and over the 4-year study period. 

Total recharge averaged 0.9 cm/d and discharge was 

0.1 cm/d (Choi and Harvey, 2000). Expressed as a percentage of water inflows, those fluxes are 31 and 3 percent, respectively, of surface water pumped into ENR 

for treatment. Discharge at ENR is ignored in further 

discussion because recharge dominated vertical fluxes. 

Ground-water flow beneath levees accounted for 

approximately 94 percent of the total recharge flux in 

ENR. Choi and Harvey (2000) showed that approximately 73 percent of the total recharge was discharged 

to the seepage canal. The seepage canal collected a 

mixture of shallow ground-water flow beneath levees 

and deep ground water flowing vertically to the surface. At sites greater than 0.5 km from the western and 

northern levees, recharge also occurred but at much 

lower rates. The average recharge at interior sites was 

approximately 0.05 cm/d, which accounted for only 

about 6 percent of the total recharge in ENR (table 16). 

Having accounted for a total of 79 percent of all 

recharge in ENR, the remaining 21 percent was 

inferred to have bypassed the seepage canal, probably 

discharging instead in nearby agricultural land. Eventually, that water would have discharged to a canal at 

another location in the EAA, to be evapotranspired or 

pumped away through a different system of canals.

Recharge varied temporally in ENR. Whereas 

flow in the seepage canal was almost constant, total 

recharge varied over approximately a factor of 2, correlating positively with surface-water levels in ENR 

(Choi and Harvey, 2000). For that reason, Choi and 

Harvey (2000) concluded that recharge was controlled 

mainly by water management rather than other factors 

(such as seasonal or interannual variation in precipitation). 

In WCA-2A, recharge also was the dominant 

interaction between the wetland and ground water. 

Averaged over 2 years throughout the entire basin, a 

recharge of 0.2 cm/d and discharge of 0.1 cm/d was 

estimated for the WCA-2A interior, which amount to 

31 percent and 15 percent of average surface-water 

fluxes through WCA-2A, respectively (table 16). Using 

the SFWMM, researchers at SFWMD estimated 

recharge in WCA-2A to be 0.04 cm/d and discharge to 

be 0.02 cm/d, equal to 6 and 3 percent of surface-water 

inflow to WCA-2A, respectively. Lower values of 

recharge and discharge from SFWMD model simulations may be the result of the larger spatial scale and 

longer temporal scale of averaging inherent in their 

methods. The present research suggests that SFWMD 

estimates are minimum estimates, and that recharge 

and discharge in WCA-2A each are approximately a 

factor of 5 larger. It is important to keep in mind that 

the difference among various estimates of recharge and 

discharge is not a reflection on the relative value of different approaches—each has its merits. The detailed 

estimates of recharge and discharge developed by the 

present study are important to shorter term and smaller 

scale water-balance studies, and studies concerned with 

water quality. The estimates of recharge and discharge 

reported here are based on relatively short data sets and 
Use of Geochemical Tracers to Identify Sources and Pathways of Water Flow in the Everglades 49

must be considered uncertain until a longer term data 

set is available. The larger scale water-balance work at 

SFWMD is well established and has primary importance to regional flow estimation. More detailed comparisons between the SFWMM and the measurements 

of the present report should be encouraged.

The main factor that caused recharge and discharge fluxes to increase at ENR relative to 

pre-drainage conditions was land subsidence, which 

was near its maximum in terms of the effects on the 

land slope in that vicinity. Another factor affecting 

recharge in ENR is the relatively large ratio of levee 

perimeter to wetland surface area (4 x 10-4 ft/ft2) compared with WCAs. That ratio was a factor of 6 smaller 

in WCA-2A (6 x 10-5 ft/ft2), suggesting a higher contribution of ground-water flow beneath levees per unit 

area of wetland. As discussed earlier, vertical fluxes 

within 0.5 km of levees usually were higher than 0.3 

cm/d, whereas fluxes in the wetland interior typically 

were less. As a result, ground-water flow beneath 

levees accounted for 94 percent of recharge at ENR 

(whereas vertical fluxes in the wetland interior were 

thought to dominate in WCA-2A), and basin-averaged 

recharge was almost 5 times larger in ENR compared 

to WCA-2A (table 16). 

The overall result of land subsidence and 

water-resources management in the north-central Everglades appears to be a general pattern of decreasing 

interactions between surface water and ground water, 

from areas where they are highest in the relatively 

small basins at the northern boundary (ENR and 

STAs), to the center of the Everglades’ largest enclosed 

basin (WCA-3A). The greatest recharge and discharge 

fluxes were observed in ENR, and fluxes decreased 

toward the interior areas of WCA-3A (fig. 21). The 

increasing size of WCAs to the south is accompanied 

by a decreasing ratio of levee length to wetland surface 

area, which is an important reason why recharge and 

discharge decrease toward WCA-3A. 

In contrast to basin-averaged recharge and discharge, fluxes in the wetland interior increased toward 

the south. Greater vertical fluxes in the wetland interior 

of WCA-2A (compared with ENR) are at least partly 

the result of greater driving forces associated with 

surface-water fluctuations in WCA-2A. Higher hydraulic conductivity of the peat in WCA-2A, compared 

with ENR, also contributed to higher vertical fluxes in 

the interior of WCA-2A. 

USE OF GEOCHEMICAL TRACERS TO 

IDENTIFY SOURCES AND PATHWAYS OF 

WATER FLOW IN THE EVERGLADES 

Attempts to manage water quality in the Everglades over the past 40 years generally have proceeded 

with little regard to the close hydrologic connection 

between surface water and ground water. Seepage of 

substantial quantities of Everglades surface water 

beneath levees was first quantified in the 1960s, and 

was shown to be an important factor depleting the 

water supply to Everglades National Park during dry 

periods. In contrast, the effect that seepage has on 

water quality has not been as widely considered. 

Although the variable sources of water inputs to the 

Everglades (for example, precipitation, agricultural 

runoff, Lake Okeechobee drainage) have been well 

documented, the chemical quality of those inflows is 

not always well known. In addition, the potential for 

ground water to function as a storage reservoir for 

Site

Total Basin Flux (cm/d) Interior Basin Flux& (cm/d)

Recharge Discharge Recharge Discharge

ENR 0.9 a (31)* 0.1 a (3) 0.05 c (6) B.D.c (–)

WCA-2A 0.04 b (6) 0.02 b (3) 0.2 c (31) 0.1c (15)

a Average for August 1994 to August 1998, from Choi and Harvey (2000).

b Average for 1965 to 1995, from South Florida Water Management District, http://www.sfwmd.gov.

c Average for February 1997 to October 1998, this study.

* Numbers in parentheses indicate percentage of surface-water inflow.

& Interior basin refers to all wetlands in basin farther than 0.5 kilometer from a levee

Table 16. Basin-scale estimates of recharge and discharge in Everglades Nutrient Removal 

(ENR) project and Water Conservation Area 2A (WCA-2A), north-central Everglades, south 

Florida 

[cm/d, centimeters per day; B.D., below detection; –, not applicable]
50 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

surface-water contaminants, or as a site of biogeochemical reactions that affect contaminants, generally is not well understood. A thorough investigation 

will require study of the vast interior areas of the Everglades, in addition to the wetlands in close proximity to 

levee boundaries.

One reason that interactions between surface 

water and ground water are poorly understood in the 

Everglades is because the interactions are difficult to 

estimate or simulate. Measuring surface-subsurface 

interactions in wetlands is always a challenge, both 

because of the difficulties of deploying hydrological 

instrumentation in wetlands, and because the fluxes 

involved generally are small enough to challenge the 

precision of the readily available methods. For those 

reasons, hydraulic measurements alone may be inadequate to characterize interactions between surface 

water and ground water in the Everglades. The vast 

area of the Everglades, however, ensures that even 

small fluxes between wetland surface water and ground 

water potentially are of great importance to chemical 

budgets and water quality. Geochemical measurements, 

themselves, offer an alternative tool for researchers that 

potentially can supplement hydraulic measurements, 

revealing long-term patterns of interactions between 

surface water and ground water, and resulting effects 

on water quality.

Approach

Variability in the geochemical composition of 

natural waters often can be used to identify distinct 

sources of water and mixing among those source 

waters. Dominant water sources and the extent of mixing are quantified on the basis of chemical similarity to 

various endmembers. Endmembers represent the 

unmixed source waters at a field site. Usually, the endmembers can be estimated from samples that show the 

extremes of variation in chemical composition. In the 

case of the Everglades, some important endmembers 

include deep ground water, wetland surface water, and 

Figure 21. Spatial trends in discharge and recharge in north-central Everglades, south Florida. Time-averaged 

precipitation, evapotranspiration, and surface flows in Everglades Nutrient Removal (ENR) project and Water 

Conservation Area 2A (WCA-2A) are plotted for comparison.

-4

-3

-2

-1

0

1

2

3

4

ENR - M104 2A-F1 2A-U3 2B-S 3A-15

Time-averaged median

25th percentile

75th percentile

ENR - M201

-9.6

0.0 0.0

DISCHARGE OR RECHARGE, IN CENTIMETERS PER DAY

Average annual precipitation

Average annual evapotranspiration

Average surface flow in ENR 

Average surface flow in WCA-2A 

Recharge Discharge

North South
Use of Geochemical Tracers to Identify Sources and Pathways of Water Flow in the Everglades 51

precipitation. Water samples with intermediate 

geochemical composition often reflect mixing between 

endmembers. The variability in geochemical composition of ground water also can represent differences in 

geochemical reactions between ground waters and the 

aquifer sediments that contain them, in addition to mixing of waters from different sources. In a given situation the ideal geochemical tracer provides information 

about flow paths and flow velocities, in addition to 

information about water source, mixing, and geochemical reactions. Depending on the aquifer system and the 

tracer, the relevant travel times that might be revealed 

include months, years, decades, centuries, or millennia.

Geochemical data from Everglades surface water 

and ground water are presented here to determine 1) 

the dominant sources of ground water and surface 

water in the north-central Everglades, and 2) locations 

of recharge and discharge and extent of vertical mixing 

between surface water and ground water. The present 

study used major ions and water-stable isotopes to 

delineate source waters and extent of mixing between 

surface water and ground water in the Everglades. A 

detailed description of the approaches used and methods is given in Harvey and others (2000).

Source of Ground-Water Recharge 

Water-stable isotopes were one of the geochemical tracers used to determine the source of water to the 

Surficial aquifer beneath the north-central Everglades. 

The isotopes showed that recharge of surface water 

from the wetlands was the dominant source of ground 

water as opposed to infiltration of rainfall through the 

unsaturated zone of a nearby upland area or seasonal 

wetland. Most samples of Everglades surface and shallow ground water suggest a substantial effect of evaporation. The primary evidence for evaporation is isotopic 

ratios of oxygen (δ18O) and hydrogen (δ2H) in ground 

water that are most similar to Everglades surface water. 

Similar to Everglades surface water, these ground 

waters are isotopically "heavy" in comparison with precipitation, and enriched in δ18O relative to δ2H according to a ratio that indicates significant evaporation of 

water prior to recharge (fig. 22). Precipitation or 

recharge water that did not experience significant evaporation would be isotopically "lighter," without the 

strong evaporation signature. Only direct recharge of 

ground water from substantially evaporated surface 

water in the wetlands can explain the stable isotope 

geochemistry of Everglades ground water collected at 

most sites. 

There are a few exceptions to the evaporative signature of Everglades surface and ground waters. In surface water, these exceptions occurred in WCA-2A after 

time periods of heavy rainfall when water levels were 

relatively low. For a brief period following those heavy 

rains, surface water was similar to precipitation in 

terms of having a relatively light water-stable isotopic 

composition. 

 Some shallow ground waters beneath central 

ENR also had the isotopically "light" stable isotopic 

composition. Wells M203-PZ, M103-PZ, M401-P, 

MP2-C and M203-P all had isotopic compositions that 

were closer to precipitation than to typical surface 

waters and ground water at nearby sampling locations. 

These shallow ground waters beneath central ENR 

appear to have been recharged by rapid infiltration of 

precipitation on farm fields of the Everglades Agricultural Area (EAA) prior to construction of the ENR wetland. Recharge through the unsaturated agricultural 

fields would have occurred quickly without sufficient 

time for evapotranspiration to occur. Because the ENR 

treatment wetland has been flooded continuously since 

late 1993, that recharge would need to have occurred 

prior to 1993, during the time of agricultural management in that location.

Distinguishing Sources of Recharge Using 

Major-ion Composition

Major-ion chemistry was useful to identify some 

of the more important sources of water to Everglades 

surface and ground water. Analyses from a sampling 

event in December 1997 are shown as points on a Piper 

diagram in figure 23. Piper diagrams show the effects 

of various factors, including major-ion composition of 

possible source waters, as well as the proportions of 

mixing between those source waters in samples. The 

effects of geochemical interactions between water and 

soil or aquifer material also may be indicated on a 

Piper diagram. Possible source waters, or endmembers, 

are identified on Piper diagrams by water samples that 

plot at the extreme edges of the sample distribution. 

Waters that are mixtures between two endmembers are 

identified as samples that plot on lines connecting the 

endmembers. Often, more than two endmembers are 

present, complicating what would otherwise be a simple mixing analysis. Even if only two endmembers are 
52 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

-25

-20

-15

-10

-5

0

5

10

15

20

25

-5 -4 -3 -2 -1 1 0 2 3 4 5

δ18O

Surface water-WCA-2A

Ground water-WCA2A

Precipitation

Ground water-ENR

Surface water-ENR

Global meteoric water line

δ 2H

Figure 22. Water-stable isotopic ratios δ 2H and δ18O from precipitation, surface water, and ground water in 

Everglades Nutrient Removal (ENR) project and Water Conservation Area 2A (WCA-2A), north-central Everglades, 

south Florida. Most isotopic values from ground water are relatively heavy (positive) on a trend with lower slope 

than the global meteoric water line, suggesting that appreciable evaporation occurred from that water. In contrast, 

precipitation and many surface-water samples are relatively light in isotopic value. Only a small group of 

ground-water samples are relatively light in isotopic value (negative). See text for interpretation.

likely to be present, accurately classifying those endmembers still may be difficult because of spatial and 

temporal variability of chemistry at a limited number of 

sampling sites.

There is no single widely accepted method of 

classifying water types by their major ion composition. 

For the present study, available classifications of water 

types in the Everglades were considered first. Howie 

(1987) used Stiff diagrams to classify ground waters in 

the southern part of the study area in Broward County. 

Upchurch (1992) suggested a descriptive classification 

of natural waters in Florida based on Piper diagrams, 

using the separate triangular graphs for cations and 

anions. Frazee (1982) developed a more specialized 

interpretive classification of south Florida waters using 

the combined cation and anion graph on Piper diagrams. In this study, the keys for both the Upchurch 

(1992) and Frazee (1982) classifications were used to 

prepare the Piper diagrams presented here (fig. 23).

Calcium and/or sodium generally dominate the 

cation composition of south Florida waters. The anion 

composition is dominated by bicarbonate and/or chloride ions. If a third cation (for example, magnesium) or 

anion (for example, sulfate) is important, the sample is 

referred to as "mixed" in its cation or anion composition. Using the descriptive classification scheme of 

Upchurch (1992), along with water-stable isotopic 

data, nine major water types relevant to the study area 

were delineated (fig. 23):

A1 – calcium bicarbonate water

F1 – calcium-sodium bicarbonate water 

F6 – calcium-sodium bicarbonate-chloride water

E5 – sodium-chloride water 

E6 – sodium chloride-bicarbonate water
Use of Geochemical Tracers to Identify Sources and Pathways of Water Flow in the Everglades 53

B2 – calcium-magnesium bicarbonate-sulfate 

water

G1 – mixed-cation bicarbonate water

G1a – mixed-cation bicarbonate water with 

“light” stable isotopic signature

G6 – mixed-cation bicarbonate-chloride water

Background - Source and Transitional Waters

Interpretations of water sources to ENR and 

WCA-2A are summarized briefly here. Interpretations 

follow those of Frazee (1982) or are slight modifications thereof. Terms used to classify the waters for this 

study that were defined by Frazee (1982) are italicized.

Water types A1, F1, E5, and B2 are the relevant 

source waters in the north-central Everglades. Type A1 

waters are Fresh Recharge Waters derived from rainfall 

and interaction with aquifer sediments, principally 

limestone. Type F1 waters are similar to A1 waters but 

are higher in ionic strength and have a greater importance of sodium either because of different aquifer sediments, greater reaction times, or mixing with different 

water. Type E5 waters are Relict Seawater derived 

from seawater intrusion into the fine sands of the bottom half of the Surficial aquifer during an earlier geologic time. The term Relict Seawater, therefore, has the 

same meaning as Frazee’s term Lateral Inflow. Type B2 

waters are Fresh Formation Waters derived from Fresh 

Recharge Waters that have much longer contact times 

with soils and aquifer sediments. Consequently, B2 

waters have a greater effect of magnesium and sulfate 

in their ion makeup. 

Transitional Water is a general term used by 

Frazee (1982) to refer either to waters that are evolving 

by geochemical reactions with aquifer sediments, or 

waters that changed their geochemical character by 

mixing with other geochemically distinct waters. 

The most frequent classification of water in ENR 

and WCA-2A were F6 waters, followed by G6, F1, E5, 

E6, and G1 waters. Many of those waters are the product of mixing among two or more endmembers. For 

example, the most common water type was F6, which 

is a mixture between Fresh Recharge Waters (F1) and 

older Relict Seawater from the base of the aquifer (E5). 

The progressively saltier mixtures along the mixing 

continuum are F1, F6, E6, and, finally, E5 waters. 

Another important mixed water type was G6, which 

shares a fresh-salt mixture in common with F6 and E6 

waters, but also has inputs of magnesium and sulfate 

from mixing with B2 waters. A good example of Transitional Waters that evolved over centuries or millennia 

in contact with aquifer sediments, with little mixing 

with other waters, are B2 waters. B2 waters are otherwise known as Fresh Formation Waters. 

 Fresh Recharge Water

Water type A1 is freshwater that originated as 

rainfall-derived recharge to the aquifer. Type A1 water 

interacted with peat sediments and sand and carbonate 

layers of the Surficial aquifer over a relatively short 

period of geologic time (decades to centuries). Dissolution of limestone was the most important water-rock 

interaction affecting the chemistry of A1 waters. Type 

F1 waters are similar to A1 waters except that F1 

waters have a higher ionic strength and more importance of sodium, which reflects either exposure to different aquifer sediments, more prolonged contact time, 

or a minor amount of mixing with Relict Seawater. 

Frazee (1982) refers to both A1 and F1 water types as 

Fresh Recharge Water. 

A1 waters are represented at the study sites by 

shallow ground waters sampled from wells along the 

Atlantic Coastal Ridge. Surface water and shallow 

ground water from WCA-2A and ENR are F1 or F6 

waters, because of a greater contribution from sodium 

and/or chloride ions. Whereas A1 waters probably represent waters recharged through unsaturated sediments 

of sandy coastal ridges, F1 waters are more likely 

derived from recharge of rainfall that fell directly onto 

the wetlands themselves. Because water-stable isotopes 

indicated that Everglades ground water principally was 

derived from recharge of wetland surface water, F1 

waters appear to be a much more important source of 

Fresh Recharge Water to the Everglades than A1 

waters.

A good model for the source of F1 waters to the 

study sites is water recharged within the 

rainfall-dominated wetlands of WCA-1. Wetlands outside of WCA-1 have other sources of water, such as 

deep ground-water discharge from the Surficial aquifer, 

drainage from Lake Okeechobee and from the Everglades Agricultural Area (EAA), with distinctly different geochemical signatures. The first of those other 

source waters discussed is discharge of Relict Seawater

from the lower half of the Surficial aquifer. 
54 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

EXPLANATION

ENR SURFACE WATER

ENR GROUND WATER

LAKE OKEECHOBEE 

SURFACE WATER

ATLANTIC COASTAL 

RIDGE GROUND WATER

EVERGLADES AGRICULTURAL 

AREA SURFACE WATER AND 

GROUND WATER

FW-I

FW-II

TCW

FW-III

FW-IV

TW-I

TW-II

TRSW

RSW

CW

FW-I

FW-II

TCW

FW-III

FW-IV

TW-I

TW-II

CW

A

B

C

D

E F

G

1 6

2

3

4

5

7

TRSW

RSW

0 0

100

CALCIUM

CATIONS

80 60 40 20

A

B

C

D

E F 0

20

40

60

60

80

80

100

100 0

100

40

20

0

40

20

60

80

60

80

40

20

6

1

2 4

5

7

CHLORIDE

ANIONS

0

20

40

60

80

80

60

40

20

100

100

100 80 60 40 20 0

0

80

60

40

20

20

40

60

80

SODIUM + POTASSIUM

CALCIUM + MAGNESIUM

SULFATE

CARBONATE + BICARBONATE

SULFATE + CHLORIDE

0 0

100

100

40

20

60

80

60

80

40

20

CHLORIDE

ANIONS

0

20

40

60

80

100

100

100 80 60 40 20 0

0

80

60

40

20

20

40

60

80

SODIUM + POTASSIUM

CALCIUM + MAGNESIUM

SULFATE

CARBONATE + BICARBONATE

SULFATE + CHLORIDE

MAGNESIUM

CALCIUM

CATIONS

80 60 40 20 0

20

40

60

60

80

80

100

100 0

100

40

20

0

MAGNESIUM

3

80

60

40

20

100

EXPLANATION

WCA-2A SURFACE WATER

WCA-2A GROUND WATER

LAKE OKEECHOBEE 

SURFACE WATER

ATLANTIC COASTAL 

RIDGE GROUND WATER

G

(A)

(B)

EVERGLADES AGRICULTURAL 

AREA SURFACE WATER AND 

GROUND WATER

Figure 23. Piper diagrams illustrating variability in major ion composition of surface and 

ground waters from Everglades Nutrient Removal (ENR) project (A), and Water Conservation 

Area 2A (WCA-2A) (B), north-central Everglades, south Florida. See text for explanation of 

classifications.
Use of Geochemical Tracers to Identify Sources and Pathways of Water Flow in the Everglades 55

Relict Seawater

Type E5 water is high ionic strength water that is 

dominated by the ion signature of seawater; for example, sodium and chloride. In surface water and ground 

water of the north-central Everglades, this signature is 

acquired by mixing with a large component of Relict 

Seawater that originally intruded the fine sands in the 

lower half of the Surficial aquifer during a geologic 

time of a relatively higher sea-level stand. E5 waters 

mostly are represented by the deep ground waters of 

the Surficial aquifer (60-200 ft deep), but E5 waters 

also are found at shallow depths in the aquifer (less 

than 60 ft deep), and on the wetland surface, because of 

mixing caused by upward discharge of Relict Seawater

from the aquifer to the wetland surface. The presence 

of E5 waters in surface water and shallow ground water 

attests to the importance of deep vertical mixing 

between surface water and ground water in some locations. Near levees, vertical mixing is especially deep, 

extending through at least the top half of the Surficial 

aquifer.

Fresh Formation Water

Water type B2 has greater effects of magnesium 

and sulfate in its ion composition, often because of 

longer periods of weathering in contact with soils and 

aquifer sediments (compared with other water types). 

Frazee (1982) refers to the water type as Fresh Formation Water, with long contact times being the main difference separating this water type from the Fresh 

Recharge Water dominated by calcium bicarbonate. 

Water type B2 is believed to be present in ground water 

beneath Lake Okeechobee and in areas surrounding 

Lake Okeechobee, including the Everglades Agricultural Area (EAA) (Frazee, 1982). Even longer contact 

times of B2 waters in the aquifer eventually would produce Connate Water (for example, classification C or D 

for cations and 3 or 4 for anions), a saltwater distinctly 

different from marine water in its ion balance. Connate 

Water also is believed to be present in ground water 

beneath Lake Okeechobee (Frazee, 1982). 

Mixed Waters and Other Transitional Waters

Many of the ground waters from ENR and 

WCA-2A plotted on the Piper diagram on a mixing line 

between F1 and E5 waters. This mixing line represents 

the continuum between rainfall-driven recharge of wetlands in the Everglades (F1), and Relict Seawater (E5). 

Another group of water samples, mostly surface 

waters and a few ground waters, had a mixed cation 

signature (for example, a G classification for cations), 

because of either mixing between water types or longer 

weathering times in the aquifer. For example, mixing of 

F6 type water with Fresh Formation Water or Connate 

Water probably accounts for classification of Lake 

Okeechobee water as G6 type water. The presence of 

Fresh Formation Water in ground water of the Lake 

Okeechobee and EAA is evident from water analyses 

reported in Parker and others (1955). Those water samples were collected in the 1940s from ditches, canals, 

and ground waters in the EAA. The range of those samples on the combined cation and anion portion of a 

Piper diagram is shown as a trapezoidal shaded area in 

figure 23.

A number of surface waters and shallow ground 

waters had the mixed cation signature of G6 or G1 

waters. Because G6 waters only were found in surface 

water or shallow ground water, it was concluded that 

these waters are mixtures of various waters, such as 

A1, F1, E5, B2, and possibly Connate Water. Lake 

Okeechobee contains G6 type water, making it possible 

that some of the G6 waters from ENR and WCA-2A, 

most likely surface water, could be predominantly 

drainage water from Lake Okeechobee. In contrast, G1 

waters were found both very shallow (15 ft) and relatively deep (60 ft or greater). The deeper G1 waters 

evolved by geochemical interactions with aquifer sediments. That geochemical change involved a transition 

from calcium-sodium bicarbonate water (F1) to calcium-sodium bicarbonate-chloride water (F6) and, 

finally, to G1 water with proportionately greater importance of magnesium (and sometimes sulfate) compared 

with G6 waters. The shift in anion composition usually 

is not as great as the cation shift, possibly because of 

bacterial sulfate reduction that removes sulfate from 

solution. 

Most of the shallow G-1 ground waters have the 

unique “light” stable isotopic composition that was 

interpreted earlier in this section as a signature of 

rainfall-driven recharge through agricultural soil. The 

mixed ion classification of that water may, in part, be 

the result of chemical inputs associated with agricultural fertilizer (Bates and others, 2002). For that reason, 

a specialized classification was created for shallow G1 

waters called G1a, which uniquely associates the water 

with rapid recharge of precipitation that fell on wetlands converted for agricultural use. 
56 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

Geochemical Tracing of Surface-Water and 

Ground-Water Interactions 

It was believed that geochemical data would help 

delineate ground-water discharge and recharge in the 

north-central Everglades. Geochemical tracers did support previous hydraulic analysis by identifying 

recharge in WCA-1 and flow beneath the levee as an 

important source of ground water to the aquifer beneath 

ENR and WCA-2A. Support for the above interpretation comes from cross-sectional plots of specific conductivity and water type (figs. 24 and 25). Those plots 

show that Fresh Recharge Water (Type-F1) has been 

recharged to substantial depths (60 ft or more) beneath 

the levees separating the study sites from WCA-1. This 

observation is consistent with hydraulic interpretations 

that water ponded at higher elevations on the WCA-1 

side of the levee drives recharge and flow beneath the 

levee.

Geochemical data provide a long-term integrated 

view of levee underflow. Of particular value is the 

information about the depth of underflow that would be 

difficult to discern from hydraulic data alone. At 

greater depths beneath the levee (100 ft) is Fresh Formation Water (type G1) that also is derived from freshwater recharge, but which has had considerably longer 

contact time with aquifer sediments. 

Ground-water geochemistry changes rapidly 

with distance away from the levee, with Relict Seawater present in the aquifer at elevations well above the 

fine sands in the bottom half of the Surficial aquifer 

(below 100 ft) where it was once thought to be 

restricted (fig. 25). Upward movement of Relict Seawater from the bottom half of the Surficial aquifer is 

caused by ground-water underflow beneath the levees. 

The potential energy that drives recharge also drives a 

corresponding discharge flux on the down gradient side 

of the levee. Those flow paths are referred to as "underflow" paths. The deepest of those underflow paths are 

deep enough to have contacted the fine sands that contain the relict saltwater, mixing with the saltwater and 

advecting some of the salts upward toward the wetland 

surface. As a result, the presence of Relict Seawater

(type E5) or Transitional Relict Seawater (type E6) 

delineates zones of discharge. In WCA-2A, the zone of 

discharge extends for 2 mi on the down gradient side of 

the levee. In ENR, the zone of discharge that is delineated by relict seawater does not occur directly down 

gradient of the levee, but instead is more centralized 

beneath the 2.5-mi-wide ENR wetland.

Are sea salts from the deeper aquifer actually 

discharged to surface water? Two surface-water samples from ENR are classified as Transitional Relict 

Seawater (type E6), indicating that relict sea salts discharge to surface water. The best example may be discharge to the seepage canal on the western side of 

ENR. The purpose of the seepage canal is to collect 

ground water that recharges within the ENR treatment 

wetland and return it to the wetland. The seepage canal 

not only captures ground water recharged in ENR but 

also ground-water discharge from the deeper parts of 

the aquifer. In contrast to ENR, discharge of relict sea 

salts to the wetland surface is not indicated by analysis 

of surface-water samples in WCA-2A. However, the 

relatively high salinity and geochemical makeup (type 

E5 and E6) of the shallowest ground waters beneath 

site F1 in WCA-2A, as well as similar data from peat 

porewater (unpublished data), suggest that relict sea 

salts are discharging to surface water at low rates. 

Shallow ground water indicated additional variability in geochemical type, in addition to Fresh 

Recharge Water, Relict Seawater, and Transitional 

Relict Seawater. A fourth common type in shallow 

ground water beneath ENR was the variation of Fresh 

Formation Water classified as type G1a. Although possessing the same overall geochemical signature as the 

water found at 100 ft beneath the levees at both ENR 

and WCA-2A, it is clear that the shallow ground water 

at approximately 15 ft beneath central ENR fundamentally is different in origin. This shallow ground water is 

the water with the very light water-stable isotopic signature that was discussed earlier in this section. This 

interpretation, also discussed earlier, is that this water 

is derived from precipitation that infiltrated rapidly on 

farm fields prior to construction of ENR. 

Surface waters at ENR and WCA-2A always 

represented water mixtures rather than pure endmembers. The most common types were Transitional, Relict 

Seawater, and Transitional Fresh Formation Water. 

Surface waters always showed some effect of mixing 

with relict sea salts determined by the relative importance of chloride (type F6). The relict sea salts were 

derived from discharge of ground water from a deep 

source in the Surficial aquifer. A major component of 

drainage or runoff from Lake Okeechobee or the EAA 

was traceable by the increased relative importance of 

magnesium in water samples (type G6). Taken alone, 

however, major ions generally will not be sufficient to 

separate water predominantly derived from Fresh Formation Water from Agricultural Recharge Water. For 
Use of Geochemical Tracers to Identify Sources and Pathways of Water Flow in the Everglades 57

9511,0561,156

26,860

1,7751,075

4,900

4,430

1,063

1,1241,203

LAYER 5-FINE SAND

LAYER 4-MEDIUM TO FINE SAND

LAYER 1-PEAT

LAYER 2-SAND

SITE

MP-3

SITE

MP-203

SITE

MP-103 SITE

MP-2

SITE

MP-1

Seepage

Canal

ENR

A'

EAST

WCA-1

A

WEST

EAA

1,056 WELL SCREEN INTERVAL, NUMBER 

REPRESENTS SPECIFIC CONDUCTANCE VALUE, IN MICROSIEMENS PERCENTIMETER

EXPLANATION

30

20

10

0

80

60

0

40

20

1,500 4,500 FEET 3,000

500

0

0 1,500 METERS 1,000

1,520

1,056

1,398 1,383

1,412

1,000

1,100

1,200

LINE OF EQUAL SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER—Dashed where approximate

1,000

1,653

17,030

1,423

SEAWATER > 17,000

Calcium sodiumbicarbonateSodium-chloride

Calcium sodiumbicarbonate chloride

Mixed cationbicarbonate chloride Sodium-chloride bicarbonateMixed cationbicarbonate F1

E5

F6

E6

G1

G6

WATER TYPES

L-7

Canal

LAYER 3-LIMESTONE WITH INTERBEDDEDSAND

1,000

1,100

1,200

1,146

Figure 24. Ground-water flow paths delineated by specific conductivity (contours) and geochemical water type (color codes) at Everglades Nutrient Removal Area (ENR), north-central Everglades, south Florida.
58 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades 1510

5

0

30

0

20

10

2 MILES 1 0

1 0 4 KILOMETERS 2 3

METERS

FEET

1,265

777

2,405

3,4023,166

5,046

2,257 1,176

1,298

10,899

1,1219891,304

LAYER 4-MEDIUM/FINE SAND

LAYER 3-LIMESTONE/SAND

LAYER 2-MEDIUM/FINE SAND

LAYER 1 A/B - PEAT/FINE SAND

SITE

U-3

SITE

F-4

SITE

F-1 

Seepage

Canal

WCA-2A WCA-1

B'

NORTHEAS

T

WCA-1

B

SOUTHWEST

WCA-3

5,000

10,000

1,000

1,000

3,000

1,200

1,629

LEVEE AND

SITE S10C

1,265 WELL SCREEN INTERVAL, NUMBER 

REPRESENTS SPECIFIC CONDUCTANCE VALUE, IN MICROSIEMENS PERCENTIMETER

EXPLANATION

LINE OF EQUAL SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER—Dashed where approximate

900

Calcium sodiumbicarbonateSodium-chloride

Calcium sodiumbicarbonate chloride

Mixed cationbicarbonate chloride Sodium-chloride bicarbonateMixed cationbicarbonate F1

E5

F6

E6

G1

G6

WATER TYPES

Hillsboro Canal

Figure 25. Ground-water flow paths delineated by specific conductivity (contours) and geochemical water type (color codes) at Water Conservation Area 2-A (WCA-2A), 

north-central Everglades, south Florida
Effect of Ground-Water and Surface-Water Interactions on Mercury Transport in the North-central Everglades 59

example, using major ions alone, a ground water with 

long residence time in the aquifer cannot necessarily be 

distinguished from precipitation that recharged quickly 

in an agricultural field of the EAA (possibly because of 

the confusing signature introduced by chemical additives in fertilizer). Water-stable isotopes had considerable value in making that distinction, however, with 

lighter isotopes identifying waters recharged on agricultural fields without substantial evaporation.

Geochemical evidence provides a long-term integrated picture of hydraulic processes in the aquifer and 

exchange with surface water. Water-stable isotopes 

indicated that the major source of water in Everglades 

ground water is recharge of evaporated surface water 

from the wetlands rather than infiltration of rainfall 

through unsaturated sediments. This observation indicates that recharge always has occurred in the 

north-central Everglades. Ground-water recharge is 

balanced by discharge, which, although small in magnitude, is a persistent source of water to flow in the 

Everglades over the long term.

Major ion chemistry and water-stable isotopes 

identified eight major water types. Four of those were 

considered to be source waters (types F1, G1, G1a, and 

E5). The other four are mixtures of the source waters. 

The distribution of those water types identified 

long-term average flow pathways of recharge and discharge. 

Particularly evident as a major factor driving 

recharge and discharge in the present-day Everglades 

were the effects of wetland compartmentalization by 

levees. Geochemical water typing provided information that was consistent with hydraulic information. 

The geochemical tracers, however, particularly were 

useful in showing the extent to which water management has caused vertical mixing throughout essentially 

the entire depth of the Surficial aquifer. 

The effect of levees is indicated by the presence 

of Fresh Recharge Water recharged from the up gradient side of the wetland at a depth of 90 ft beneath the 

levee, whereas down gradient of the levee, relict sea 

salts have been entrained by the deepest underflow 

paths and transported upward to be discharged to surface water. The effect of levees on interactions between 

surface and ground water, therefore, extends vertically 

throughout at least the top three-fifths of the Surficial 

aquifer, and horizontally for at least 1 mi away from the 

levees. These observations support the hydraulic interpretation earlier in this report that water management 

has increased interactions between surface water and 

ground water in the north-central Everglades. 

EFFECT OF GROUND-WATER AND 

SURFACE-WATER INTERACTIONS ON 

MERCURY TRANSPORT IN THE 

NORTH-CENTRAL EVERGLADES

A major part of research on aquatic cycling of 

mercury concerns chemical reactions in the subsurface 

(Gilmour and others, 1998). For example, the conversion of inorganic forms of mercury to methylmercury is 

a critical step that makes mercury more bioavailable in 

aquatic ecosystems. This reaction is thought to occur 

predominantly in the subsurface; for example, in porewater of the wetland peat (Gilmour and others, 1991). 

In order to understand the biogeochemical controls on 

mercury methylation, it also is necessary to understand 

physical transport processes that affect movement of 

mercury between surface water and peat porewater. 

Diffusion often has been investigated as an important 

mechanism that releases methylmercury from porewater into surface water, whereas advective transport 

(with discharging or recharging water) less often has 

been considered (Krabbenhoft and Babiarz, 1992). 

Mercury mobility with recharging and discharging ground water in the Everglades is evaluated in this 

section. Discharge is the upward flow of ground water 

through peat into surface water, whereas recharge is the 

downward flow of surface water through peat into the 

aquifer. In addition to potentially transporting mercury, 

discharge and recharge also may affect mercury cycling 

by transporting other constituents that could affect 

chemical transformations of mercury. An example is 

transport of sulfate, which potentially affects mercury 

cycling through the effect on sulfate-reducing bacteria 

and sulfide levels, and their effects on methylation 

rates. The specific objectives of this part of the study 

are listed below:

1.) Characterize the distribution of dissolved total 

mercury and methylmercury (HgT and MeHg) 

in the Everglades Nutrient Removal (ENR) 

project and Water Conservation Area 2A 

(WCA-2A) in surface water, peat porewater, 

and ground water. 

2.) Relate patterns in mercury distribution to factors such as aquifer lithology, ancillary 

geochemistry, and source of recharge water. 
60 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

3.) Quantify ground-water exchange fluxes of 

mercury to improve the overall mercury mass 

budget at the ENR. 

ENR is a relatively large constructed wetland 

(3,815 ac) in the north-central Everglades built as a 

prototype for testing the effectiveness of a larger set of 

constructed wetlands called Stormwater Treatment 

Areas (STAs) designed to remove excess nutrients from 

agricultural runoff. A concern developed among 

water-resources managers that STAs might release 

large amounts of inorganic mercury to dissolved phases 

that subsequently would be exported in surface-water 

outflow. Much attention was focused on the ENR 

project because it represented an area of the Everglades 

where that concern could be tested by developing a 

reliable mercury balance. Previous hydrological work 

in ENR (for example, Choi and Harvey, 2000) demonstrated clearly that recharge and discharge of ground 

water had to be considered, in order to develop a reliable budget for mercury (or any chemical constituent 

that differs in its average concentration in surface 

water). The general approach used here was to combine 

recent estimates of recharge and discharge in ENR 

(Choi and Harvey, 2000) with mercury data from 

ground water and surface water to compute wetland to 

ground-water fluxes.

Although ENR is relatively large for a constructed wetland, it is small in comparison with hundreds of thousands of acres that make up the larger 

basins in the north-central Everglades. A disadvantage 

of working primarily at ENR is that it does not represent spatial variability across diverse Everglades landscapes. For that reason, the mercury distribution in 

WCA-2A also was investigated. Data from WCA-2A 

are not as comprehensive as from ENR, but, nonetheless, serve to broaden the relevance and applicability of 

conclusions given here. 

Approach

Mercury data were used in the present investigation in two major ways: to assess spatial patterns in 

concentrations and to refine mercury mass balances. 

Two data sets were used for those purposes. Each data 

set is described below.

Data set I 

Data were combined from various collaborating 

research projects to address the mercury distribution. 

Data sources from ground water are described first. A 

team of USGS and SFWMD researchers collected filtered samples for total mercury (HgT) and methylmercury (MeHg) analyses from a total of 19 sites in ENR 

and WCA-2A. A total of 39 wells were sampled along 

with the surface water at each site. Research sites and 

sampling procedures are summarized briefly here. 

Sampling wells encompassed various depths throughout the top half (100 ft) of the Surficial aquifer. A total 

of 24 shallow wells (approximately 25 ft deep) were 

sampled, 8 intermediate-depth wells (approximately 60 

ft deep), 6 deeper wells (approximately 90 ft deep), and 

one 180-ft-deep well. All wells and accompanying surface waters were sampled on five site visits between 

June 1997 and June 1998. Detailed sampling procedures and all mercury data from the ground-water sampling effort are reported in Harvey and others (2000). 

Mercury analyses from porewater were made 

available by the ACME (Aquatic Cycling in Mercury in 

the Everglades) research group, a group consisting primarily of USGS scientists but also other cooperating 

investigators. Those porewater samples had been 

acquired and analyzed separately from the 

ground-water samples and were analyzed in a different 

laboratory (USGS, Middleton, Wis.), following the 

procedures described in King (2000). The procedures 

used to collect those samples are described briefly here. 

Filtered peat porewater samples were acquired approximately three times per year between May 1995 and 

January 1999. Porewater was collected from discrete 

depths in the peat (approximately 5-cm vertical resolution) over total depths ranging from 20 to 50 cm below 

the peat surface. Fewer sites were sampled for peat 

porewater than for ground water, but sampling of peat 

porewater extended over a longer time period (David 

Krabbenhoft, U.S. Geological Survey, oral commun., 

2001). 

Data set II 

For the purpose of improving the ground-water 

component of the ENR mercury budget, a second data 

set was obtained from the SFWMD mercury program 

(South Florida Water Management District, 1999). 

SFWMD conducted detailed sampling of mercury in 

surface water and ground water at ENR between 1995 

and 1999. The data discussed here are a subset of a 
Effect of Ground-Water and Surface-Water Interactions on Mercury Transport in the North-central Everglades 61

very large database produced through their efforts 

(Miles and Fink, 1998; South Florida Water Management District, 1999). All available SFWMD data from 

monitoring wells and from the seepage canal were used 

in this study. The seepage canal receives ground-water 

discharge on the western side of ENR and returns it to 

the project inflow on the northern side. Also, SFWMD 

data collected from one surface-water site representative of the volume-averaged concentration of mercury 

in recharging surface water was used in this study. The 

SFWMD data used here is referred to as data set II.

Data set II included approximately 155 

surface-water analyses (including filtered and unfiltered splits) collected biweekly at surface-water site 

M203 (fig. 2). The rationale for choosing that site was 

that it provided a good estimate of volume-averaged 

concentration of mercury in recharging ground water. 

Ground-water analyses in data set II are represented by 

82 filtered samples collected from 14 wells emplaced 

beneath the eastern and western perimeter levees 

(Rohrer, 1999; Harvey and others, 2000). The shallow 

wells used for collection of data set II were approximately 25 ft below land surface, with screens emplaced 

in a sandy limestone layer. These are the ground waters 

most involved in recharge and discharges fluxes (Choi 

and Harvey, 2000), and were, therefore, a priority for 

sampling. In addition, 15 samples from the seepage 

canal, some filtered and some unfiltered, were used. 

Sampling procedures for mercury data that are part of 

data set II are given by South Florida Water Management District (1999). 

Laboratory Analyses 

All of the mercury data from Harvey and others 

(2000), as well as the additional analyses from data set 

II, were collected and analyzed according to SFWMD 

protocols as part of their mercury program (South Florida Water Management District, 1999). Samples for 

HgT were analyzed by the Florida Department of Environmental Protection prior to September 1997, and for 

the remainder of the project by Frontier Geosciences 

(Seattle, Wa.), a contract laboratory specializing in 

mercury analyses. MeHg analyses were conducted by 

Frontier Geosciences for the duration of the project. 

Approximate detection limits were 0.2 and 0.02 ng/L 

for HgT and MeHg, respectively. All mercury data analyzed by contract laboratories and corresponding information are kept by the SFWMD as part of their 

mercury research program (Darren Rumbold, South 

Florida Water Management District, written commun., 

1999). 

An important consideration for interpreting data 

from the SFWMD mercury program is a problem with 

filter contamination. Extensive testing by SFWMD 

during the summer of 1998 revealed that the capsule 

filters used until that time randomly contaminated 

some HgT analyses, typically by increasing concentrations by 20 to 50 percent (Darren Rumbold, South Florida Water Management District, written commun., 

1999). Testing indicated that the percentage of the filtered samples affected was low, less than 20 percent. 

Unfortunately, the contamination problem was discovered late in the sampling program. As a result of the 

contamination, all of the HgT analyses prior to August 

1998 potentially are affected. Fortunately, because 

samples are affected at random and because the proportion of samples affected is relatively small, the contamination problem is not large enough to invalidate the 

major trends identified in this investigation. Interpretations that are based on relatively large data sets are 

valid because all samples have an equal probability of 

being affected. However, sample contamination is 

expected to have introduced noise to the data set that 

may obscure subtle trends that otherwise may have 

been observable. 

It is important to note that no MeHg analyses 

were affected by the contamination problem, nor were 

any HgT analyses from peat porewater affected. Also, a 

different type of filter was used for peat porewater collections, and that filter type passed extensive tests for 

contamination (David Krabbenhoft, U.S. Geological 

Survey, oral commun., 2001).

Distribution of Mercury in Ground Water, 

Surface Water, and Peat Porewater 

Concentrations of filterable mercury species in 

the Everglades reflect many processes, including 

sources, affinity for the local substrate and resulting 

distribution coefficient, and rates of transformation by 

biogeochemical processes. A simple comparison of 

mercury concentrations among sites provides initial 

indications about the most important processes affecting spatial variation. Hypotheses are explored further 

through relations with ancillary geochemical or hydrological factors.

HgT concentration was highest in peat porewater 

and lowest in ground water (fig. 26a). Median concen-
62 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

0.01

0.1

1

10

0.001

0.01

0.1

1

10

0.1

1

10

100

TOTAL MERCURY, IN NANOGRAMS PER LITER METHYLMERCURY, IN NANOGRAMS PER LITER METHYLMERCURY/TOTAL MERCURY, IN NANOGRAMS PER LITER

SURFACE

 WATER

1-PEAT

2-SAND

3-LIMESTONE

4-SAND5-SAND

(A)

(B)

(C)

Detection limit

Detection limit

ENR

WCA-2A

max

upper quartile

median

lower quartile

min

trations of HgT in peat porewater were 5 ng/L at ENR 

and 2 ng/L at WCA-2A. Surface-water concentrations 

of total mercury were approximately 1 ng/L at both 

sites. Median concentrations of HgT in ground water 

generally were between 0.5 and 0.7 ng/L. The exception was one deep well (180 ft) representing a layer of 

fine sand near the bottom of the Surficial aquifer at 

ENR. Samples from this well had a slightly higher HgT

concentration than other well samples, with a median 

concentration of 0.9 ng/L determined for four sampling 

events. 

MeHg concentration also tended to be higher in 

peat porewater and surface water compared with 

ground water (fig. 26b). The median concentration in 

peat porewater was about 0.05 ng/L in both ENR and 

WCA-2A. Median concentrations in surface water 

were 0.04 and 0.07 ng/L in ENR and WCA-2A, respectively. In ground water, median concentrations of 

MeHg generally were at or very near to the detection 

limit (between 0.02 and 0.03 ng/L) 

 Site-specific studies in the Everglades normally 

show that dissolved MeHg concentrations are highest 

in porewater from the top 5 cm of peat (Gilmour and 

others, 1998). Similar median concentrations of MeHg 

in porewater and surface water in the box plots of figure 26 may be misleading. Depth-averaging of MeHg 

in the peat lumps together relatively high porewater 

concentrations of MeHg in upper layers of peat with 

undetectable levels of dissolved MeHg from lower layers. 

Ratios of MeHg to HgT are potential indicators 

of the extent that the available mercury has been methylated. A formal analysis would require corrections for 

the relative affinities of HgT and MeHg for solids, as 

well as the production and removal rates of different 

mercury species by specific biogeochemical reactions. 

This analysis is beyond the present study scope and 

only concentration ratios are discussed. The ratio 

MeHg/HgT was highest in surface water (0.04-0.07), 

intermediate in shallow ground water (0.03–0.05), and 

lowest in peat porewater (0.01–0.02) (fig. 26c). A low 

ratio in peat porewater likely was the result of 

depth-averaging in peat because most porewater samples are below the layer of active methylation. Ratios in 

surface water were relatively high as compared with 

porewater because MeHg diffuses into surface water 

from the top 5 cm of peat where active methylation 

occurs. Median ratios of MeHg /HgT in most ground 

waters generally were lower than surface water (less 

than 0.04), which does not strongly suggest active 

Figure 26. Distribution of dissolved total mercury (A), 

methylmercury (B), and ratio of methylmercury to total 

mercury (C) in surface water and ground water of the 

Everglades Nutrient Removal (ENR) project and Water 

Conservation Area 2 (WCA-2), north-central Everglades, 

south Florida. 
Effect of Ground-Water and Surface-Water Interactions on Mercury Transport in the North-central Everglades 63

methylation in ground water. The exception was the 

upper sand layer of the aquifer at WCA-2A (geologic 

layer 2), where the median ratio was slightly higher and 

similar to surface water (0.05).

Median concentrations of both HgT and MeHg 

showed trends of decreasing concentrations with depth 

in the aquifer to near detection levels (fig. 26). Ground 

water, therefore, appears not to be an appreciable 

source of dissolved mercury to the peat or surface 

water (compared to other sources such as atmospheric 

deposition). Instead, it appears more likely that surface 

water and peat are potential sources of mercury to the 

Surficial aquifer.

Relation to Ground-Water Geochemical 

Type and Source of Recharge

Simple variables, such as location, or geologic 

characteristics of aquifer layers, generally were unable 

to explain substantial variability in mercury distribution. For that reason, other potentially important variables were examined, including sulfate concentrations, 

geochemical water type, and source of recharging 

ground water. 

Relation to Sulfate 

Many investigations of mercury cycling in 

aquatic environments have identified a close linkage 

with sulfur cycling (Gilmour and others, 1991; Branfireun and others, 1996). For that reason, mercury and 

sulfate (SO4

2-) concentrations were plotted and analyzed. Other investigators acknowledge that sulfide, 

rather than sulfate, usually is a better indicator of the 

methylation rates in the Everglades (Gilmour and others, 1998). Unfortunately, sulfide in ground water was 

not measured during this study.

MeHg was almost always detectable in surface 

water, where SO4

2- concentrations ranged from tens to 

hundreds of mg/L SO4

2-. There was obvious correlation 

between MeHg and SO4

2- in surface water (fig. 27). 

MeHg was only infrequently detected in ground water 

at ENR and concentrations were not related to SO4

2- in 

any obvious way. Discounting wells U1-GW4 and 

U3-GW3 (considered as outliers), the only detectable 

MeHg in WCA-2A ground water (up to 0.06 ng/L) was 

measured in wells with undetectable (less than 1 mg/L) 

SO4

2-. Extremely high values of MeHg in repeat measurements from U1-GW4 (up to 8 ng/L) appear to be 

the result of a contaminated well. A single very high 

observation of MeHg measured on one visit to well 

U3-GW3, although not necessarily indicative of a contaminated well, was also treated as an outlier in the data 

set. 

Source of Recharge Water 

At ENR many of the ground-water samples with 

highest concentrations of MeHg tended to belong to the 

geochemical subtype referred to as “agricultural 

recharge water.” The source of agricultural recharge 

water was rainfall onto agricultural soils that infiltrated 

downward through the unsaturated zone to the water 

table. Agricultural recharge water is identified mainly 

through the much lower (more negative) values of δ18O 

and δ2H that are characteristic of rainfall that infiltrates 

quickly into soils before extensive evaporation occurs. 

Recharge of this type could have occurred only while 

the ENR site was managed as farmland. Agricultural 

recharge water was rare in ground-water samples from 

the Everglades, which most often have the positive values of δ18O and δ2H associated with the evaporation 

that occurs in wetland surface water. The other indicator of agricultural recharge water is major ion chemistry that reflects greater representation of calcium, 

magnesium, and sulfate relative to sodium, potassium, 

and chloride in ground-water chemistry, compared to 

other ground waters that underlie the Everglades.

Although not as unique an association with the 

agricultural recharge water type, there was a tendency 

for HgT concentrations to be higher in agricultural 

recharge water compared to other ground waters and 

surface waters at ENR (fig. 28). For example, of the 

measurements of HgT that were relatively high at ENR 

(greater than 2 ng/L), almost all were associated with 

agricultural recharge water. However, 50 percent of the 

wells where HgT concentration was above 0.5 ng/L 

were not associated with agricultural recharge water. 

Although the highest values of HgT were associated 

with agricultural recharge water, water type did not 

explain the distribution of HgT in ground water, 

because 50 percent of the detectable HgT measurements were associated with non-agricultural recharge 

water. Therefore, unlike MeHg, HgT appears to be 

present in all shallow ground water beneath the 

north-central Everglades, regardless of source. 

The difference in concentrations between ground 

water recharged in agricultural and wetland settings 

were more pronounced for MeHg than HgT. For example, samples from wells that had the highest concentrations of MeHg (up to 0.2 ng/L) and the highest ratios of 
64 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

MeHg/HgT (up to 0.4), almost were all agricultural 

recharge waters (fig. 28b). MeHg essentially only was 

detectable in ENR ground water that had been 

recharged under agricultural-management conditions 

(fig. 28b). Eight of 17 measurements in wells with that 

water type had MeHg concentrations between 0.03 and 

0.2 ng/L. None of the other samples from wells 

recharged from wetland surface water contained MeHg 

concentrations greater than 0.03 ng/L.

The data suggest that the source of MeHg is 

present only in ground water recharged in certain areas 

of the Everglades previously converted to agriculture. 

Why should the presence of MeHg be associated 

uniquely with agricultural recharge water, whereas HgT

is present in all types of ground water beneath the Everglades? Either agricultural management increased 

recharge of MeHg, or created ground-water conditions 

under which methylation was more efficient. Plots of 

MeHg compared with HgT indicate variable ratios of 

MeHg/HgT (fig. 29), and, therefore, do not help answer 

remaining questions. These questions cannot be 

answered with the results from the present study but 

deserve further attention as part of possible future 

studies.

A few qualifications are appropriate about the 

extent to which agricultural recharge water beneath 

ENR is representative of agricultural management 

throughout the Everglades Agricultural Area (EAA). 

The ENR site was farmed for many years prior to the 

construction of the treatment wetland. Agriculture 

management of the ENR site began after the initial conversions of wetlands to agricultural fields in the early 

1900s. WCA-1 was constructed during the period 

1949–1950. Because of its position adjacent to 

WCA-1, the ENR vicinity was difficult to drain and, 

therefore, probably was wetter than many other parts of 

the EAA. Also, re-flooding of the ENR in 1993 that 

began operation of the treatment wetland was not a typical agricultural practice in the EAA. Based on the 

work of other investigators, an increase in surface-water MeHg concentration is expected after 

re-flooding of a wetland following a period of extended 

"dry down" of the peat (Kelly and others, 1997). It is 

possible that re-flooding at ENR affected the transport 

of MeHg out of the peat; however, that episodic 

recharge cannot explain the high values of MeHg 

observed in agricultural recharge water beneath ENR 

because water-stable isotope and ion tracers demon0.01

0.1

1

10

1 10 100 1,000 10,000

SULFATE CONCENTRATION, IN MILLIGRAMS PER LITER

ENR-Ground water

WCA-Ground water

ENR-Surface water

WCA-Surface water

METHYLMERCURY, IN NANOGRAMS PER LITER

Detection limit

Figure 27. Dissolved methylmercury and sulfate concentrations in surface water and ground water of Everglades 

Nutrient Removal Area (ENR) and Water Conservation Area 2A (WCA-2A), north-central Everglades, south Florida. 

Dotted lines surround potentially unreliable data from well U1-GW-4 and U3-GW3. See text for explanation.
Effect of Ground-Water and Surface-Water Interactions on Mercury Transport in the North-central Everglades 65

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

METHYLMERCURY, IN NANOGRAMS PER LITER

Detection limit

ENR - Ground water

ENR - Surface water

ENR - Ground water with isotopic 

signature of agricultural recharge 

δ18Ο

(B)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

δ18Ο

ENR - Ground water

ENR - Surface water

ENR - Ground water with isotopic 

signature of agricultural recharge 

TOTAL MERCURY, IN NANOGRAMS PER LITER

Detection limit

(A)

Figure 28. Dissolved mercury and oxygen isotopic ratio in surface water and ground water of Everglades Nutrient 

Removal Area (ENR) and Water Conservation Area 2A (WCA-2A), north-central Everglades, south Florida. Total 

mercury (A) and methylmercury (B). Note that methylmercury values are higher for ground waters that are relatively 

"light" in their isotopic ratios.
66 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

strate that high MeHg ground water was recharged 

prior to flooding of ENR. Therefore, a preliminary conclusion that high values of MeHg in agricultural 

recharge water beneath ENR may be representative of 

all EAA ground water is warranted. More observation 

and analysis of ground-water mercury in the EAA will 

be required to determine a final conclusion.

Ground-water Component of the Mercury 

Budget at ENR 

The South Florida Water Management District 

recently (1999) developed a budget for mercury inputs 

and outputs for ENR. This budget includes atmospheric 

inputs of mercury by wet deposition, inputs with surface water pumped into the ENR, export by 

surface-water pumping at the outflow point, and gaseous evasion from surface water and plants. The data 

and procedures for developing the overall budget are 

discussed in detail in South Florida Water Management 

District (1999). However, the District was unable to 

close the mercury budget between surface water, wetland soil, and the Surficial aquifer (L. Fink, South Florida Water Management District, oral commun., 1998). 

The present work extends the previous budget by 

improving the estimate of ground-water recharge of 

mercury. In addition, preliminary determinations 

regarding the mobility of mercury in ground water 

were made by comparing budget estimates with the 

mercury load in a canal that collects ground water from 

beneath ENR. 

Computation of Mercury Fluxes

The approach used here was to combine 

time-averaged recharge and discharge fluxes at ENR 

developed by Choi and Harvey (2000) with 

time-averaged mercury concentrations to compute mercury fluxes. Discharge of mercury to the ENR was 

computed by multiplying the average ground-water 

mercury concentrations in eastern-side wells by the 

ground-water discharge flux. Recharge was computed 

similarly using concentrations from wells on the western side of ENR. The average surface-water concentrations of HgT and MeHg (at site M203) were 1 ng/L and 

0.1 ng/L, respectively (table 17). Wells on the eastern 

side of ENR, where ground-water discharge occurs, 

had an average HgT concentration of 0.7 ng/L and 

undetectable MeHg (less than 0.02 ng/L). In contrast to 

the eastern side, ground water on the western side had 

the highest average concentration of HgT (1.4 ng/L). 

MeHg also was undetectable in the western wells. The 

difference in mercury concentrations between recharging and discharging sides of ENR are shown graphically in figure 30.

The steady-state approach to calculating a mercury budget assumes that flow and concentration either 

do not vary substantially with time or are well 

described by average values. Both recharge and mercury concentrations vary over time in ENR. Recharge 

fluctuated by approximately a factor of 2 about the 

mean value. Those fluctuations were smooth, however, 

occurring on timescales of weeks to months, which 

were well described by the biweekly estimation procedure for recharge used by Choi and Harvey (2000). 

Mercury sampling was less frequent than hydrologic 

measurements (approximately monthly), and various 

gaps of 2 months are present in the data. Mercury concentrations in surface water varied more than an order 

of magnitude if occasional spikes in concentration are 

included (fig. 31). Concentration spikes resulted for 

both filtered and unfiltered HgT data, reflecting the 

effects of particle re-suspension on surface-water concentrations. HgT concentration values that were filtered 

sometimes were higher than unfiltered values for the 

same day, which may reflect the contamination problem discussed earlier. 

Data collection was too sparse to establish definitively whether ground-water HgT in the western wells 

increased over the 4-year study period. Observing 

breakthrough of HgT in those wells is the expected 

result of the increased recharge of HgT that began after 

the ENR wetland became operational. Determining the 

time of HgT breakthrough is important because it 

would help characterize interactions between mercury 

and aquifer sediments, and resulting retardation during 

transport. Unfortunately, the number of sampling 

events is too sparse, particularly near the beginning of 

the measurement period, to determine breakthrough 

time (fig. 31). MeHg in ground water showed no obvious differences over time or between locations 

(fig. 30). Because no other major long-term trends 

were apparent in recharge or in mercury concentrations, time-averaging both recharge and mercury concentrations data was judged to be a reasonable 

first-order approach to budgeting ground-water fluxes 

at ENR. 
Effect of Ground-Water and Surface-Water Interactions on Mercury Transport in the North-central Everglades 67

0.01

0.1

1

10

0.1 1 10

TOTAL MERCURY, IN NANOGRAMS PER LITER

METHYLMERCURY, IN NANOGRAMS PER LITER

Detection limit

Detection limit

WCA-2A-Ground water

WCA-2A-Surface water

(B)

0

0.1

0.2

0.3

0.4

0.5

0.6

TOTAL MERCURY, IN NANOGRAMS PER LITER

METHYLMERCURY, IN NANOGRAMS PER LITER

ENR-Ground water

ENR-Surface water

Detection limit

Detection limit

0 12 345

(A)

Figure 29. Dissolved methylmercury and total mercury, Everglades Nutrient Removal Area (ENR) (A) and Water 

Conservation Area 2A (WCA-2A) (B), north-central Everglades, south Florida. Dotted lines surround potentially 

unreliable data from well U1-GW4 and U3-GW3. See text for explanation.
68 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

The seepage canal on the western and northern 

sides of ENR collects ground water that was recharged 

in ENR. Average HgT concentrations are based on a 

small number of samples (15) from the seepage canal. 

The 5 filtered samples actually had a higher mean concentration of HgT (0.8 ng/L) than the 10 unfiltered samples (0.4 ng/L). The higher concentrations may have 

resulted from two sources: 1) contamination of the filtered HgT analyses, or 2) lower HgT concentrations in 

the seepage canal earlier in the study (1994 and 1995), 

before breakthrough of HgT in ground-water discharge 

occurred. The present analysis used all HgT analyses 

available in the seepage canal, whether filtered or unfiltered, which resulted in an average value of HgT of 0.6 

ng/L for the seepage canal. Dissolved MeHg was undetectable (less than 0.02 ng/L) in the seepage canal.

Mercury Fluxes with Recharging and Discharging 

Water in ENR

The resulting mercury flux computations are 

reported in table 18 with reference to the overall mercury balance at ENR (South Florida Water Management District, 1999). Ground-water discharge to ENR 

from the eastern side (WCA-1) was not appreciable, 

contributing approximately 0.5 and 0.3 percent of HgT

and MeHg to ENR, respectively. However, recharge on 

the western side of the ENR is a major pathway for output of HgT from ENR. Recharge of HgT accounted for 

an output of 70 g/yr, or 10 percent of all inputs to ENR. 

Recharge of MeHg was negligible, either below detection, or, if values of MeHg close to detection levels are 

used, accounting for no more than export of 1 g/yr (3 

percent of inputs). 

The finding that HgT mercury is recharged in 

substantial quantities to the sand and limestone aquifer 

Sites Years 

sampled

Total 

number of 

observations

Mean Hg 

concentrations1

Standard

deviations

n HgT

(ng/L)

MeHg

(ng/L)

HgT

(ng/L)

MeHg

(ng/L)

Westside wells

MP3-D 97-98 4 0.8 0.02 0.6 0.005

P02A 96-98 18 1.1 0.02 0.7 0.005

P03A 96-98 9 1.4 0.02 0.7 0.005

P04A 96-98 11 1.5 0.02 0.8 0.005

P05A 96-98 13 2.7 0.02 3.5 0.005

P06A 96-98 13 1.8 0.02 1.1 0.006

P07A 96-98 14 1.0 0.02 0.6 0.007

Average between sites 1.4 0.02 1.1 0.006

Eastside wells

MP1-C 97-98 4 0.5 0.02 0.3 0.005

MP1-D 97-98 4 0.9 0.02 0.8 0.005

P10A 95-96 4 0.7 0.02 0.3 0.004

P12A 95-96 4 0.6 0.02 0.1 0.004

P13A 95-96 10 1.0 0.02 0.9 0.01

Average between sites 0.7 0.02 0.5 0.006

Seepage canal 97-98 5 0.8 0.02 0.5 0.006

Seepage canal - Unfiltered 94-99 10 0.4 0.05 0.1 0.05

Seepage canal - All data 94-99 15 0.6 0.04 0.3 0.004

Surface water - ENR-203 95-99 155 1.0 0.01 0.7 0.14

1 All concentrations are for filtered samples unless otherwise noted.

Table 17. Mercury concentrations in shallow ground water and surface water at 

Everglades Nutrient Removal (ENR) project, north-central Everglades, south Florida

[HgT, total mercury; MeHg, methylmercury]
Effect of Ground-Water and Surface-Water Interactions on Mercury Transport in the North-central Everglades 69

0.00

0.02

0.04

0.06

0.08

0.10

0.12

P01B P13A P12A P10A P08B P14B MP1C MP1D P02A P03A P04A P05A P06A P07A P11A MP3-D

SITE IDENTIFICATION

METHYLMERCURY CONCENTRATION, 

IN NANOGRAMS PER LITER

Detection limit

6/7/95

9/6/95

12/5/95

3/6/96

6/4/96

9/5/96

12/4/96

3/5/97

6/4/97

9/97

12/97

3/98

6/98

9/98

12/98

(B)

0.0

2.0

4.0

6.0

8.0

10.0

12.0

P01B P13A P12A P10A P08B P14B MP1C MP1D P02A P03A P04A P05A P06A P07A P11A MP3-D

SITE IDENTIFICATION

6/7/95

9/6/95

12/5/95

3/6/96

6/4/96

9/5/96

12/4/96

3/5/97

6/4/97

9/97

12/97

3/98

6/98

9/98

12/98

Eastern wellsdischarge side of

ENR 

Western wellsrecharge side of

ENR 

Eastern wellsdischarge side of

ENR 

Western wellsrecharge side of

ENR 

TOTAL MERCURY CONCENTRATION, 

IN NANOGRAMS PER LITER

Detection limit

(A)

Figure 30. Dissolved total mercury (A) and methylmercury (B) in shallow wells at Everglades Nutrient Removal project 

(ENR), north-central Everglades, south Florida. Wells are grouped by eastern and western sides of the wetland, where 

ground-water discharge and recharge occur, respectively.
70 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

9/1/94 3/2/95 8/31/95 2/29/96 8/29/96 2/27/97 8/28/97 2/26/98 8/27/98 2/25/99 8/26/99

DATE

METHYLMERCURY, IN NANOGRAMS PER LITER

(B)

0.0

1.0

2.0

3.0

4.0

9/1/94 3/2/95 8/31/95 2/29/96 8/29/96 2/27/97 8/28/97 2/26/98 8/27/98 2/25/99 8/26/99

DATE

M203–SURFACE 

WATER

P05A–WELL

P06A–WELL

P04A–WELL

P03A–WELL

P02A–WELL

P07A–WELL

M203-unfiltered

surface water

Seepage canalunfiltered surface 

water 

Seepage canalfiltered surface

water

EXPLANATION

TOTAL MERCURY, IN NANOGRAMS PER LITER

(A)

M203–SURFACE 

WATER

P05A–WELL

P06A–WELL

P04A–WELL

P03A–WELL

P02A–WELL

P07A–WELL

M203-unfiltered

surface water

Seepage canalunfiltered surface 

water 

Seepage canalfiltered surface

water

EXPLANATION

Figure 31. Time course of total mercury (A) and methylmercury (B) concentrations in Everglades Nutrient Removal 

project (ENR), north-central Everglades, south Florida. 
Effect of Ground-Water and Surface-Water Interactions on Mercury Transport in the North-central Everglades 71

is consistent with vertical profiles of HgT concentrations in peat. A vertical profile in peat shows relatively 

equivalent concentrations of HgT through the entire 

depth of peat, which supports the conclusion that HgT

is recharged to the aquifer (fig. 32). In contrast, MeHg 

does not appear to be recharged through the peat. The 

steep decline in MeHg concentration with depth in peat 

is presumed to result from the increasing importance of 

demethylation over methylation, compared to downward advection of MeHg through peat. A peat profile 

from western ENR where MeHg only was detected in 

the top 5 cm of the peat is consistent with the negligible 

recharge flux of MeHg (table 18 and fig. 33). 

In addition to calculating ground-water exchange 

fluxes, it also was possible to estimate the change in 

mercury storage in peat. For the purposes of this study, 

change in mercury storage in peat is estimated as the 

difference in advection of mercury downward across 

the peat surface and recharge of mercury out of the 

base of the peat into the aquifer. Downward advection 

of surface-water mercury into peat is computed similarly to ground-water recharge, except surface-water 

concentrations are multiplied by the recharge flux 

instead of ground-water concentrations. Downward 

advection accounted for movement of 50 g/yr, or 8 percent of total inputs, of HgT from ENR surface water to 

peat porewater. For MeHg, downward advection into 

peat accounted for 5 g/yr, or 20 percent of inputs of 

MeHg, respectively (table 18). After computing storage 

estimates, it was apparent that HgT was released from 

peat at a rate of 20 g/yr and MeHg was stored at a rate 

of 4 g/yr, which are equal to rates of 3 and 10 percent 

of the inputs to ENR, respectively.

Caution is required in evaluating rates of mercury storage in peat calculated during this study. These 

calculated rates do not consider the effects of sedimentation of particulate matter with solid phase mercury 

onto the peat surface, or the effects of upward diffusion 

of mercury back to surface water. For example, King 

(2000) showed that upward diffusive fluxes probably 

exceed downward advective fluxes of mercury at ENR. 

Nonetheless, storage estimates calculated here can be 

used as a preliminary interpretation of the changes in 

mercury processing in the peat. The rate of release of 

total mercury from solid phases on peat was calculated 

as the difference in the flux from the peat to the aquifer 

and the flux from surface water to the peat. Partitioning components indicate that approximately 70 percent 

of the HgT recharged to the aquifer was derived from 

downward advection of surface water into peat, 

whereas the remainder (30 percent) came from release 

of Hg from solid phases in peat. Release of particulate 

mercury from peat into recharging water was not 

expected, because of the high affinity of mercury for 

sorption sites on peat. On the other hand, release of 

HgT from organic sediments has been observed in littoral zones of lakes that receive sedimentation of labile 

organic matter with bound mercury (Krabbenhoft and 

RECHARGE AND DISCHARGE FLUXES OF MERCURY1

AND SUBSURFACE STORAGE CHANGES 

 FLUX

 (grams/year)

 PERCENTAGE OF 

INPUT TO ENR2

HgT MeHg HgT MeHg

a - Discharge from ground water to peat, eastside 3 0.1 0.5 0.3

b - Recharge from ENR surface water, westside

 b1 - ENR surface water to peat 50 5 8 20

 b2 - ENR peat to ground water 70 1 10 3

c - Apparent storage change in peat3 -20 4 3 10

d - Seepage canal flux4 20 1 3 3

e - Apparent storage change in Surficial aquifer3 50 0 8 0

Table 18. Annual budget for recharge, discharge, and subsurface storage of dissolved mercury in 

Everglades Nutrient Removal (ENR) project, north-central Everglades, south Florida, 1994-98. 

[Concentrations and fluxes were averaged for the period 1994-98; concentrations are the average of filtered samples unless otherwise 

noted; HgT, total mercury; MeHg, methylmercury; a, average concentration in eastside wells x average ground-water discharge; b1, 

average concentration at M203 x average ground-water recharge; b2, average concentration in westside wells x average ground-water 

recharge; c, calculated as (b1 - b2 + a); d, average concentration in westside wells x average flow in seepage canal; e, calculated as 

(b2 - d - a)]

1 Ground-water recharge and discharge fluxes are from Choi and Harvey (2000).

2 Sum of mercury inputs by atmospheric wet deposition and surface-water pumping.

3 Positive and negative numbers indicate gains or losses of dissolved mercury from storage because of chemical association with sediment.

4 Computed by using all filtered and unfiltered mercury data because of the relatively small number of samples.
72 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

M201 M103 M104-unvegetated

0

5

10

15

20

25

30

M104-vegetated

0 2 6 10 14 16 4 8 12 0 2 6 10 14 16 4 8 12 0 2 6 10 14 4 8 12 0 2 6 10 14 4 8 12

0 0.5 1 1.5 2 0.5 1 1.5 2 0.5 1 1.5 2 0.5 1 1.5 2 000

TOTAL MERCURY, IN NANOGRAMS PER LITER

METHYLMERCURY, IN NANOGRAMS PER LITER

DEPTH IN PEAT, IN CENTIMETERS

0

5

10

15

20

25

30

DEPTH IN PEAT, IN CENTIMETERS

January 1997

April 1997

July 1997

EXPLANATION

No data

No data

No data

No data

No data

No data

No data

No data

No data

No data

No data

No data

No data

No data

No data

No data

No data

No data

No data

No data

No data

No data

(B)

(A)

Figure 32. Profiles of estimated concentrations of dissolved total mercury (A) and methylmercury (B) in peat 

porewater at Everglades Nutrient Removal (ENR) project, north-central Everglades, south Florida. Dissolved 

concentrations were estimated by King (2000), who used measurements of solid phase mercury 

concentrations and sediment bulk density, along with estimates of the liquid-solid distribution coefficient at 

selected depths from the study sites.
Effect of Ground-Water and Surface-Water Interactions on Mercury Transport in the North-central Everglades 73

300 100

20

50*

70 3

10

300

Seepage

canal

50

30 8

?

0

?

0

1

5*

1 0.1

0

Seepage

canal

0

2

19

3 FLUX

CHANGE IN STORAGE

Flux does not consider

diffusion and sedimentation.

Fluxes in gray are from

South Florida Water 

Management District (1999)

EXPLANATION

*

(A) (B)

SURFACE WATER

PEAT

AQUIFER

Babiarz, 1992). Other factors may be important in 

causing HgT release in ENR peat. First, the storage of 

mercury may have been exceptionally high, building up 

during many years of agricultural farming in oxic soils 

that then became anoxic following re-flooding (Kelly 

and others, 1997). Second, ancillary constituents 

recharged with surface water or released by sediment 

diagenesis after re-flooding may affect the binding constants for mercury. For example, King (2000) found 

that both dissolved inorganic sulfur compounds and 

sulfur-bearing dissolved organic carbon compounds 

may compete with mercury for surface-binding sites in 

peat, effectively decreasing the partitioning coefficient 

for mercury on peat.

MeHg appears to be either strongly retained in 

the peat or rapidly degraded, judging from both storage 

calculations and profiles of MeHg in peat. Much of the 

retention in peat porewater may be temporary, because 

of biodegradation and the steep diffusion gradient that 

may transport MeHg back to surface water. King 

(2000) found that upward diffusion of MeHg back to 

surface water probably exceeds the downward recharge 

flux of MeHg into the peat. 

Evidence for Reactivity of Mercury in Peat 

and in the Surficial Aquifer

What is the fate of HgT recharged to the aquifer? 

It is possible that some of the HgT was retained in 

ground-water flow paths beneath the western boundary 

levee. The average concentration of total mercury in 

the seepage canal (0.6 ng/L) was less than half of the 

average for wells beneath the western and northern 

levees (1.4 ng/L) (table 17), suggesting the possibility 

that mercury is retained by geochemical reactions in 

ground water. If shallow recharge in ENR were the 

only source of water to the seepage canal (and if the 

concentrations were no longer changing with time), it 

Figure 33. Average ground-water fluxes and storage rates (grams per year) of total mercury (A) and methylmercury (B) 

compared with surface-water and atmospheric fluxes in Everglades Nutrient Removal Area (ENR), north-central 

Everglades, south Florida. Surface-water and atmospheric wet deposition fluxes are from South Florida Water 

Management District (1999). Budget represents a 4-year average, 1994-98.
74 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

only would be necessary to divide the seepage canal 

concentration by the concentration detected in western 

wells to determine the extent of mercury retention. A 

preliminary conclusion from these data could be drawn 

by assuming that mixing between shallow 

ground-water recharge and deeper ground water from 

another source was minimal. However, it would be 

more prudent to attempt to account for mixing with 

deeper ground water.

Choi and Harvey (2000) found that, on average, 

the seepage canal captures most of the recharging 

ground water in ENR (approximately 73 percent). During periods with of low water levels in ENR, recharge 

of ENR water closely matched flows in the seepage 

canal. During periods of high water levels in ENR, 

there is an additional component of recharge in ENR to 

deep flow paths not captured by the seepage canal. 

There is increased flow in the seepage canal at those 

times (approximately 20 percent increase), and chemical data from the seepage canal shows that the additional flow represents discharge from deeper ground 

water with higher chloride and lower mercury concentrations. The task of accounting for mixing between 

shallow and deeper ground water was accomplished 

using a combined water and chloride balance for the 

seepage canal. The water balance suggests that, on 

average, discharge from deeper ground water to the 

seepage canal makes up approximately 10 percent of 

the flow in the seepage canal. The deeper ground water 

that discharges to the seepage canal has a chloride concentration approximately equal to 370 mg/L (the highest measured chloride concentration in the seepage 

canal; Harvey and others, 2000). The resulting mass 

balance on chloride in the seepage canal simulated an 

average concentration of 170 mg/L. This concentration 

is similar to the measured average concentration (Harvey and others, 2000). Based on this average chloride 

concentration, an HgT concentration of 1.3 ng/L was 

computed in the seepage canal for the situation where 

discharge of deep ground water (with undetectable 

HgT) is the only important factor affecting HgT concentrations (that is, chemical mercury reactions were 

ignored). Comparing the computed concentration with 

the average measured concentration of HgT in the seepage canal (0.6 ng/L) indicates that approximately 50 

percent of the HgT was removed by chemical reactions 

in the shallow part of the Surficial aquifer or in the bottom sediments of the seepage canal.

Therefore, based on the computed budget, it was 

determined that total mercury is stored by geochemical 

retention during passage through the aquifer sediments. 

The simplest type of chemical reaction that would 

explain storage in the aquifer is sorption of mercury to 

peat or aquifer sediments (sand and limestone). Sorption would cause retardation in the transport of HgT; 

however, complete breakthrough of mercury eventually 

would be expected if sorption were completely reversible. Breakthrough of HgT in western levee wells at 

ENR appeared to have occurred relatively quickly (2 

years or less), because a steady plateau concentration 

of HgT had been reached by the time those wells were 

sampled routinely. Based on these data, breakthrough 

of HgT also would have been expected to be detected in 

the seepage canal during the 4-year study period, but 

that was not the case. An interpretation that is consistent with these observations is that retention of HgT is 

non-reversible on aquifer solids. Another interpretation 

is that non-reversible retention of HgT occurred on 

organic sediments as ground water discharged across 

the bottom of the seepage canal. The present data are 

not sufficient to draw a final conclusion. Detailed 

examination of core material and bench-top experimentation would be needed to make further evaluations.

Improving future ground-water budgets for mercury should be investigated. The present investigation 

used a time-averaged approach to develop the mass 

budget. This approach was simple and considered a 

first step. Also, this approach had the advantage of consistency with other budget estimates that are reported 

as annual estimates. Calculating time-averaged fluxes 

was also the most straightforward means to deal with 

problems such as gaps in mercury data and random 

contamination of a small percentage of filterable HgT

concentrations (South Florida Water Management District, 1999). Future work may determine that a 

time-dependent analysis is justified. In any case, the 

annually averaged approach is a good first step to refine 

specific questions, develop preliminary conclusions, 

and establish alternatives for further research.

Additional work also is needed to estimate diffusive fluxes of mercury from peat to surface water, and 

sedimentation of mercury to the wetland surface, 

because those fluxes could be significant compared to 

recharge fluxes. For example, estimates of mercury diffusion from ENR sediments suggest that upward diffusive fluxes of mercury from peat to surface water 

probably exceed the downward recharge fluxes (King, 

2000). More work is needed to determine how diffusion and sedimentation will interact with ground-water 

fluxes to affect the mercury balance in peat. 
Summary and Conclusions 75

SUMMARY AND CONCLUSIONS

Water-resources management in the Everglades 

has evolved from the original purpose to protect against 

floods and to conserve water during dry periods, to also 

include protecting and restoring natural functions and 

characteristics. The main effect that digging canals and 

draining wetlands had was to reduce the water-storage 

capacity of this subtropical peatland. Over time, the 

decreased water storage contributed to the degradation 

of a natural balance between highly seasonal water 

flows, pristine low-nutrient freshwaters, and a unique 

assemblage of specially adapted plants and wildlife. 

The first half of the twentieth century saw the many 

adjustments to the changing hydrology that caused a 

general drying out of the Everglades system. The second half of the twentieth century began with the completion of a system of levees that enclosed Water 

Conservation Areas (WCAs). For a number of years 

following completion of WCAs, water tended to be 

stored at levels that were too high for the ecosystem 

during wet periods, resulting in problems such as the 

drowning of tree islands. Equally undesirable was an 

approach to managing floods that involved conveying 

vast amounts of freshwater (that the pre-drainage Everglades previously could have stored) through canals to 

be flushed to the Atlantic Ocean. During excessively 

dry periods control structures were shut to keep water 

levels at minimum acceptable levels in WCAs, which 

had the effect of curtailing the southerly flow of water 

to the Everglades National Park (ENP). Even during 

times between storms and droughts, problems persisted 

with water management in the Everglades. Some of 

these problems included shallow seepage beneath 

levees that occurred at all times and deeper 

ground-water flows increased from the Everglades to 

outside areas after an extended period of ground-water 

pumping in well fields to the east. Thus, southerly 

flows to ENP were depleted even during times of typical water-level conditions. The Comprehensive Everglades Restoration Plan, authorized by Congress in 

2000, sought to address the large, complex, and expensive to remediate water-management problems.

Except for seepage studies beneath the eastern 

boundary levees, interactions between surface water 

and ground water have not been extensively investigated in the Everglades. More work is needed on accurately estimating recharge and discharge, and the 

effects on the water balance and water quality of the 

Everglades. Investigations of the factors that control 

recharge and discharge also are important. A problem 

of special concern in the Everglades is characterizing 

recharge and discharge in the vast wetland interior 

areas, where few studies have been completed. Interactions between surface water and ground water in interior areas, and their modification because of 

water-resources management, are relevant to the protection of fresh ground-water resources beneath the 

vast interior of the Everglades.

In an effort to learn more about surface-water 

and ground-water interactions in the Everglades, the 

U.S. Geological Survey (USGS) and South Florida 

Water Management District (SFWMD) developed an 

agreement to undertake a detailed study of those interactions in selected areas of the north-central Everglades. The present study focused its efforts on the 

most highly manipulated part of the Everglades system—areas of the north-central Everglades that had 

been converted to farming during the past century and 

now returned to management as constructed wetlands, 

called Stormwater Treatment Areas (STAs).

The present investigation determined the extent 

to which interactions between surface water and 

ground water have increased in the north-central Everglades as a result of water-resources management. On 

the western side of the WCAs, and in the Stormwater 

Treatment Areas, or STAs, there has been a dramatic 

shift since pre-drainage times in the direction of horizontal ground-water flow and in the magnitude of 

recharge. The direction of ground-water flow shifted 

southwest to northwest as a consequence of the steepening of land and water-table slopes that followed 

extensive land subsidence in the Everglades Agricultural Area, or EAA. Another factor that was important 

in changing interactions between surface water and 

ground water was the abrupt change in water level (typically 2.5 ft or more) across levees that increased driving forces for recharge and discharge. Rapid releases of 

large quantities of surface water between basins also 

had the effect of increasing recharge and discharge by 

propagating surface waves (and subsurface pressure 

waves through the aquifer), causing temporary periods 

of increased recharge often followed by temporary 

periods of increased discharge.

Recharge is the dominant interaction between 

surface water and ground water in the northern Everglades when averaged over space and time. Net 

recharge is the result of increased hydraulic gradients 

caused by land subsidence and decreasing water-table 

elevations outside of the WCAs. Basin-averaged 
76 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

recharge at the Everglades Nutrient Removal (ENR) 

project (0.9 cm/d) exceeded recharge at Water Conservation Area 2A (WCA-2A) (0.2 cm/d) by more than a 

factor of 4. Recharge exceeded discharge in both ENR 

and WCA-2A, by a factor of 10 at ENR, and by a factor 

of 2 in WCA-2A. Recharge in both ENR and WCA-2A 

accounted for a volumetric flux of water equal to 

approximately 30 percent of surface water pumped into 

each basin.

Recharge and discharge increased since 

pre-drainage conditions in the vicinity of ENR as a 

result of water-management conditions. The estimated 

change was from approximately no net flux under 

pre-drainage conditions to 0.9 cm/d net recharge under 

present water-management conditions. Changes in net 

recharge and discharge probably were not as dramatic 

in WCA-2A, from no net flux under pre-drainage conditions to approximately 0.08 cm/d net recharge under 

present conditions. An increase in net recharge since 

pre-drainage conditions mainly has been controlled by 

large-scale and long-term changes, such as subsidence 

in the agricultural areas and increasing ground-water 

pumping from well fields to the east of the Everglades.

Site-specific estimates of recharge and discharge 

in ENR and WCA-2A ranged between detection limits 

(0.04 cm/d) and 20 cm/d. The general pattern of 

recharge and discharge measurements showed that 

interactions between surface water and ground water 

decrease from the north Everglades toward the 

north-central Everglades. The largest recharge and discharge fluxes were observed in ENR. From ENR, 

recharge and discharge decreased consistently toward 

the wetland interior of WCA-3A. What explains the 

pattern of decrease from north to south? Recharge and 

discharge fluxes were highest within 0.5 km of levees 

(always greater than 0.3 cm/d), whereas vertical fluxes 

in the wetland interior rarely exceeded 0.3 cm/d. The 

order of magnitude difference in vertical fluxes near 

and far from levees suggests that basin-averaged fluxes 

are predictable from the ratio of the length of levee 

perimeter to the surface area of each basin, which 

decreases from the northern extent of the Everglades 

(ENR and STAs) towards the WCA-3. Greater hydraulic conductivity in WCA-2A peat compared with ENR 

also affected recharge and discharge. Differences in 

hydraulic properties of peat appear to be explained by 

peat compaction in wetlands managed for agriculture 

for appreciable time (decades).

Vertical fluxes in the interior areas of the wetland 

basins were relatively small and varied over time, frequently changing direction. To put these fluxes in the 

perspective of other water balance fluxes, recharge and 

discharge in interior areas were usually less than 50 

percent of average precipitation and evapotranspiration 

(0.4 cm/d and 0.35 cm/d, respectively). Nevertheless, 

recharge and discharge in the wetland interior still are 

quite important. For example, recharge in the interior 

of WCA-2A accounted for a volumetric flux equal to 

30 percent of surface flow through that basin.

Little is known about the factors that control vertical exchange in the wetland interior. Hydrogeologic 

model simulation showed that recharge and discharge 

in the wetland interior are not explained by water-level 

differences across levees. Instead, it is more likely that 

the local and regional water-surface slopes, controlled 

by topography and water releases between basins, have 

a greater effect on recharge and discharge in the wetland interior. The factor discussed in great detail in this 

report was the effect of large releases of water in 

WCA-2A that transmit waves of surface water through 

the wetlands, and induce pressure gradients that cause 

cycles of recharge and discharge. Another important 

factor is the layering of different aquifer sediments in 

the aquifer, which transmits surface-pressure perturbations at different rates. The relation between water 

releases and hydraulic properties of aquifer sediments 

and peat drives vertical exchange in ways that are not 

yet fully understood. It is clear, however, that recharge 

and discharge are related closely with fluctuating 

surface-water levels. Reversal between recharge and 

discharge in the wetland interior may not affect the 

long-term average water balance; however, those vertical fluxes could contribute to net exchange of solutes, 

including contaminants, between wetlands and ground 

water. Future investigations will require better field 

measurements and more sophisticated modeling to test 

the importance of such factors in controlling 

surface-water and ground-water interactions.

Geochemical evidence provides a long-term integrated picture of hydraulic processes in the aquifer and 

exchange with surface water. Water-stable isotopes 

indicated that the major source of fresh water in Everglades ground water is recharge of evaporated surface 

water from the wetlands rather than infiltration of rainfall through unsaturated sediments. This observation is 

consistent with the interpretation that both recharge and 

discharge have always occurred in the Everglades, with 

recharge occurring at time periods of relatively high 

surface-water levels and discharge occurring when 

surface-water levels were low.
Summary and Conclusions 77

Major ion chemistry and water-stable isotopes 

identified eight major water types in the north-central 

Everglades. Four of those types were considered to be 

source waters. The other four are mixtures of the 

source waters. The distribution of those water types 

identified long-term average flow pathways of recharge 

and discharge. Particularly evident as a major factor 

driving recharge and discharge in the present-day Everglades was the effect of wetland compartmentalization 

by levees. Geochemical water typing provided information that was consistent with hydraulic information. 

The geochemical tracers, however, particularly were 

useful in showing the extent to which water management has caused vertical mixing throughout essentially 

the entire depth of the Surficial aquifer. The effect of 

levees is indicated by the presence of Fresh Recharge 

Water recharged to a depth of at least 90 ft on the 

up-gradient (headwater) side of the levee, and Relict 

Seawater discharged on the down-gradient (tailwater) 

side of the levee. The effect of levees on interactions 

between surface and ground water, therefore, extends 

vertically throughout at least the top half of the Surficial aquifer (100 ft), and horizontally for at least 1 mi 

away from the levees. These observations support the 

hydraulic interpretation that water management has 

increased interactions between surface water and 

ground water in the north-central Everglades. 

A wide-ranging assessment of the distribution of 

mercury in surface water, peat porewater, and ground 

water found no evidence that ground water was a significant source of mercury to surface water. Rather, 

peat and surface water appear to be a source of mercury 

to ground water, transported with recharging ground 

water. Maximum values of total mercury (HgT) in shallow ground water were approximately equal to corresponding measurements in surface water at both ENR 

and WCA-2A. Methylmercury (MeHg) was detectable 

only in certain shallow wells at ENR and WCA-2A. 

Those wells had MeHg concentrations that were 50 

percent and 10 percent of the highest MeHg concentrations in surface water at ENR and WCA-2A, respectively. Whereas HgT was detectable throughout the 

Surficial aquifer, MeHg was mainly present at detectable concentrations only in ground water recharged at 

the ENR site when that area was managed for agriculture.

Variable SO4 concentrations do not co-vary with 

MeHg concentrations at either ENR or WCA-2A. 

Source of recharge water generally is thought to cause 

variability in mercury concentrations. For example, 

MeHg only was detectable in shallow ground water 

beneath ENR in wells where major ion chemistry and 

water-stable isotopic ratios indicated that agricultural 

recharge water was the source. Eight of 17 samples 

from wells with agricultural recharge water had detectable MeHg, with values as high as 0.2 ng/L. In contrast, 

well samples with wetland recharge water all had very 

low values of MeHg (less than 0.04 ng/L). Type of 

recharge water was not as important for HgT, because 

measurements greater than 0.5 ng/L were distributed 

equally between the agricultural recharge water and 

wetland recharge water. 

Ground-water fluxes of mercury were determined as part of a steady-state hydrologic and mercury 

balance in ENR. The resulting mercury flux computations indicated that ground-water discharge to ENR 

from the eastern side (WCA-1) was not appreciable, 

contributing approximately 0.4 and 0.3 percent of HgT

and MeHg to ENR, respectively. However, recharge on 

the western side of the ENR was a major pathway for 

export of HgT from ENR. Recharge of HgT accounted 

for an export of 68 g/yr, or 10 percent of all inputs to 

ENR. Recharge of MeHg was negligible, either below 

detection, or, if values of MeHg close to detection levels are used, accounting for no more than export of 

1 g/yr (3 percent of inputs). 

Based on the mass budget for Hg and other evidence, it was concluded that recharge of HgT was augmented with release of more HgT during downward 

transport through the peat. The primary evidence for 

this conclusion is that average concentrations of HgT

measured in shallow ground water were higher than 

average values measured in surface water. Independently measured profiles of HgT in peat porewater corroborate that conclusion, showing that HgT

concentrations are high throughout the entire depth of 

peat. Therefore, it appears that solid-phase mercury 

stored in the peat is being mobilized and recharged to 

the aquifer. Mass-balance estimates suggest that 70 

percent of the recharged HgT comes from surface 

water, whereas 30 percent comes from release from the 

peat. In contrast to results for HgT, it was found that 

MeHg was not recharged to the aquifer. Support for 

this conclusion comes from measurements of MeHg in 

shallow ground water that are below detection, and 

from porewater profiles in peat that show relatively 

high concentrations of MeHg are restricted to very near 

the peat surface. 
78 Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central Everglades

Chemical data and water-stable isotopic ratios 

indicate that most surface water recharged in ENR is 

discharged to a seepage canal on the western and northern side of ENR. Transport of recharged water through 

the aquifer to the seepage canal appears to take place in 

a matter of weeks to months, with only relatively minor 

mixing with deeper ground water. Measurements of 

HgT in the seepage canal suggested that HgT had not 

yet discharged to the canal at the end of the 4-yr study 

period (1994-98). Because the travel time between 

points of recharge in ENR and discharge in the seepage 

canal was relatively short (weeks to months), it was 

concluded that mercury was retained or delayed in its 

transport through the aquifer by interaction with aquifer sand or limestone or fine organic materials at the 

base of the seepage canal.
References Cited 79

REFERENCES CITED

Abtew, W. and Mullen, V., 1997, Water budget analysis for 

the Everglades Nutrient Removal Project: Technical 

Memorandum, South Florida Water Management 

District, WRE #354, 21 p.

American Society of Testing and Materials, 1991, Standard 

test method for particle-size analysis of soils, D-422: 

Philadelphia, Pa., 6 p.

Barlow, P.M. and Moench, A.F., 1998, Analytical solutions 

and computer programs for hydraulic interaction of 

stream-aquifer systems: U.S. Geological Survey 

Open-File Report 98-415A, 85 p.

Bates, A.L., Orem, W.H., Harvey, J.W., and Spikier, E.C., 

2002, Tracing sources of sulfate in the Florida 

Everglades: Journal of Environmental Quality, v. 31, 

p. 287-299.

Beach, D.K., 1982, Depositional and diagenetic history of 

Pliocene-Pleistocene carbonates, northwestern Great 

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