publications > scientific investigations report > surface-water and ground-water interactions > introduction > purpose and scope

The Everglades is a subtropical coastal wetland that extends 160 km from Lake Okeechobee to Florida Bay in southeastern Florida (fig.1). Before water management began in the Everglades, large quantities of fresh surface water moved southward by overland sheet flow through the broad wetland system, ultimately discharging to the Atlantic Ocean, Florida Bay, or the Gulf of Mexico, depending on the particular flow path through the wetlands. Beneath the wetlands is ground water flowing in limestone and sand sediments of the Biscayne and Gray Limestone aquifers, known collectively as the surficial aquifer system (fig. 1b).

Beginning about 1910, construction of canals began in the Everglades initially for the purpose of drainage and flood control. The early canals extended southeast from Lake Okeechobee to the Atlantic Ocean (Light and Dineen, 1994). With the passage of time came growing concerns that the Everglades needed to be managed for water supply in addition to being managed for flood control. Beginning in the 1950s, additional systems of canals and levees narrowed the main flow-way and completely encircled parts of the Everglades, creating a series of enclosed basins called Water Conservation Areas (WCAs). The purpose of the WCAs went beyond just flood control, and included water storage for later delivery to the growing population of the lower east coast of Florida as well as to Everglades National Park. The construction and management of the WCAs (and the associated drainage and subsidence of areas outside) have altered the Everglades ecosystem in profound ways. Decreasing surface-water flows and deteriorating water quality are blamed for declines in wading bird populations, disappearance of tree islands, and replacement of native plant communities by cattails (Jensen and others, 1995; McCormick and others, 1998; Rutchey and Vilchek, 1999). In the past 20 years, these concerns have fueled wide-ranging discussions on how to improve water management in the Everglades. In 2000, Congress approved the Comprehensive Everglades Restoration Plan (CERP), with the goal to restore (to the extent possible) predrainage conditions in the central Everglades.

Figure 1. Central Everglades and adjoining areas, south Florida, showing in (A) locations of Water Conservation Areas (WCA), Everglades Nutrient Removal (ENR) project, and Stormwater Treatment Areas (STA) and showing in (B) generalized hydrogeologic features across Broward County, in the central Everglades, South Florida. [larger image]

Evaluating the success of restoration efforts depends on reliable hydrologic information, including quantification of interactions between surface water and ground water. Surface water and ground water in the Everglades are exchanged across the wetland ground surface by processes known as recharge (flow from surface water to ground water) and discharge (flow from ground water to surface water). Recharge and discharge are generally assumed to have been relatively small under the predrainage conditions in Everglades, as a result of the small natural topographic gradients and the wide expanse of wetland available for dissipation of floodwater. The principal topographic features in the central part of the predrainage Everglades are related to the ridge and slough topography, which gave the Everglades a corrugated appearance by alternating between sawgrass ridges and sloughs on spatial scales of hundreds of meters. Only recently has there been speculation about how the topography of ridges and sloughs, and tree islands, may have evolved, and presently be maintained, due to the complex feedbacks between hydrologic driving forces, sediment transport and accumulation, carbon and nutrient dynamics, and plant performance (National Research Council, 2003, Science Coordination Team, 2003). The earliest observations that interactions between surface water and ground water could be important to Everglades hydrology began in the 1950s with concerns raised about large amounts of "seepage" (or ground-water underflow) that began to occur beneath the eastern boundary levee that separated the remaining wetlands of the Everglades from the growing urban population to the east (U.S. Army Corps of Engineers, 1952). Although still a major concern for water managers, seepage is not the only issue requiring a better understanding of surface-water and ground-water interactions. For example, the deeper ground water beneath the Everglades is high in dissolved salts due to its origin as entrapped sea water during higher sea level stands in an earlier geologic time period. Increased recharge and discharge is bringing more dissolved salts into surface water, and the increasing load of salts is contributing to an upset of subtle biological and geochemical dependencies that influence plant community structure in this unique ecosystem (McCormick and others, 1998). Furthermore, changes in surface-water and ground-water interactions may be involved in storing phosphorus and other surface-water contaminants that are currently entering the Everglades. A thorough understanding of surface-water and ground-water interactions is essential to understanding how long this legacy of contamination could last, and how far it could be transported downstream in the Everglades under "restored" flows. Concerns are not just for the ecosystem. For example, municipal water budgets indicate that an important source of drinking water comes from recharge of central Everglades water in into the Biscayne aquifer (particularly in WCA-3A) and eastward movement toward domestic well fields. There is relatively little understanding, however, of the source areas, flow paths, and travel times required for Everglades surface water to reach domestic water-supply well fields. There is also little understanding of the role of ground-water discharge in sustaining sensitive wetland ecosystems during drought.

From the standpoint of water quality, surface-water and ground-water interactions could be important in affecting much more than just transport and storage of phosphorus. More information is needed about how pore water in Everglades peat and shallow ground water functions as a reservoir not only for phosphorus, but for a host of surface-water contaminants including sulfate from agricultural drainage, atmospheric-derived mercury, dissolved organic carbon, dissolved salts from discharge of deep ground water of marine origin with relatively high sulfate and chloride concentrations, and volatile organic carbons of uncertain origin (Krabbenhoft and others, 1998; Bates and others, 2002; Harvey and others, 2002). Increasingly contaminated Everglades surface waters are cycled back and forth between wetland surface water and the shallow ground-water system by the processes of recharge and discharge. Over time, that exchange of water between the surface and subsurface is having the effect of replacing what was previously a layer of very high-quality, fresh ground water near the top of the aquifer with contaminated surface water (Harvey and others, 2002). Both the physical mechanism of contaminant storage in the aquifer and the chemical reactions that occur there may affect contaminant mobility. Contaminants stored in ground water potentially can return to surface water with discharging ground water long after restoration management improvements have been implemented. As stated above, little is known about how the current distribution of contaminants in Everglades waters and soils will spread under higher "restored" flows. The potential for these legacies of contamination to affect future water quality in the Everglades is significant, and predicting those effects requires a better understanding of how to quantify surface- and ground-water interactions, and how to determine the processes controlling the magnitude of those interactions.

A previous USGS Open-File report (Harvey and others, 2000) describes many the methods and materials used in the overall investigation in great detail, including all of the specifics regarding borehole drilling methods, geophysical measurements, sampling of ground-water geochemistry, and design and operation of shallow piezometers and seepage meters. That report provided appendixes that include all of the data collected prior to September 1998. Another recent report by Harvey and others (2002) documented hydrogeologic and ground-water geochemical patterns in the central Everglades and used that information to explain the occurrence and fate of mercury in Everglades ground water.

In an effort to improve understanding of interactions between ground water and surface water in the Everglades and its consequences for water quality, the U.S. Geological Survey (USGS), began a second cooperative research investigation in 1999 with the South Florida Water Management District (SFWMD). Together, those organizations conducted a detailed investigation of hydrologic interactions between surface water and ground water in Water Conservation Area 2A (WCA-2A), a 42,492-ha basin in the central Everglades. Fieldwork for the investigation was completed in 2002.

Purpose and Scope

This report presents the results of an investigation to develop reliable methods of quantifying surface- and ground-water interactions in the central Everglades, south Florida. The focus of the investigation is Water Conservation Area 2A (WCA-2A), with additional information presented from the Everglades Nutrients Removal (ENR) Project, and from single sites in WCA-2B and WCA-3A (fig. 1). In this investigation, three new methods were used to quantify recharge and discharge in the interior wetland areas of the central Everglades. These methods were (a) the Darcy-flux calculation approach, based on measured vertical gradients in hydraulic head and hydraulic conductivity of peat, to calculate vertical fluxes between ground water and surface water; (b) modeling the vertical transport and decay of the naturally occurring short-lived radium isotopes ²²⁴Ra and ²²³Ra through peat; and (c) modeling the transport and decay of naturally occurring and "bomb-pulse" tritium (³H) in surface water and ground water. The report includes discussion of the physical factors that affect recharge and discharge in the central Everglades, including effects of geologic materials and their hydrogeologic properties; seasonal and interannual climate fluctuations; and effects of water management, including ponding of water at different elevations across levees, as well as the release of large pulses of surface water between WCAs through water-control structures.

One purpose the report is to compare results acquired by testing different methods of quantifying recharge and discharge side by side in WCA-2A wetlands. Several of the new methods are based on modeling the distribution of naturally occurring solute tracers (radium, tritium, and tritium-helium ratios) that are transported with water in the central Everglades. Cross-comparisons between those estimates and other independently acquired estimates based on hydraulic measurements provide insight about advantages and limitations of each method, and about the effect of differences in the spatial and temporal averaging of each method on overall results. To help place the new recharge and discharge estimates in the perspective of previous understanding in the Everglades, the new estimates were also compared with recharge and discharge estimates from the South Florida Water Management Model (South Florida Water Management District, 1999).

In addition to the above-stated research purposes, the present report also functions as an outlet for data sets not previously published in the prior reports. Appendix 1 provides detailed locations of research sites and additional information about wells and piezometers. Appendix 2 expands previously published data sets on hydraulic conductivity in peat and in sediments that are transitional to the underlying aquifer. Appendix 3 reports hydraulic-head calibration data collected at surface-water recorders and wells. Appendixes 4-17 illustrate the measured water levels in surface water and wells for a period of record that is typically 1998-2002. Appendixes 18-21 report all of the geochemical data collected in WCA-2A after 1998.

In the next several sections background information is given about hydrologic setting, hydrogeology, and a summary of previous investigations of interactions between surface water and ground water in the Everglades.