USGS Publications Warehouse

Abstract

Recharge and discharge are hydrological processes that cause Everglades surface water to be exchanged for subsurface water in the peat soil and the underlying sand and limestone aquifer. These interactions are thought to be important to water budgets, water quality, and ecology in the Everglades. Nonetheless, relatively few studies of surface water and ground water interactions have been conducted in the Everglades, especially in its vast interior areas. This report is a product of a cooperative investigation conducted by the USGS and the South Florida Water Management District (SFWMD) aimed at developing and testing techniques that would provide reliable estimates of recharge and discharge in interior areas of WCA-2A (Water Conservation Area 2A) and several other sites in the central Everglades. The new techniques quantified flow from surface water to the subsurface (recharge) and the opposite (discharge) using (1) Darcy-flux calculations based on measured vertical gradients in hydraulic head and hydraulic conductivity of peat; (2) modeling transport through peat and decay of the naturally occurring isotopes 224Ra and 223Ra (with half-lives of 4 and 11 days, respectively); and (3) modeling transport and decay of naturally occurring and 'bomb-pulse' tritium (half-life of 12.4 years) in ground water. Advantages and disadvantages of each method for quantifying recharge and discharge were compared. In addition, spatial and temporal variability of recharge and discharge were evaluated and controlling factors identified. A final goal was to develop appropriately simplified (that is, time averaged) expressions of the results that will be useful in addressing a broad range of hydrological and ecological problems in the Everglades. Results were compared with existing information about water budgets from the South Florida Water Management Model (SFWMM), a principal tool used by the South Florida Water Management District to plan many of the hydrological aspects of the Everglades restoration. A century of water management for flood control and water storage in the Everglades resulted in the creation of the Water Conservation Areas (WCAs). Construction of the major canals began in the 1910s and the systems of levees that enclose the basins and structures that move water between basins were largely completed by the 1950s. The abandoned wetlands that remained outside of the Water Conservation areas tended to dry out and subside by 10 feet or more, which created abrupt transitions in land-surface elevations and water levels across the levees. The increases in topographic and hydraulic gradients near the margins of the WCAs, along with rapid pumping of water between basins to achieve management objectives, have together altered the patterns of recharge and discharge in the Everglades. The most evident change is the increase in the magnitude of recharge (on the upgradient side) and discharge (on the downgradient side) of levees separating WCA-2A from other basins or areas outside. Recharge and discharge in the vast interior of WCA-2A also likely have increased, but fluxes in the interior wetlands are more subtle and more difficult to quantify compared with areas close to the levees. Surface-water and ground-water interactions differ in fundamental ways between wetlands near WCA-2A's boundaries and wetlands in the basin's interior. The levees that form the WCA's boundaries have introduced step functions in the topographic and hydraulic gradients that are important as a force to drive water flow across the wetland ground surface. The resulting recharge and discharge fluxes tend to be unidirectional (connecting points of recharge on the upgradient side of the levee with points of discharge on the downgradient side), and fluxes are also relatively steady in magnitude compared with fluxes in the interior. Recharge flow paths are also relatively deep in their extent near levees, with fluxes passing entirely through the 1-m peat layer and interacting with a substantial portion (greater than 30 m) of the ground water in the underlying sand and limestone aquifer. The recharged water flows beneath the levees and is discharged in an adjacent basin or outside the Everglades, and therefore contributes to the basin-scale water balance in WCA-2A. Unlike recharge and discharge near levees, fluxes in the interior areas of WCA-2A are highly unsteady in magnitude and frequently undergo reversals in direction. Because of the highly transient nature of these fluxes, the depth of exchange between surface water and ground water in the wetland interior was not as deep (< 8m) as locations in the vicinity of levees. In contrast to levee-driven fluxes, the fluxes in the interior of WCA-2A probably are only important to seasonal (or shorter) timescale variations in the basin-scale water balance. This is because recharge and discharge in the interior of WCA-2A are too shallow and too far from levees to cause a net exchange with areas outside the basin. Although the recharge and discharge fluxes in WCA-2A's interior are smaller on a per unit area basis compared fluxes near levees, they are nevertheless the dominant interaction between surface water and ground water in WCA-2A when considered as a whole. Dominance of surface-water and ground-water interactions in the interior wetlands results from the very large ratio of wetlands in the interior compared with wetlands close to the levees. A simple hydrogeological model accurately predicted the effect of water-level differences across levees on recharge and discharge, but the model was insufficient to explain why recharge and discharge were also significant in the wetland interior. The pattern of recharge and discharge fluxes was at a maximum near the levee (approximately 2 cm/day), and decreased exponentially with distance until modeled fluxes became insignificant. Agreement between modeled and measured results deviated beyond a distance of 600 m, with the model predicting that recharge and discharge fluxes would decline to insignificance while measurements in the WCA-2A interior (based on head measurements and Darcy-flux calculations) showed that recharge and discharge fluxes remained significant throughout the basin (ranging generally between 0.2 and 1 cm/day, or approximately a factor of two to ten times smaller than the maximum flux near levees). These interior fluxes are the dominant interaction between surface water and ground water in WCA-2A because of the large ratio of interior wetland area compared to wetland area near levees. Recharge and discharge in the WCA-2A wetland interior reversed in direction on weekly, monthly, and annual timescales according to a 5-year time series (1997-2002) of hydraulic data. Ground-water discharge tended to occur during average to moderately dry conditions when local surface-water levels were decreasing. Recharge tended to occur during moderately wet periods or during very dry periods just as water levels began to increase. The cyclic variation in recharge and discharge is driven by the differential responses of surface water and ground water to annual, seasonal, and weekly trends in precipitation and operation of water-control structures. For example, a meteorological event such as heavy rainfall in one area of the Everglades causes fluctuations in the surface-water level that are transferred to other nearby areas. The surface-water and ground-water systems have different response times to these perturbations. It is these differential response times to perturbations in water level that, along with hydraulic conductivity of peat soil, determine the magnitude and direction of vertical fluxes across the wetland surface. One of the unintended effects of water management involves the growing number and capacities of water pump and spillway operations. These operations have increased the range of surface-water level fluctuations in the interior areas of the water conservation areas relative to the predrainage Everglades. Following major releases of surface water between basins, gravity waves move toward the central parts of the basin that cause relatively high frequency fluctuations in surface-water levels and ground-water hydraulic heads. Since head fluctuations are not instantaneous (and propagate at different rates in the surface water and ground-water systems), there are concomitant fluctuations in vertical hydraulic gradients which cause the magnitude and direction of vertical fluxes to alternate between recharge and discharge as the gravity waves move toward the center of the WCAs. The highly transient nature of surface-water fluctuations and ground-water responses in the interior parts of the wetland interior causes fluctuations in recharge and discharge on a variety of timescales. Quantifying the time-averaged behavior of recharge and discharge was an important goal of the present study, and environmental solute tracers were potentially well suited to accomplishing that task. Comparison between results gained using short-lived radium isotopes as a tracer in peat pore water, and tritium in ground water, showed that most recharged water in the Everglades only moves through relatively shallow flow paths in the peat before being discharged back to surface water. Only a small proportion of the total amount of recharged water (a few percent) enters the deeper flow paths that pass through the sand and limestone aquifer. The exceptions are wetlands within a half kilometer or so of levees, where the percentage of recharged water flowing through the sand and limestone aquifer is considerably higher (50% or more). A comparison with Darcy-flux calculations demonstrated the advantage of environmental solute tracers in avoiding the problem of accurately estimating hydraulic conductivity in the sediments. Tracer methods also have challenges, in particular the problem of differing sensitivities of various tracers. Due to the relatively narrow range of sensitivity each tracer has to a particular timescale of recharge and discharge, a single tracer will generally only will be appropriate for characterizing a subset of the total recharge and discharge fluxes. This is the result of the differential detection capability of the various tested tracers across the very broad distribution of residence timescales of recharged water. The flux estimates acquired by tracers in the present study varied over two orders of magnitude (0.01-2 cm/d), with the differences reflecting the portion of total recharge that a particular tracer is sensitive to. It is important to note that there is no measurement of recharge and discharge (tracer based or hydraulic) that is not affected by issues of scale dependence. For example, the Darcy-flux calculations discussed earlier also produce results that are scale dependent, as illustrated by a comparison between estimates for WCA-2A that were averaged for two different timescales (daily and annually) and estimates that were averaged spatially for two different spatial scales (meters and kilometers). Thus, there is no single measure of recharge and discharge in the wetland interior that can be scaled to all possible problems of interest. Understanding scale dependence is beneficial to a comparison of the results of this study with the results of the South Florida Water Management Model (SFWMM), a hydrological model used extensively by the South Florida Water management District (SFWMD) to design many of the hydrological aspects of the Everglades restoration. Because the SFWMM is spatially discretized on a 2-mile by 2-mile square grid, and because recharge and discharge are often estimated from modeling results by averaging on annual or longer timescales, the SFWMM also is subject to scale dependence in its results. Like tritium modeling, longer runs of the SFWMM generally provide results that reflect longer timescale and deeper subsurface interactions between surface water and ground water. For example, a decadal timescale run of the SFWMM (1979-1990 'calibration' simulation) produced an estimate of recharge and discharge (0.03 cm per day) that was consistent with tritium modeling (0.01 cm per day). Decreasing the length of a model run for the SFWMM appears to increase its sensitivity to shorter term interactions between surface water and ground water. For example, a shorter (5-year) run of the SFWMM (1991-1995 'verification' simulation) produced an estimate of recharge and discharge that was consistent with Darcy-flux calculations in this investigation (0.1 compared with 0.2 cm per day for the Darcy-flux calculations, respectively). The scale dependence of measurements of recharge and discharge requires that investigators choose a technique that is appropriately matched to spatial and temporal scale of the questions being addressed. For example, studies considering transport of contaminants and chemical reactions in surface water of the Everglades will need to pay particular attention to fast-timescale water exchange with pore water in the peat. On the other hand, studies concerned with questions about long-term recharge in the Everglades and its role in supplying water to well fields will need to pay attention to longer term exchanges between surface water and ground water in the sand and limestone aquifer, both beneath the immediate area of interest and outside the basin. Investigators can be most confident in their estimates of surface-water and ground-water interactions if a combination of hydraulic methods and more than one tracer with differing sensitivities to long and short timescale processes are used. Using what has been learned about the differing sensitivities of each method, the results can be combined to characterize the full distribution of timescales involved in exchange between surface water and ground water in the Everglades.