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Modeling decadal timescale interactions between surface water and ground water in the central Everglades, Florida, USA

2. Study area

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The research was conducted in Water Conservation Area 2A (Fig. 1A), a large (42,525 ha) wetland basin in the central Everglades. Levee construction to enclose WCA-2A began in the 1950s, and the basin was completely enclosed by levees and canals by 1963 (Light and Dineen, 1994). Presently, WCA-2A shares boundaries with WCA-1 to the north and northeast, the Everglades Agricultural Area to the west and northwest, lands developed for light industry and residential areas to the east, WCA-2B to the south, and WCA-3A to the southwest (Fig. 1B). The following paragraphs provide a brief summary of aspects of hydrogeology and water management in the central Everglades that are necessary to interpret study results.

Surface-water flow in the Everglades has been extensively modified and is managed to control floods and accommodate water needs of a large and rapidly growing urban area to the east of the Everglades, as well as agricultural uses of water on the northwest side. What remains of the Everglades north of Everglades National Park have been compartmentalized into a series of very large artificial basins called Water Conservation Areas (WCAs) that are greater than 40,000 ha in area. The source of surface flow in the WCAs is from canals that drain from Lake Okeechobee and from the Everglades Agricultural Areas (EAA), from direct precipitation, and from ground-water discharge to the wetlands or to the canals that drain into the wetlands (Fig. 1). Surfacewater flow passes from each conservation area to the next, moving southward through the wetlands, canals, culverts, and spillways until flow eventually reaches Florida Bay. Along the way, surface-water flow is augmented by precipitation and discharge from ground water, and depleted by evapotranspiration and by recharge to ground water. Due to the decreased water storage capacity of the altered wetland system, a large amount of surface water must occasionally be pumped eastward through canals to the Atlantic Ocean during periods of high water.

map showing location of Water Conservation Area 2A, map showing location of generalized hydrogeologic cross section and diagram illustrating layering of hydrogeologic section
Fig. 1. Location of data collection sites in Water Conservation Area 2A (WCA-2A), central Everglades, south Florida (A). Location of WCA- 2A within the larger Everglades system (B). Water Conservation areas are shown in light grey and location of a generalized hydrogeologic section labeled B-B'. Hydrogeologic cross-section shows the approximate location of the present study in relation to the Surficial aquifer system in southeast Florida (in white) and layering of two of its named aquifers (Biscayne and Gray limestone aquifers) and intervening semi-confining units (C). At the bottom boundary is a confining unit (shown in grey) that separates the Surficial aquifer from the deeper Floridan aquifer. Generalized hydrogeologic cross-section is from Reese and Cunningham (2000). [larger image]

Although wetlands of the WCAs occasionally dry out, they normally have standing water at depths that typically range from 15 cm to 1.2 m. Beneath the surface water in WCA-2A is a layer of organic peat approximately 1 m thick that was formed from the incomplete decomposition of sawgrass, water lilies, and other emergent plants. The peat is fibrous with a low mineral or ash content, usually less than 10% (Gleason and Stone, 1994). The peat is in direct contact with the top of the Surficial aquifer that underlies the Everglades. In some areas, the peat is separated from the aquifer by a relatively thin layer of calcareous mud. In other areas, peat is in direct contact with a limestone layer or sand layer at the top of the Surficial aquifer.

The Surficial aquifer beneath the central Everglades is approximately 60 m thick in eastern Broward County (Fig. 1C) and is composed of layers of variable thickness of sand, shell, and limestone (Reese and Cunningham, 2000). The Surficial aquifer overlies an aquitard called the Hawthorn Formation that restricts hydrologic communication with the deeper Floridan aquifer. When averaged over the entire depth of the Surficial aquifer, hydraulic conductivity is relatively high in the coastal ridge to the east of the Everglades and declines to the west (Fish and Stewart, 1991). The marked decrease in depth-averaged hydraulic conductivity from east to west in Palm Beach County accompanies the geological change from high porosity limestones and coarse sands in the east to limestones with variable degrees of cementation and finer sands in the western part of the Everglades (Miller, 1987; Harvey et al., 2002). In general, there is not as significant a decrease in hydraulic conductivity from east to west in the top 10 m of the Surficial aquifer (Harvey et al., 2002). The peat overlying the Surficial aquifer has a hydraulic conductivity that is approximately 2 orders of magnitude lower than the sand and limestone aquifer layers that lie beneath (approximately 30 cm d-1 compared with 2500 cm d-1).

Fig. 2A illustrates the lithology along the research transect where tritium was sampled in ground water. The deepest wells extended a little more than halfway (~35 m) into the Surficial aquifer, near the point of transition from the Fort Thompson formation in the Biscayne aquifer to the Tamiami formation, a semiconfining layer. At the top of the section is a 1-m layer of peat with undifferentiated freshwater marl and sand present just beneath it, and below that are layers of coarse sand (~5 m thick), limestone with sand stringers (~6-m thick), and sand transitioning to fines sand (~21-m thick). More detailed data and interpretation of the lithology are available in Harvey et al. (2002).

detailed hydrogeologic cross-section along research transect in Water Conservation Area 2A
Fig. 2. Detailed hydrogeologic cross-section along research transect in Water Conservation Area 2A (section A-A' shown in Fig. 1) where the model was applied (A), showing formation names and lithologic descriptions from Harvey et al. (2002). Conceptual model system showing one-dimensional transport of naturally occurring tritium tracer in surface water (B), where v equals the average surface-water velocity, d equals average surface-water depth, and D equals the longitudinal dispersion coefficient in surface water. Interaction between ground water and surface water is specified by a vertical exchange flux (qE) that determines the rate at which surface water is exchanged for ground water in a well-mixed layer of the aquifer with thickness dGWtheta-1, where dGW is the depth of water storage in the aquifer with detectable tritium, and theta is the aquifer porosity. [larger image]

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