Home Introduction - Purpose - Hydrologic Setting - Site Information Hydrogeology of NC Everglades Quantifying Recharge and Discharge Use of Geochemical Tracers Effect of GW and SW Interactions Summary References PDF Version

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

Figure 2. (Left) Area in red is shown enlarged in image on the right. (Right) 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. [click for larger image]

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 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.

Figure 3. (Left) Area in red is shown enlarged in image on the right. (Right) 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. [click for larger image]

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.

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