The lack of reliable information about exchange between surface and ground water in the Everglades not only leaves the water balance uncertain in many areas of the Everglades, but it also hampers progress in determining chemical sources, chemical residence times, and the fate of nutrients and mercury in the Everglades. The present project recently completed emplacement of hydrological instrumentation at 17 sites in interior areas of the Everglades where nutrient and mercury dynamics are being investigated. The sites are located in the Everglades Nutrient Removal Area in Palm Beach County (ENR) and in Water Conservation Area 2a in Palm Beach and Broward Counties (WCA-2a). Two seepage meters were installed near existing research platforms at each site and two to four research wells were emplaced nearby to depths that range between 15 and 180 feet below ground surface. Surface-water level recorders and data logging instrumentation were also added. Measurement of vertical hydraulic gradients and vertical water fluxes demonstrated that significant vertical exchange occurs between the surficial aquifer system, wetland peat, and surface water at many of the research sites. Exchange fluxes ranged from as low as 0.03 centimeters per day (the limit of detection for seepage meters in this system) to as high as 30 cm/d in some areas, which is approximately two orders of magnitude higher than average daily precipitation and evapotranspiration for the area. The direction of exchange varied spatially at ENR (upward on the eastern side, downward on the western side) and varied seasonally at WCA-2a (upward flow in the mid-summer through fall during the wet season and downward flow in winter, spring, and early summer during the dry season). Why do exchange fluxes between surface and ground water predominantly vary spatially at the ENR and temporally in WCA-2a? ENR is a relatively small constructed wetland (4,000 acres) reclaimed from agricultural land that has been subjected to considerable excavation of drainage canals. The ENR wetland site is located between an impoundment where water is retained at high level (WCA-1) and a region in the Everglades Agricultural Area (EAA) where ground water is pumped away to maintain a low water table. WCA-2a is much larger in extent (105,000 acres) and is affected less by local pumping, drainage, excavation, or managed water levels in canals. Our interpretation is that the predominance of spatial variation in vertical exchange fluxes in the ENR reflects the proximity of its interior areas to canals, where surface-water levels are stabilized at high levels in some areas and low levels in other areas and then are maintained at those levels to achieve management goals. At the ENR the trend in upflow on the eastern side of the project and downflow on the western side varies only minimally with time because of the nearly constant water levels in WCA-1 and the EAA. The pronounced seasonal variability in vertical exchange at WCA-2a may also reflect management influences; for example the timing of water releases from WCA-1 into WCA-2a. However, the fact that the highest upward hydraulic gradients occur in WCA-2a in the wet season, when surface-water levels are highest, is not consistent with a control on vertical fluxes in WCA-2a that is imposed by the release of surface water from WCA-1. In contrast to the situation at ENR, seasonal changes in fluxes at WCA-2A suggest an overriding influence of regional climatic variability and the regional water balance rather than local management of water levels.

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