Concerns about the flow and chemical quality of water in the Everglades have opened new discussion on how best to manage the conveyance of surface water through this ecosystem. Plans call for improving chemical quality of water entering the compartmentalized basins of the north and central Everglades, called Water Conservation Areas (WCA's), as well as maintaining surface flows through WCA's to Everglades National Park. Several restoration efforts are already underway, including the construction of large constructed wetlands called Stormwater Treatment Areas (STA's) at the northern terminus of the Everglades to intercept agricultural drainage and to remove excess nutrients. Evaluating the success of ongoing restoration efforts and planning for future restorations depends on reliable hydrologic information, including a better understanding of the role of interactions between surface water and ground water. The purpose of the present project is to (1) quantify hydrologic fluxes between surface water and ground water in areas where limited prior information exists, and (2) use new estimates of hydrologic fluxes to improve the accuracy of hydrologic budgets and chemical mass balances for constituents such as mercury, sulfate, and nutrients. Investigations are underway in three principal areas. The first area is the 4,000-acre Everglades Nutrient Removal (ENR) area, a prototype STA; the second area is WCA-2A, a 105,000-acre basin with a long history of accumulating excess nutrients; and the third area is central Taylor Slough, where the goal of ongoing restoration efforts is to maintain southerly flows through the slough to Florida Bay while protecting chemical quality and increasing flood protection for the agricultural and residential areas to the east of the Park. The main users of the data are the Everglades Restoration Department and Planning Department of SFWMD (South Florida Water Management District).

Our findings are summarized below:

1. Management of water levels in the compartmentalized WCA's has enhanced interactions between surface water and ground water by establishing and maintaining water-level differences across levees that drive substantial underflow beneath the levee through the permeable limestone of the surficial aquifer. We know from published work that water fluxes are significant through the permeable bottoms of canals immediately adjacent to levees, but our results are some of the first to indicate significant fluxes through the less-permeable wetland peat. Although lower in magnitude than hydrologic fluxes through canal bottoms, vertical fluxes through peat are important because they occur over a much larger area of the wetlands, extending miles from the levee. For example, the relatively high water levels that are maintained in WCA-1 drive ground-water flow under the L-7 and Hillsboro levees and upward into the eastern part of the ENR and the northern part of WCA-2A, respectively. Downward flow occurs in central and western ENR, central and eastern WCA-2B, and central WCA-2A.

2. In addition to man's influence on interaction of surface water and ground water, another major control is the distribution of transmissive and restrictive zones for ground-water flow in the aquifer. A limestone layer near the top of the surficial aquifer is the main conduit that links recharge areas in WCA-1 with discharge areas in the ENR and WCA-2A. The transmissive limestone layer is located between +10 and -30 feet NGVD (1929 National Geodetic Vertical Datum) and has horizontal hydraulic conductivities ranging between 150 cm/d (centimeters per day) in the denser parts of the limestone to greater than 6500 cm/d in well-indurated parts. Underlying the limestone is coarse sand to -70 feet NGVD grading to fine sand at the base of the surficial aquifer at -180 feet NGVD. Horizontal hydraulic conductivities in sands range from 30 cm/d in the fine sands to greater than 6,500 cm/d in the coarsest sands. At the top of the surficial aquifer is 2 to 4 feet of organic wetland sediment (peat) that acts as a hydraulically restricting layer that impedes vertical flow. The median estimate of vertical hydraulic conductivity of peat in northern Everglades sites was 17 cm/d, which is similar to that of a very fine sand, but lower by several orders of magnitude lower than hydraulic conductivities of the limestone and sand aquifer that underlies the peat.

3. Measured vertical fluxes through peat in the northern Everglades ranged from less than 0.04 cm/d (detection limit) to 10 cm/d, which, at the upper limit, is more than an order of magnitude higher than average daily precipitation or evapotranspiration (approximately 0.5 cm/d). Fluxes generally decreased with distance from levees toward central areas of the compartmentalized WCA's, where the direction and magnitude of vertical fluxes responds more to regional influences. The net, long-term interaction between ground water and surface water in the WCA's appears to be a small downward flux from surface water to ground water. The net downward flux is most likely a response to the long-term effect of storing water and maintaining relatively high water levels within the WCA's. Outside the Everglades, water levels have tended to decline over the past 50 to 80 years due to groundwater withdrawals and subsidence in some areas.

4. Vertical hydrologic fluxes between surface water and ground water were determined by direct measurements using seepage meters and indirect estimates based on modeling of vertical transport of chloride in peat. Neither measurement technique worked at all study sites. Seepage-meters were most reliable at sites where vertical hydraulic gradients in the peat were greater than 0.1. The average uncertainty of vertical flux estimates determined from replicate seepage-meter measurements was 50 percent. Modeling vertical transport of chloride in porewater to estimate hydrologic fluxes was generally a less reliable technique, due to difficulties in specifying fluctuations in chloride concentrations in surface water for the upper boundary condition. Currently we are quantifying the role of vertical hydrologic fluxes as sources and sinks for sulfate and mercury in WCA-2A and ENR, respectively.

5. Surface flow in Taylor Slough in the eastern part of Everglades National Park is augmented by surface water and shallow ground-water flowing southeasterly from the pine islands into Taylor Slough. Some of the southerly flow of water becomes ponded behind the Old Ingraham Highway, but there is clear evidence that at all times of the year a substantial amount of water flows beneath the highway through the porous road bed and aquifer, emerging in Taylor Slough. A historical canal usually referred to as the Rookery Canal also plays a role in distributing the shallow drainage across Taylor Slough. Environmental chemical tracers are being used to estimate the volumetric rate of ground-water inflow to Taylor Slough.

(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)