VARIABLE-DENSITY SURFACE WATER/GROUND WATER FLOW AND SOLUTE TRANSPORT MODELING FOR THE SOUTHERN EVERGLADES OF FLORIDA

Restoration for the Florida Everglades relies heavily on hydrologic models that simulate optimum water management design. To support the restoration effort, the U.S. Geological Survey is developing two integrated surface water/ground water flow and solute transport models for Everglades National Park, Big Cypress National Preserve, and surrounding areas. Both models provide hydrologic input to biological models that simulate population dynamics for threatened or endangered species. The SICS hydrologic model, developed for the Taylor Slough basin, has a 300-meter grid spacing and encompasses 900 square kilometers. The TIME model, which encompasses Taylor Slough and Shark Slough, has a 500-meter grid spacing and covers 6800 square kilometers. Overland flow in the wetlands is simulated using the SWIFT2D hydrodynamic code. The SEAWAT code is used to simulate variable-density ground water flow in the underlying permeable Biscayne aquifer. The surface and ground water models are explicitly coupled through a leakage flux and an associated leakage concentration, which allow the exchange of fluid and salt mass between the wetland and aquifer. Model results show that spatial variations in leakage rates correspond with the position of the freshwater/saltwater interface in the aquifer, located 5 to 25 kilometers inland from the Gulf of Mexico or Florida Bay. Fresh ground water flows toward the coast, mixes with saline ground water, and discharges into the wetland. Model results suggest that leakage rates are temporally variable. Surface water recharges the aquifer after heavy rainfall events. As surface water stages decline in response to runoff and evapotranspiration, ground water discharges upward into the wetland. The exchange of salt mass caused by leakage affects the simulated wetland salinity, an important parameter for biological models.