ABSTRACT

Marine estuaries often rely on significant quantities of freshwater to sustain their ecosystems. Most estuaries typically receive most of their freshwater from surface runoff, but during dry periods, submarine groundwater discharge can provide relatively substantial amounts of freshwater. Effective management of marine estuaries, therefore, requires a thorough understanding and quantification of both runoff and submarine groundwater discharge. The ability to quantify submarine groundwater discharge is complicated by the effects of density variations in coastal groundwater. Fresh groundwater flows toward the coast and mixes with saline groundwater in the freshwater/saltwater transition zone before it discharges to a marine water body. Development of a variable-density groundwater flow model is one method used to estimate rates of submarine groundwater discharge and to numerically simulate the coastal groundwater flow processes. Although this method contains a large degree of uncertainty, it seems to be a reasonable approach for estimating large-scale rates of submarine groundwater discharge.

In southern Florida, new water management practices have been proposed torestore the ecosystem and to protect a diverse marine resource contained within Biscayne Bay. Some proposed alternatives could impact the timing and quantity of groundwater that discharges to the bay. As part of a study initiated in 1996, groundwater discharge rates to Biscayne Bay were quantified and new numerical tools were developed to evaluate the hydrologic effect of proposed management practices. Regional-scale rates of submarine groundwater discharge were quantified by calibrating a three-dimensional, variable-density, groundwater flow model using field measurements of head, canal baseflow, and the position of the freshwater/saltwater transition zone. This regional-scale model simulates groundwater flow for nearly a 10-year period. Two-dimensional, vertical cross sectional models were constructed to determine the dispersive properties of the Biscayne aquifer and to simulate the local-scale patterns of submarine groundwater discharge. The SEAWAT code, which is a combined version of MODFLOW-88 and MT3DMS, was used for the variable-density simulations.

Results from the numerical simulations suggest that groundwater discharges to Biscayne Bay at an average rate of about 2 x 10⁵ m³/day for a 100 km length of shoreline. When compared with measured runoff rates, simulated groundwater discharge constitutes about 5 percent of the freshwater discharge to the bay. During the 1989 and 1990 dry seasons, the simulated rate of fresh groundwater discharge exceeded measured surface-water runoff. During the wet season, fresh groundwater discharge is less than 1 percent of the measured surface-water runoff. Results from the model also suggest that the northern half of Biscayne Bay receives nearly 100 percent of the fresh groundwater discharge. Simulated rates of groundwater discharge to the southern half of the bay are minimal because of a low-lying coastal water table.

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(This abstract was taken from the proceedings of The First International Conference on Saltwater Intrusion and Coastal Aquifers -- Monitoring, Modeling, and Management, Essaouira, Morocco, Workshops -- April 18-21, 2001, Conference -- April 23-25, 2001)