Large quantities of fresh ground water were discharged to Biscayne Bay prior to the 20th century. There are several historical accounts of ships that replenished their potable water supplies by drawing freshwater directly from Biscayne Bay. Beginning in the early 1900ís, an extensive network of drainage canals was constructed in southeastern Florida to lower the water table, reduce flooding, and increase the available land for agriculture and building development. These canals significantly altered the hydrology of southeastern Florida, lowering the water table and thus reducing ground-water discharge to Biscayne Bay. The current water-table altitude and gradient are not high enough to discharge large quantities of ground water into Biscayne Bay.

The flow of freshwater into Biscayne Bay is vital to the health of the estuarine ecosystem. In addition to the freshwater quantity, the timing of the freshwater input also is important. Prior to canal development, Biscayne Bay would have received most of its freshwater as discharge from upward ground-water flow through bottom sediments. These ground-water discharges to the bay are believed to have been relatively constant throughout the year, only fluctuating with the water-table head. In contrast, much of the current freshwater flow to Biscayne Bay occurs as pulse events of surface-water discharge at coastal canal structures, with most of the discharge occurring during the wet season (June to October). In 1995, 80 percent of the total canal flow to Biscayne Bay occurred during the wet season.

Water managers in southern Florida currently are evaluating alternative strategies for managing water resources. Some of these strategies involve changes to the canal operations that would route more water to Everglades National Park. In April 1996, the U.S. Geological Survey began a study to characterize the Biscayne aquifer near the Biscayne Bay western shoreline, quantify ground-water discharge to Biscayne Bay, and develop scientific tools that can be used to evaluate how changes in water-management scenarios would affect the ground-water discharge to Biscayne Bay.

Monitoring wells were installed in the Biscayne aquifer to help characterize the hydrogeology at the coast. These wells were installed in transects perpendicular to shore at three different locations: Coconut Grove, Cutler Ridge, and Mowry Canal. Each transect has nested well clusters both onshore and offshore. Cores were collected during the drilling of each well. The collection of water-level and salinity data from these wells began in March 1998. The data are being used to characterize the shape and position of the freshwater-saltwater interface and calibrate numerical models of ground-water flow.

Reliable and accurate methods for the direct measurement of ground-water discharge to Biscayne Bay do not exist. For this reason, variable-density, numerical flow models, which represent ground-water flow and the transport of salt, are being developed at two different scales. First, a regional three-dimensional flow and transport model will be used to simulate the flow of ground water toward the coast. This model will cover most of Miami-Dade County with a finite-difference grid, aligned parallel to the coast and a constant cell size both horizontally and vertically. The model will be used to simulate the regional flow of ground water toward Biscayne Bay during 1997 and 1998 and will be calibrated to measured water levels, canal flows, and salinity patterns using a monthly or weekly time step. Although this model will simulate variable-density ground-water flow, the resolution of the model grid cells will not be detailed enough to adequately characterize the freshwatersaltwater interface and ground-water discharge to Biscayne Bay. Instead, results from the model will be used to define the inland boundary conditions for three high-resolution, cross-sectional models.

A two-dimensional, vertical, cross-sectional model will be developed for each of three transects where water-level and salinity data are being collected. The purpose of the cross-sectional models is to quantify the discharge to Biscayne Bay by calibrating the models to water-level and salinity data. As ground-water discharge is expected to vary over tens of meters, these models will have a 10 x 1-meter-cell size in the x and z directions, respectively. Tidal effects will be simulated by changing the water level at the bay boundary during hourly or quarter-hourly time steps. Simulations will represent the period of time that data were collected in the field. Results from these models will be presented as ground-water discharge and salinity of the discharge as functions of time and distance from the shore. These discharge results will allow water managers to determine the effects of varying inland conditions on the distribution and timing of fresh ground-water discharge to Biscayne Bay.

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