SOFIA - Ground-Water Discharge to Biscayne Bay

One of the main goals of Everglades restoration is to “get the water right” by modifying the existing canal network and management operations. There is concern that the modifications could adversely affect salinities in Biscayne Bay, which hosts a wide range of estuarine organisms. To evaluate the effects of the modifications on Biscayne Bay, the U.S. Army Corps of Engineers is constructing a hydrodynamic circulation model. To achieve a reasonable calibration, the model requires the accurate specification of freshwater discharges to the bay. The two most important mechanisms for freshwater discharge to Biscayne Bay are thought to be canal discharges and submarine ground-water discharge from the Biscayne aquifer. Canal discharges are routinely measured and recorded, but few studies have attempted to quantify the

Figure 1. Finite-difference model grid for the regional-scale model used to simulate variable-density ground-water flow to Biscayne Bay from 1989 to 1998. Click for larger image.

rates and patterns of submarine ground-water discharge. In 1996, the U.S. Geological Survey initiated a project, in cooperation with the U.S. Army Corps of Engineers, to quantify the rates and patterns of submarine ground-water discharge to Biscayne Bay. These rates have been incorporated into the circulation model, which is currently under development by the Corps.

To quantify rates and patterns of submarine ground-water discharge, field studies were combined with numerical modeling techniques. The field studies were performed at three transects: Coconut Grove, Cutler Ridge, and Mowry Canal. At each transect, monitoring wells were installed inland and offshore to characterize the interface between fresh and saline ground water. Data from the offshore monitoring wells indicate that ground water beneath Biscayne Bay has relatively low salinity values along the coast. At distances of about 300 to 500 meters from shore, however, ground-water salinities are nearly equal to seawater salinities. This suggests that most of the ground-water discharge is confined to a narrow band along the coast. The salinity data also support the conceptual model of cyclic ground-water flow within the freshwater/saltwater interface being driven by density variations. Ground-water discharge rates to Biscayne Bay were quantified by constructing variable-density ground-water flow models that use the assumption of an equivalent porous medium. The models were developed with a code called SEAWAT, which is a combination of MODFLOW and MT3D. The numerical models were constructed at the local and regional scales. Local-scale models were developed and calibrated using the field data collected at the Coconut Grove and Cutler Ridge transects. Results from the local-scale models were used to guide the development and calibration of the larger, regional-scale model (fig. 1). The regional-scale model covers most of Miami-Dade County and represents the period from January 1989 to October 1998. The model includes the following hydrologic stresses: recharge, evapotranspiration, canal interaction, interaction with surface water in the Everglades, municipal well fields, and ground-water discharge to Biscayne Bay.

Results from the regional-scale model provide important information about the rates and patterns of ground-water discharge to Biscayne Bay. The model results suggest that ground-water discharge rates are probably 3 to 10 percent of the total discharge from the coastal canals (fig. 2).

The results also suggest that most of the ground-water discharge to Biscayne Bay occurs along the northern half of the coast, where the Atlantic Coastal Ridge is directly adjacent to the bay. Ground-water discharge rates for southern Biscayne Bay are nearly zero because the low-lying areas prevent the water table from rising more than a few tenths of a meter above sea level. The importance of tidal canals also was highlighted by the model. Simulated ground-water discharges to the tidal portions of the Miami, Coral Gables, and Snapper Creek Canals are similar in magnitude to submarine ground-water discharge directly to Biscayne Bay.