SURFACE WATER AND SEAWATER INTERACTIONS IN THE COASTAL ENVIRONMENT OF BISCAYNE BAY, SOUTHEAST FLORIDA

Groundwater flow in a coastal environment generally consists of the seaward flow of freshwater overlying the landward flow of seawater, with dispersion across the interface resulting in a transition zone. Tidal fluctuations can result in significant changes to the flow system, especially in the vicinity of the discharge zone. If the coastal environment includes surface water bodies, interactions between the surface water and fresh-to-saline groundwater result in complex daily and seasonal effects. These interactions were investigated at a site located within the City of North Miami, Florida as part of the development of a former landfill into a high end residential/commercial development. The site includes several freshwater lakes adjacent to a mangrove system consisting of both fresh and saline surface water and groundwater ultimately discharging to Biscayne Bay. Hydraulic characteristics were obtained from a variety of aquifer tests and natural gradient tracer tests that were primarily conducted with rhodamine dye. Qtracer and SEAWAT 2000 were used to analyze the tracer tests. SEAWAT 2000 was also used to model groundwater flow under various seasonal periods including the five-year and 25-year storm events, which results in the injection of surplus storm water into the Biscayne Aquifer. The model demonstrated that the groundwater flow system is significantly altered by the presence of, and the associated water level fluctuations in the lakes, especially during the storm events. Vertical flow and mass transport is significantly increased as a result of the lakes and associated seasonal fluctuations. The density of brackish water in the mangroves would be expected to result in vertical flow. However, the mangrove system appears to limit vertical discharge due to the low permeability sediments deposited in this environment.

Organic decomposition of the landfill material has generated an ammonia plume whose treatment was evaluated with an experimental prototype permeable reactive barrier. Various models of this environment have also been used to simulate full-scale remedial combinations of slurry walls and reactive barriers.