The U.S. Army Corps of Engineers (COE) is planning to construct gated spillways and culverts to allow for the restoration of natural sheetflow conditions to Everglades National Park (ENP). These proposed changes may further affect the hydrologic conditions of ENP and other parts of the ecosystem, thus leading to the following questions: (1) Is ground water flowing to Biscayne Bay a significant component of the water budget in south Florida?
(2) Would the quantity of ground water flowing to Biscayne Bay be greatly affected by changes in the operation of gates and control structures in canals?
(3) How much change in ground-water discharges to Biscayne Bay has occurred due to modifications to the hydrologic system?
Quantification of ground water flowing to Biscayne Bay is needed as input to two interagency projects: the South Florida Ecosystem Restoration Program and the Biscayne Bay Feasibility Study. The principal objective of the Biscayne Bay Feasibility Study is to investigate ongoing construction/dredging projects and propose solutions to alleviate adverse factors that affect the bay and to aid in the development of guidelines for future management of the natural resources of Biscayne Bay. The Biscayne Bay Feasibility Study includes the implementation of a surface- water circulation model which will be developed by the Waterway Experi- mental Station of the COE. Quantification of ground-water discharges to Biscayne Bay is needed as input to the bay water circulation model.
Core samples will be collected and analyzed to determine the geologic and hydrogeologic characteristics of the subsurface material underlying the bay and near the coast. Gamma and electromagnetic induction resistivity logs will be run at selected wells to aid in the determina- tion of the geohydrologic horizons and the definition of salinity profiles.
Cores were obtained and monitor wells were installed at 29 sites along three transects in Biscayne Bay in 1996. An additional 12 cores and wells are planned onshore during 1997.
A model analysis will be performed to further define the saltwater- freshwater mixing zone and to estimate ground-water flow to Biscayne Bay. A preliminary ground-water flow model and a refined model will be used for the analysis. Some historical and seasonal simulations (including near predevelopment conditions) will be performed. Additionally, an attempt will be made with the model to estimate the location of the saltwater-freshwater zone.
A preliminary ground-water flow model will be constructed using the SWIP code as modified by Merritt in 1994. The model solves ground-water flow equations accounting for fluid density and viscosity dependence on temporal changes of pressure, temperature, and solute transport. Density changes are important because the differences in density between the fresh ground water and the saline bay water affect flow and water levels. Data from an existing ground-water flow model, which includes most of Dade County from Tamiami Trail south, will be transferred to the new proposed model grid using a geographic information system (GIS).
The existing southern Dade County model is a comprehensive model that subdivides the surficial aquifer in four layers (one layer is used to represent overland flow) and considers the effects of rainfall, evapo- transpiration, canal water levels, canal control structures, areal and vertical changes in permeability, aquifer thickness, production wells, and tidal boundaries. All of the parameters from the southern Dade County model to be used in the southwestern part of the preliminary model will also be generated for the northern and northeastern parts of the model grid using GIS technology. Hydrogeologic data from the southern Dade County model will be extrapolated to the area of the grid that covers Biscayne Bay. For purposes of this preliminary model, the boundary conditions used will be constant head or pressure, whichever is determined to be appropriate for the steady-state simulation. Water- level data from wells, canals, and tidal gage sites will be used to represent the boundaries. This preliminary model will be used to select the locations for drilling test holes and observation wells and provide an initial estimate of ground-water flow to Biscayne Bay.
Data collected from the new wells will be used to test and refine the model calibration. The boundary conditions for the refined model will be represented using time-varying water levels in wells, canals, and tidal gate sites that will be converted to pressure values using a correction for density of the different model layers and water salinities. A sensitivity analysis will be performed, focusing on how the variation of the model parameters will affect ground-water flow to Biscayne Bay.