The Fernandina permeable zone contains brackish water in parts of Duval County, Florida. Upward flow from the Fernandina permeable zone to the upper zone of the Lower Floridan aquifer increases chloride concentrations in ground water in parts of Duval County. Numerical models of the ground-water flow system in parts of Duval, St. Johns, and Clay Counties, Florida, were used to (1) estimate the vertical flows between the low-quality water of the Fernandina permeable zone and the high-quality water of the upper zone of the Lower Floridan aquifer in the vicinity of Deerwood 3 and Brierwood well fields, based on 2000 ground-water withdrawal rates; (2) determine how such vertical flows change as several scenarios of injection, withdrawal, and intervening rest periods are simulated in the two well fields; and (3) evaluate the effects of changes in less certain hydraulic parameters on the vertical flows between the Fernandina permeable zone and the upper zone of the Lower Floridan aquifer. The ground-water flow system was simulated with a four-layer model using MODFLOW-2000, which was developed by the U.S. Geological Survey. The first layer consists of specified-head cells simulating the surficial aquifer system with prescribed water levels. The second layer simulates the Upper Floridan aquifer. The third and fourth layers simulate the upper zone of the Lower Floridan aquifer and the Fernandina permeable zone, respectively. Average flow conditions in 2000 were approximated with a steady-state simulation. The changes in upward flow from the Fernandina permeable zone due to periods of injections and withdrawals were analyzed with transient simulations. The grid used for the ground-water flow model was uniform and composed of square 250-foot cells, with 400 columns and 400 rows.
The active model area encompasses about 360 square miles in parts of Duval, St. Johns, and Clay Counties, Florida. Ground-water flow simulation was limited vertically to the bottom of the Fernandina permeable zone. The steady-state ground-water flow model was calibrated using time-averaged 2000 heads at 20 control points. Environmental-water heads in the Fernandina permeable zone were calculated for wells with variable water density. Transmissivity of the Upper Floridan aquifer, the upper zone of the Lower Floridan aquifer, and the Fernandina permeable zone, and the leakance of the intermediate confining unit, the middle semiconfining unit, and the semiconfining unit were obtained from regional ground-water flow models and adjusted until a reasonable fit between simulated and computed heads was obtained.
The calibrated hydraulic properties from the steady-state ground-water flow model, and the calibrated storage coefficient from the transient model, were used to simulate hypothetical transient scenarios of injection, withdrawal, and intervening rest periods to assess changes in flow between the Fernandina permeable zone and the upper zone of the Lower Floridan aquifer. Based on the simulated flows between the Fernandina permeable zone and the upper zone of the Lower Floridan aquifer and the 18 million gallons per day of water available for injection, the reversal of the prevailing upward flow from the Fernandina permeable zone was not achieved. However, steady-state and transient simulations indicate that the upward flow of water from the Fernandina permeable zone could be reduced by as much as 64 percent, from 0.11 to 0.04 cubic foot per second, if only injection periods are simulated.