ABSTRACT: The hydrogeology of Lake County and the Ocala National Forest in north-central Florida was evaluated (1995-2000), and a ground-water flow model was developed and calibrated to simulate the effects of both present day and future ground-water withdrawals in these areas and the surrounding vicinity. A predictive model simulation was performed to determine the effects of projected 2020 ground-water withdrawals on the water levels and flows in the surficial and Floridan aquifer systems.
The principal water-bearing units in Lake County and the Ocala National Forest are the surficial and Floridan aquifer systems. The two aquifer systems generally are separated by the intermediate confining unit, which contains beds of lower permeability sediments that confine the water in the Florida aquifer system. The Floridan aquifer system has two major water-bearing zones (the Upper Floridan aquifer and the Lower Floridan aquifer), which generally are separated by one or two less-permeable confining units.
The Floridan aquifer system is the major source of ground water in the study area. In 1998, ground-water withdrawals totaled about 115 million gallons per day in Lake County and 5.7 million gallons per day in the Ocala National Forest. Of the total ground water pumped in Lake County in 1998, nearly 50 percent was used for agricultural purposes, more than 40 percent for municipal, domestic, and recreation supplies, and less than 10 percent for commercial and industrial purposes.
Fluctuations of lake stages, surficial and Floridan aquifer system water levels, and Upper Floridan aquifer springflows in the study area are highly related to cycles and distribution of rainfall. Long-term hydrographs for 9 lakes, 8 surficial aquifer system and Upper Floridan aquifer wells, and 23 Upper Floridan aquifer springs show the most significant increases in water levels and springflows following consecutive years with above-average rainfall, and significant decreases following consecutive years with below-average rainfall. Long-term (1940-2000) hydrographs of lake and ground-water levels and springflow show a slight downward trend; however, after the early 1960's, this downward trend generally is more pronounced, which corresponds with accumulating rainfall deficits and increased development.
The U.S. Geological Survey three-dimensional ground-water flow model MODFLOW-2000 was used to simulate ground-water flow in the surficial and Floridan aquifer systems in Lake County, the Ocala National Forest, and adjacent areas. A steady-state calibration to average 1998 conditions was facilitated by using the inverse modeling capabilities of MODFLOW-2000. Values of hydrologic properties from the calibrated model were in reasonably close agreement with independently estimated values and results from previous modeling studies. The calibrated model generally produced simulated water levels and flows in reasonably close agreement with measured values and was used to simulate the hydrologic effects of projected 2020 conditions.
Ground-water withdrawals in the model area have been projected to increase from 470 million gallons per day in 1998 to 704 million gallons per day in 2020. Significant drawdowns were simulated in Lake County from average 1998 to projected 2020 conditions: the average and maximum drawdowns, respectively, were 0.5 and 5.7 feet in the surficial aquifer system, 1.1 and 7.6 feet in the Upper Floridan aquifer, and 1.4 and 4.3 feet in the Lower Floridan aquifer. The largest drawdowns in Lake County were simulated in the southeastern corner of the County and in the vicinities of Clermont and Mount Dora. Closed-basin lakes and wetlands are more likely to be affected by future pumping in these large drawdown areas, as opposed to other areas of Lake County. However, within the Ocala National Forest, drawdowns were relatively small: the average and maximum drawdowns, respectively, were 0.1 and 1.0 feet in the surficial aquifer system, 0.2 and 0.8 feet in the Upper Floridan aquifer, and 0.3 and 0.8 feet in the Lower Floridan aquifer.
Projected 2020 withdrawals from the Floridan aquifer system caused decreases from average 1998 conditions in the following simulated flows: combined rates of excess evapotranspiration and excess overland runoff (which represent evapotranspiration and overland runoff that occur in excess of their assumed minimum rates); ground-water discharge to streams, lakes, and wetlands; and springflow. The largest simulated flow decreases for first- or second-magnitude springs in Lake County were at Apopka (28 percent), Seminole (12 percent), and Bugg Springs (9 percent). The largest simulated flow decrease for first- or second-magnitude springs in the Ocala National Forest was at Juniper Springs (4 percent).
Particle-tracking analyses were used to delineate areas that contribute recharge to selected springs. Based on average 1998 conditions, the contributing area for Apopka Spring covers approximately 30 square miles and has an average contributing recharge flux of 15 inches per year, and the contributing area for Alexander Springs covers approximately 76 square miles and has an average contributing recharge flux of 18 inches per year. The contributing area for Alexander Springs changed little as a result of projected 2020 conditions because relatively little pumping exists in the vicinity of the spring's contributing area. However, the size of the contributing area for Apopka Spring decreased to 26 square miles and the average contributing recharge flux decreased to 13 inches per year as a result of projected 2020 conditions.