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The Floridan aquifer system underlies parts of South Carolina, Georgia, and Alabama and all of Florida. Two areas have experienced increases in ground-water withdrawals from the Floridan aquifer system: the Dougherty Plain in southwest Georgia and the Fort Walton Beach area of northwest Florida. In southwest Georgia, seasonal withdrawals for irrigation increased from about 47 billion gallons in 1977 to about 76 billion gallons in 1980, and to about 107 billion gallons in 1981 a drought year. However, these withdrawals have not resulted in appreciable water-level declines in the Floridan aquifer system in southwest Georgia. Long-term waterlevel declines within the upper part of the Floridan aquifer system, characterized by a widespread cone of depression, have resulted from moderate pumpage for municipal use in southern Okaloosa and Walton Counties, Florida. Total pumpage amounted to approximately 5.7 billion gallons in 1978 (15.5 million gallons per day). At the center of the cone of depression near Fort Walton Beach, water levels have declined more than 140 feet below predevelopment levels. In southwest Georgia, the Floridan aquifer system is overlain by sandy clay residuum that has an average thickness of approximately 50 feet and was derived from the chemical weathering of the Ocala Limestone. In the western panhandle of Florida, the aquifer is overlain by the Pensacola Clay, which varies in thickness from 50 to 450 feet. Above the Pensacola Clay is a surficial sand-and-gravel aquifer ranging in thickness from less than a few tens of feet in the easternmost area to more than 400 feet in the westernmost part of the panhandle. Mean annual precipitation is about 51 and 63 inches, respectively, in the Dougherty Plain area of southwest Georgia and in northwest Florida. Because the Floridan aquifer system is thinly covered in southwest Georgia, during September to May streamflow and ground-water levels are directly correlated with precipitation. Water-level peaks usually occur about 1 month after major precipitation peaks. In northwest Florida, the aquifer system is generally deeply buried. Thus, although ground-water levels respond to variations in rainfall, these responses are changes in artesian pressure rather than additions of water to the saturated and unsaturated zones. Aquifer-test data indicate that transmissivities of the Upper Floridan aquifer in southwest Georgia range from 2,000 to 300,000 feet squared per day and storage coefficients range from 2*10'4 to 3* 10'2. In southern Okaloosa and Walton Counties, Florida, transmissivities based on aquifer tests range from 250 to 25,000 feet squared per day and storage coefficients range from about 2xlO"5 to 6*10-4 . Pesticides were detected in water from 10 wells open to the residuum and 4 wells open to the Upper Floridan aquifer in southwest Georgia. None of the samples from the wells in the Floridan aquifer contained pesticide concentrations exceeding the recommended limits for public drinking water supplies. Throughout southwest Georgia and adjacent Florida, water is low in chloride (less than 10 milligrams per liter) except for coastal Okaloosa and Walton Counties, where chloride concentrations exceed 500 milligrams per liter. Three two-dimensional finite-difference numerical models the subregional flow model, the Dougherty Plain flow model, and the Fort Walton Beach flow model were developed to simulate ground-water flow in the Floridan aquifer system in the study area. The subregional flow model is intended to simulate the major features of the flow system. The detailed Dougherty Plain and Fort Walton Beach flow models are intended to simulate the effects of ground-water development in these areas. The subregional flow model, covering an areal extent of 20,000 square miles, was used to obtain aquifer hydraulic parameters and to establish aquifer recharge-discharge relations prior to development. The detailed Dougherty Plain flow model was used to simulate the potentiometric surface and to evaluate water-level declines during drought conditions over a 4,400-square-mile area of southwest Georgia. The Fort Walton Beach flow model was used to simulate water-level declines from 1941 to 1978 and to evaluate water-level declines to the year 2000 within a 1,700-square-mile area of southern Okaloosa and Walton Counties, Florida. Simulation of a consecutive 3-year hydrologic drought in southwest Georgia, with total pumpage of 339 billion gallons, resulted in a mean water-level decline of 26 feet. In some areas, water levels declined from a few feet to as much as 10 feet below the top of the Upper Floridan aquifer. Pumpage of 1,224 billion gallons resulted in mean water-level declines of 33 feet. In about 30 percent of the modeled area, water levels declined from a few feet to as much as 10 feet below the top of the aquifer. A 10-year simulation under average recharge conditions with pumpage increased to 287 billion gallons per year resulted in mean water-level declines of 4 feet and a 30-percent reduction of the present-day (1980) Floridan aquifer system ground-water discharge to streams. In southern Okaloosa and Walton Counties, Florida, pumpage of 15.5 million gallons per day was increased to 20.5 million gallons per day in March 1978 to simulate the effect of development in the year 2000. Simulated declines in the year 2000 for the Upper Floridan aquifer ranged from 40 to 60 feet in Fort Walton Beach and from 20 to 30 feet in Destin, Valparaiso, Niceville, and Mary Esther. Relocating four Santa Rosa Island wells to areas of higher transmissivity resulted in smaller water-level declines.

u.s. geological survey professional papers/u.s. geological survey professional paper

Hydrogeology and simulated effects of ground-water development of the Floridan aquifer system, southwest Georgia, northwest Florida, and southernmost Alabama