EVALUATION OF THE DOWNWARD MIGRATION OF SALTWATER TO THE UPPER FLORIDAN AQUIFER IN THE SAVANNAH, GEORGIA, AND HILTON HEAD ISLAND, SOUTH CAROLINA, AREA

The Upper Floridan aquifer underlies all of Florida, most of the Georgia and Alabama Coastal Plain, and large parts of coastal South Carolina. The aquifer is composed primarily of carbonate rock of varying permeability that ranges in age from middle Eocene to early Miocene. In the study area, the Upper Floridan aquifer is confined above by the upper confining unit of Miocene age that, in turn, is overlain by more recent undifferentiated surficial sediments. Prior to groundwater development, potentiometric heads in the Upper Floridan aquifer ranged between 5 and 35 feet above mean sea level throughout most of the study area. However, by 1998 withdrawals totaled approximately 80 million gallons per day in the Savannah, Georgia and Hilton Head Island, South Carolina, area. The cone of depression created by the 1998 pumpage lowered the potentiometric surface below mean sea level over an area of about 1,200 square miles overlain by saltwater marshes, rivers, and the Atlantic Ocean. To determine if saltwater was migrating downward, pore water was extracted from the upper confining unit and analyzed for dissolved chloride concentration. Pore-water analyses indicated a trend of high chloride concentrations near the top of the upper confining unit that decreased with depth at both the offshore and onshore locations. The results of the pore-water analyses indicated the downward migration of surficial saltwater through the upper confining unit overlying the Upper Floridan aquifer. Darcy's Law was used to estimate the volume of downward flow of saltwater through the upper confining unit. This approach indicated that an area of approximately 382 square miles east and northeast of Savannah, Georgia, may be contributing 7.7 million gallons per day of downward flow to the Upper Floridan aquifer. Because this area may affect water quality in the Upper Floridan aquifer in the future, it is considered to be an area of concern. A one-dimensional solute-transport equation was used to simulate the future arrival times for a given concentration of chloride to reach the top of the Upper Floridan aquifer. These simulations, conducted on 110 cells each having an area of 4 square miles, predicted that the arrival times for saltwater having a chloride concentration of 500 milligrams per liter ranged from as early as 25 years ago to 113 years from 2005.