THE FLORIDAN AQUIFER ON THE GEORGIA/SOUTH CAROLINA COAST: USING HIGH-RESOLUTION GEOPHYSICS TO LOCATE AREAS SUSCEPTIBLE TO SEAWATER INTRUSION

The regional Upper Floridan aquifer (UFA) is the principal source of groundwater for coastal Georgia and southern South Carolina. A progressive increase in groundwater use since the late 1800s has resulted in a 5800 km² cone of depression on the aquifer's potentiometric surface that is centered on Savannah, GA, and underlies eight coastal counties. Of current concern is the potential for downward and lateral movement of saline waters from estuaries and shelf areas into the UFA in locations where stratigraphy and hydrogeologic conditions are conducive to the transport of seawater through the seabed and into the aquifer. As part of the Sound Science Initiative's Interim Strategy for managing saltwater intrusion into the UFA, the Georgia Environmental Protection Division is conducting a multi-year water resources investigation on the GA-SC coast. A significant component of that investigation involves geophysical mapping to identify sites of potential seawater intrusion into the uppermost parts of the UFA.

We used marine seismic-reflection surveys to identify and semi-quantitatively rank eleven Areas of Concern (AOCs) where the UFA is most susceptible to seawater intrusion. Each AOC has an enhanced susceptibility to intrusion because (1) the UFA is present at shallow depth, (2) the overlying Miocene aquitard is absent, (3) the potentiometric surface on the UFA is at or below mean sea level, and (4) the overlying water column is saline. The location and seawater intrusion susceptibility associated with each AOC will be utilized in aquifer testing and modeling by state and federal agencies in order to better quantify water and solute transport within the UFA. Approximately 4 years from now, an increased understanding of this coastal plain aquifer will form the sound scientific basis upon which coastal Georgia's future groundwater management plans can be constructed.