GROUNDWATER QUALITY IN THE GORDON AQUIFER NEAR CHARLESTON, SOUTH CAROLINA

The Floridan aquifer system is extensive in size and serves as a significant source of fresh water for the Southeastern United States. The system underlies most of Florida and large portions of coastal regions in Georgia. A smaller, but productive area supplies water to southern coastal areas of South Carolina. In South Carolina, the Floridan aquifer system is composed of Tertiary-aged sediments separated into three distinct aquifers. The Upper and Middle Floridan aquifers are predominately upper and middle Eocene carbonate rocks downdip, with increasing siliclastics updip. The deeper Gordon aquifer underlies the Middle Floridan aquifer and consists of lower Eocene and upper Paleocene carbonates and siliclastics. Groundwater flow throughout is generally in the southeastward direction moving towards the coast. The Floridan aquifer system is heavily used and areas of lower potentiometric pressure have altered the natural regional flow regime, causing saltwater intrusion to become a common and persistent problem for coastal areas that rely on groundwater.

The current study focuses on the Gordon aquifer in the coastal counties near Charleston, South Carolina. Historical data described groundwater as a soft, alkaline, sodium bicarbonate type with areas of high fluoride concentrations and brackish water in areas near the coast. Wells open primarily to the Gordon aquifer were sampled during 2016 and 2018. A combination of domestic, public supply, and dedicated monitoring wells were purged and sampled, or vertically profiled for specific conductance to obtain water quality information. Water samples were collected and analyzed for field and laboratory water quality including the analysis of major cations and anions to determine the chemical composition. Results from the analyses were used to characterize the aquifer, locate the freshwater-saltwater interface, and compare to past studies in the same region. The results show similar trends in chemical quality, but a significant landward shift in the location of the freshwater-saltwater interface. These analyses can serve as baseline data and should be used as a comparison to identify future changes that may occur in water chemistry due to pumping.