Position of the freshwater-saltwater interface in a coastal confined aquifer

Abstract:

Delineating the position of the freshwater-saltwater interface is necessary for understanding submarine groundwater discharge and for assessing risks associated with saltwater intrusion, but most studies of the freshwater-saltwater interface focus on shallow surficial aquifers. Groundwater dynamics in coastal aquifers at depths greater than 5 m below land surface have rarely been assessed. The freshwater-saltwater interface in deeper confined aquifers is frequently positioned offshore, where it is difficult to locate or study. Our investigation was located at North Inlet, a wide, intertidal wetland southeast of Georgetown, SC. This site was ideal for studying the freshwater-saltwater interface at the embayment scale because the first major confined aquifer under the wetlands was accessible. Using electrical resistivity tomography, we were able to image the upper 20 to 30 m of sediment, including the first major confined aquifer. At North Inlet a 75 km² island is bounded by extensive marsh and relict and modern barrier islands extending ~10 km seaward to the Atlantic Ocean. Surface and groundwater salinity throughout the marsh is 35 ppt. In order to get maximum depth penetration, surveys were conducted along dry land on the island and marsh surface with 10 m electrode spacing. Measured apparent resistivity varied in all surveys. A 20 Ohm-m resistivity layer at 25 m depth suggests that freshwater extends 335 m from the mainland. The presence of this freshwater indicates regional groundwater flow under the marsh or high rates of infiltration during precipitation events. Groundwater was saline to brackish (~10 Ohm-m) 1 km further out into the marsh at the next relict beach ridge at depths of 20 to 30 m. These results indicate that embayment scale groundwater flow plays an important role in providing low salinity pore-water to the marsh from beneath. The presence of fresh/brackish groundwater hundreds of meters from the mainland suggests that coastal confined aquifers transport freshwater significant distances. Freshwater could exist in analogous confined aquifers at shallow depths under the seafloor in other coastal systems.