Electrical resistivity imaging of groundwater dynamics in coastal aquifers in the southeastern U.S.

Abstract

Groundwater flow in coastal aquifers is influenced by hydrologic processes that operate over a wide range of temporal scales, including storm events, tides, seasonal variations in precipitation and evapotranspiration (ET). Groundwater salinity can be a useful first-order indicator of the balance between these flow processes as saline seawater mixes with freshwater from uplands/islands (Figure 1). Electrical resistivity imaging is highly useful for detailed, spatially continuous measurement of subsurface conductivity to investigate the spatial and temporal variation in groundwater salinity. Approximately monthly repeat resistivity surveys were conducted at two locations on Crabhaul Creek, at the margin of a salt-marsh estuary in South Carolina, and single surveys were conducted at a marsh hammock in Econfina River State Park, FL and a beach at Sapelo Island, GA. Resistivity models at Crabhaul Creek and Sapelo Island were corroborated by simultaneous measurements of salinity in nearby piezometers that showed excellent agreement. The freshwater-brackish water interface was clearly imaged by resistivity in each case, and used to constrain hydrologic models and guide well installation.