AQUIFERS, AQUITARDS, INCISED PALEOVALLEYS, AND SEAWATER INTRUSION: A CASE STUDY FROM A REGIONAL AQUIFER-AQUITARD SYSTEM ON THE GEORGIA-SOUTH CAROLINA COAST

The Upper Floridan aquifer (UFA) beneath the southeastern U.S. Atlantic coastal plain and inner continental shelf supplies over 50% (at least 1.3 x 10⁹ liters/day) of the water requirements for the coastal GA-SC region, a volume that has increased steadily since water was first pumped from the aquifer at Savannah, GA in the late 1800s. A large cone of depression (~50 km radius; apex at -30 m MSL) now exists on the aquifer’s potentiometric surface and is centered beneath Savannah. Where a regional Miocene-age aquitard overlying the UFA is thin or absent within the cone, the opportunity exists for seawater to migrate downward through the seabed and recharge the aquifer. This scenario may lead to salt contamination of groundwater supplies in the long term (decades to centuries) for a large coastal population exceeding 0.5 million people.

In groundwater investigations, geophysics is traditionally used from the well bore to identify well-adjacent formation characteristics and formation-fluid properties. Our project involved the relatively unique application of a large-scale (3600 km²) seismic reflection survey to a groundwater investigation in the coastal/marine environment. We used seismic data to map subsurface depth and topography of the UFA and map the extent and integrity of the overlying Miocene-age aquitard. These data were then used to identify eleven locations where seawater may be intruding into the UFA beneath the lower coastal plain and inner shelf. These locations, occurring within or on the edge of the Savannah cone of depression, mark where modern tidal channels or Quaternary incised paleovalleys impinge upon the aquifer and locally remove the Miocene aquitard. At all locations, the UFA is separated from the oceanic water column by a variably (0–17 m) thick, non-confining, Quaternary section. Significantly, ten of the locations are associated with incised paleovalleys of the Savannah River system (probably incised during oxygen isotope stage 2 and stage 6 sea-level lowstands) that track in a sinuous path across (and beneath) the inner continental shelf and locally intersect high points on the UFA. Our results from the GA-SC coast illustrate that mapping Quaternary lowstand drainage systems, and understanding complex incised-valley fills, can have significant applied environmental-geology applications.