Recent scientific studies regarding the availability, occurrence, and management of groundwater in the Coastal Plain of South Carolina have increased the stratigraphic and hydrogeologic knowledge of this physiographic region.  Despite these studies, the inner margin of the Coastal Plain has been largely neglected and remains poorly understood.  To investigate the stratigraphy and hydrogeology of the inner margin, an unconfined aquifer of an interfluvial ridge in northwest Aiken County, South Carolina was characterized by developing a stratigraphic framework and a numerical groundwater flow model.
    Interpreting and correlating 28 surface exposures, 25 geophysical well logs, 9 auger holes, and 2 geologic cores developed the stratigraphic framework.  Three unconformities distinguishable in geophyiscal logs and surface exposures provided stratigraphic control.  Two unnamed Cretaceous formations, including an undocumented clay unit, were successfully correlated in the study area for the first time.  The clay unit was found to be mappable across the study area and provides a marker bed to separate and correlate the Cretaceous formations.  The up-dip edge of the clay forms a low-permeable zone within the aquifer where yields to production wells are expected to be poor.  Subsurface mapping of potentially important hydrogeologic units was based on the stratigraphic correlations and provided an effective means to determine the anisotropy of the aquifer for model calibration.  Analyses of pumping test and permeability test data concluded conceptual model development.
    Groundwater Vistas, a groundwater modeling interface, was used to design a numerical model based on the conceptual model, and to execute MODFLOW-SURFACT, an improved version of MODFLOW for unconfined flow.  The intent was to determine the sources and sinks of the study aquifer in order to assess the availability of groundwater for supply purposes.  The model indicates that precipitation received at the interfluvial ridge provides the only significant source of water to the aquifer.  The groundwater flows through the sand ridge and, impeded by the low-permeable basement rock, flows laterally until it discharges at the adjacent streams that define the interfluve.  The flowpath analysis indicates the study aquifer is isolated with respect to other sources of groundwater, resulting in the susceptibility of the aquifer to droughts.