GEOLOGICAL CONTROLS ON THE DISTRIBUTION OF SUBMARINE GROUND WATER DISCHARGE, LONG BAY SOUTH CAROLINA

Submarine groundwater discharge (SGD) has been identified as an important pathway for transport of dissolved nutrients and contaminants to near-shore waters. Little is known, however, about the spatial distribution of SGD seeps. Discharge may be a diffuse phenomenon, or focused in geologically controlled hotspots along the inner shelf. In an effort to assess the spatial distribution of SGD, a combination of geophysical and geochemical tracer methods was employed. Electrical resistivity and seismic surveys were conducted along the inner shelf of Long Bay, South Carolina during a series of cruises in October, 2005 and November, 2006. During the November 2006 cruise water temperature, salinity, pH, dissolved oxygen, and radon samples were also collected. Track lines were oriented both transverse to and along the axes of known paleochannels. Apparent resistivity values are higher in magnitude, with larger variations near shore, and become weaker and more uniformly distributed with increasing distance from shore. Conversion of electrical profiles into SEGY format allows for direct comparison with seismic profiles. Geologic features interpreted from the seismic records that correlate with resistivity anomalies include: 1) breaches in strata that are incised by paleochannels, 2) sea floor outcroppings of gently dipping layers and 3) miscellaneous discontinuities in seismic reflectors. These features range in size from tens of meters to kilometers in horizontal distance and can extend from the sediment-water interface to depths of >10 meters. Near shore resistivity anomalies can exceed 10 ohm-m, suggesting sub-bottom sediments in some locations are saturated with brackish to nearly fresh pore water. Further offshore, lower resistivity values (<1 ohm-m) suggest more saline pore fluids. Ongoing research is focused on the integration of geochemical parameters with geophysical data in an effort to refine SGD flux estimates.