PRELIMINARY ESTIMATES OF GROUNDWATER DISCHARGE AND NUTRIENT INPUTS TO THE SURF ZONE OF LONG BAY, SOUTH CAROLINA

Groundwater discharge from land to sea provides a significant, yet often overlooked pathway for delivery of dissolved nutrients and contaminants to nearshore waters. Recent research is focused on refining groundwater and associated dissolved chemical species flux estimates, geological controls on flow pathways, and geological/geochemical contributions to the chemistry of the pore water. In Long Bay, South Carolina, continuous electrical resistivity profiles suggest that mixing between ocean water and upland-derived fresh water occurs to a large extent within the surf zone and shoreface.

Long Bay, SC, experiences episodic short-term hypoxia during summer and fall. One factor responsible for this hypoxia is the input of nutrients to near-shore waters. Among the possible sources of nutrients is shallow groundwater seepage of nutrients across the beach face into the surf zone. A beach perpendicular transect of 1 meter deep wells was installed adjacent to Apache Pier (Myrtle Beach, SC) from the swash zone to approximately 100 meters inland and sampled over two complete tidal cycles. The groundwater salinity gradient ranged from approximately 30 to 2 ppt. All wells showed suboxic conditions and maxima in dissolved ammonia and nitrite, soluble reactive phosphate and iron at mid-range salinity. The latter indicates a maximum in microbially-driven anaerobic decomposition and a concomitant release of nutrients into the groundwater. Measurements of radon are used to estimate the net movement of groundwater seaward from the beach. Preliminary results from Rn activity measurements indicate groundwater fluxes in the range of 5 cm/day. These values are typical of the South Atlantic Bight and are on the order of riverine discharge. Groundwater discharge estimates are in turn used to derive estimated fluxes of N, P and metals to the surf zone. A final goal of the project is to develop a conceptual geological/biogeochemical model describing the mechanisms and magnitude of groundwater discharges to the near-shore waters of Long Bay, SC.