PREDICTION AND VERIFICATION OF A STRONG PULSE OF SUBMARINE GROUNDWATER DISCHARGE ONTO THE SOUTH CAROLINA, USA, CONTINENTAL SHELF, AND ITS EFFECT ON DISSOLVED OXYGEN CONCENTRATIONS

Based largely on natural radionuclide tracers, Submarine Groundwater Discharge (SGD) has become recognized as a major component of estuarine and coastal hydrologic systems. This discharge may occur nearshore or across the continental shelf on tidal or longer timescales. SGD is composed of terrestrial groundwater, seawater that has circulated within the aquifer, and reaction products generated as these fluids react chemically with aquifer solids. These reaction products — including radionuclides — serve as tracers, allowing us to detect and quantify SGD on regional and global scales. This discharge is recognized to supply nutrients, carbon, and metals to coastal waters. Less recognized is the effect SGD has on dissolved oxygen (DO) concentrations in these waters due to the presence of H₂S, NH₄⁺, CH₄, and DOM, which often occur at 50 to 1000 µM concentrations. Reducing DO in coastal waters to <150 µM induces biological stress on many organisms; reducing it to <60 µM can be deadly.

In this talk we will review how radionuclides are used in SGD studies and illustrate their use with a recent study that predicted a strong release of SGD based on wind data, which was then verified with radium tracers and records of heat transport through the sediments. We learned that this discharge reduced DO concentrations in bottom waters to <130 µM for at least ten days after the discharge. Coastal waters up to 12 km offshore from Charleston to Myrtle Beach, SC, were affected. The presence of reducing compounds in SGD was likely responsible. Similar results have been reported from the Mississippi Bight, Yangtze estuary, and earlier off Myrtle Beach. More investigations of the SGD oxygen demand are required to document the importance of this effect on the health of estuarine and coastal waters.