Submarine Groundwater Discharge to the Continental Shelf in the South Atlantic Bight

Marine tracer studies have revealed that submarine groundwater discharge (SGD) to the global ocean likely exceeds river discharge, including ²²⁶Ra measurements in the South Atlantic Bight (SAB) suggesting that the SGD contribution far from shore may be as much as three times the volume of river discharge. Previous studies have suggested that these fluxes could be supported by a widespread fluid flux across the seafloor of the broad continental shelves, but such fluxes have not been confirmed or quantified far from the coast. In the summer of 2014, we installed a well field off the coast of Charleston, SC that covers an area 8 km wide and reaching from 5 km to 20 km offshore. The wells are instrumented with loggers that record the temperature of the bottom water and subseafloor temperatures at strategic depths at 20 minute intervals. The resulting thermal profiles reveal seasonal flushing to depths exceeding 50 cm below the seafloor between 8 and 20 km from shore during (which?) season. Flushing was not evident at wells in parallel transects 5 km away. If flushing affected a swath of seafloor 5 km wide, it would follow that more than 1 x 10¹⁰ L of groundwater was converted to ocean water when flushing was initiated. Subseafloor porewaters from the region are highly enriched in nutrients and Ra compared to river water and seawater, suggesting that this flushing may be an important source of nutrients to coastal waters.