SUBMARINE GROUNDWATER DISCHARGE TO THE CONTINENTAL SHELF IN THE SOUTH ATLANTIC BIGHT

Our knowledge of the exchange of solutes at the land-ocean interface beyond a few km offshore is meager due to limited data availability. In this zone poreflow has the potential to be strongly influenced by ocean currents and waves. 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 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 bottom water and subseafloor temperatures at strategic depths at 20 minute intervals. Cores and chirp seismic surveys revealed that paleochannels, which have previously been considered likely locations of focused SGD, contained significant thicknesses of mud. A regionally extensive mud confining layer was also identified. Thermal data from the wells were used as input for a 1-D thermal model using an apparent thermal diffusivity and reveal seasonal flushing to depths exceeding 50 cm below the seafloor between 8 and 20 km from shore. 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.