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 broad continental shelves, but such fluxes have not been confirmed or quantified far from the coast. We installed a well field off the coast of the Isle of Palms near Charleston, SC that covers an area 8 km wide and reaches 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. We used a numerical model to determine groundwater flow rates from the thermal data. The model has the ability to simulate heat transport due to conduction, advection, and hydrodynamic exchange (rapid flushing of seawater through the upper few cm of seafloor sediments). The results indicate spatially- and temporally-variable groundwater activity throughout the region. Groundwater velocities were generally lower in the southwest region of the well field (between 5 m/yr upward and 5 m/yr downward flow) compared to the northeast region (up to 15 m/yr upward flow). Hydrodynamic exchange persisted toward the southwest part of the region, while little to no hydrodynamic exchange was detected toward the northeast. Furthermore, there is an increase in upward groundwater flow velocity toward the end of the summer in most of the wells. Subseafloor porewaters from the SAB region are highly enriched in nutrients and Ra compared to river water and seawater, suggesting that SGD is an important source of nutrients to coastal waters.