Annual Patterns and Observations of Submarine Groundwater Discharge and Porewater Exchange in the South Atlantic Bight

Submarine groundwater discharge (SGD) and porewater exchange are a major source of nutrients to the benthos in sandy continental shelves. Increasing evidence suggests that the majority of this flow occurs far (>5 km) offshore, where it is very challenging to observe. We developed novel and cost-effective methods to monitor the periodicity and magnitude of co-occurring SGD and porewater exchange in this challenging setting. Thermal time-series measurements were collected below the sandy seafloor 10-15 km offshore in the South Atlantic Bight (SAB) during the summers of 2018, 2019, and 2020. New instrumentation was deployed with improved thermal resolution on the upper 20 cm of benthic sediments along with sediment level loggers that track accretion and scour to model porewater exchange with increased accuracy. With these new methods we were able to detect pulses of SGD each summer ranging in velocities from 1 cm/day to 8 cm/day along with co-occurring porewater exchange. SGD and porewater exchange were clearly linked to annual meteorological cycles and storm events in the SAB. Pulses of SGD and porewater exchange were recorded between July and August each year and coincided with prevailing winds related to annual shifts of the Bermuda-Azores High. Additional pulses of SGD and porewater exchange occurred 2-3 times in the late summer every year, induced by major storms and hurricanes. With this data we can begin to predict patterns of groundwater flow and porewater exchange in the SAB.