37-9 Wind Events Drive Major Episodes of Nutrient-Rich Submarine Groundwater Discharge Kilometers Offshore in Continental Shelves: Field Observations from the South Atlantic Bight

Session: Innovations in Research of Groundwater-Surface Water Interactions over Multiple Spatio-Temporal Scales

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Nearly 30 years of radium-based studies show that saline (salinity greater than 10) groundwater discharges to the coastal ocean in volumes similar to global river discharge and contributes significant nutrients to the coastal ocean, but the mechanism for this flow was slow to be identified. Our early heat-tracer studies showed that significant seawater-groundwater exchange occurs far (10 to 15 km) offshore from confined aquifers in response to wind-driven variations in sea level and had the potential to explain the Ra evidence, but the findings relied on data from only four sites and two summers. Here we describe new results obtained using refined heat-tracer methods and three summers of observations from an expanded area. Thermal time series measurements were collected below the sandy sea floor 10 to 15 kilometers offshore of Charleston, SC, during the summers of 2018-2020. Up to 7 pulses of saline (seawater salinity) submarine groundwater discharge (SGD) occurred each summer, with discharge velocities of 1 to 8 centimeters per day. Pulses of SGD coincided with upwelling favorable winds related to annual shifts of the North Atlantic subtropical high and storm activity. We observed spatial and temporal variability between sites 5-10 kilometers apart. Cumulative summertime SGD in 2019 corresponded to 65-70% of the radium-based estimates of SGD offshore of South Carolina. Geochemical analysis confirmed two-way seawater-groundwater exchange and high nutrient concentrations in SGD compared to the overlying water column and river water. The δN15 of total dissolved nitrogen and δ13C of dissolved inorganic carbon in groundwater suggests significant contributions from mineralized organic matter, with a geochemical composition distinct from overlying seawater. This mechanism for significant seawater-groundwater exchange is likely common globally, because upwelling events, storms, and confined aquifers are common to coastal environments around the globe.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10657

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