Submarine groundwater discharge (SGD) is now recognized as an important pathway for the introduction of nutrients, carbon, and metals into coastal ocean waters. The full impact of SGD is uncertain, however, because we do not understand all the driving mechanisms. Whereas most studies of SGD have focused at the coastline, increasing evidence from the South Atlantic Bight indicates that the majority of SGD occurs far (1-70 km) offshore. Mechanisms for discharge far from shore are not yet well understood. Observations from a cluster of seafloor wells 20 km offshore of North Carolina suggest that this discharge reflects slow upward migration of saline fluids from depth. These porefluids are also affected by rapid flushing of sandy seafloor sediments by seawater during storms or migration of cold ocean currents. This project will test the hypothesis that these flow processes are common and drive significant fluid exchange over large areas of the continental shelf. The project will also estimate the geochemical fluxes of radium, nutrients and carbon across the seafloor. The study area will be a 150 square-km area that reaches 20 km offshore near Charleston, South Carolina. Geophysical surveys will map seafloor bathymetry and identify buried sedimentary structures that could channel groundwater traveling upward toward the seafloor. Thermal arrays will be installed to depths of 2-5 m below the seafloor at 10 locations. Data from these arrays will indicate the depth and frequency of rapid ocean-driven flushing and quantify possible long-term upward flow from depth. Three pairs of wells installed in a shore-perpendicular transect will supply pressure data that will provide independent estimates of fluid fluxes. The wells will also be sampled for nutrients, carbon, and radium tracers, which will indicate mixing between radium-poor seawater and radium-enriched pore waters. Geochemical fluxes across the seafloor will be determined based on calculated fluid fluxes and observed geochemical compositions.
The realization that groundwater discharging directly to the ocean supplies significant quantities of nutrients, carbon, and metals to coastal waters represents a paradigm shift in our understanding of geochemical fluxes to the coastal ocean systems. Previously, only inputs from rivers, the atmosphere, and upwelling (exchange with the deeper ocean) were considered. Now, increasing evidence suggests that the volume of saline groundwater that discharges across broad continental shelves is at least as large as river discharge, but the flow mechanisms and chemical compositions of groundwater discharge in offshore regions are very poorly understood. It is essential that studies of submarine groundwater discharge focus farther offshore in order to understand these processes. This work has the potential to spur significant revisions of textbook views of the hydrologic cycle and geochemical budgets for the ocean. In particular, saline groundwater may be supplying significantly greater nutrients (nitrogen and phosphorus) than has previously been realized. These nutrients affect the fertility of coastal ecosystems, which include economically significant fisheries, and may contribute to outbreaks of harmful algae blooms as well as ocean 'dead zones'. The work is designed to include significant student participation, including participation by under-represented groups.
