Spatial Patterns in Subsurface Salinity and Submarine Groundwater Discharge Into Indian River Bay, Delaware

Severe eutrophication in Indian River Bay, Delaware is related to direct discharge of groundwater and associated nutrients into the estuary. The fresh and saline patterns of submarine groundwater discharge are controlled by many factors, including subsurface geologic heterogeneity. A buried paleochannel, expected to control both flow and mixing in the subsurface, was identified by offshore chirp seismic profiling along the southern shoreline of the bay at Holts Landing State Park. Offshore streaming resistivity measurements, confirmed by offshore multi-level salinity profiles, indicate that a zone of freshened groundwater exists beneath fine-grained channel infill. Direct measurements of submarine groundwater flux and salinity using seepage meters demonstrated that discharge was primarily saline and exhibited substantial heterogeneity. Freshened groundwater discharge was observed within a submerged interfluve along the paleochannel edge, indicating that fresh groundwater flow is impeded by fine-grained paleochannel infill and discharges along channel edges. Such patterns of groundwater flow result in complex flowpaths and subsurface mixing that may influence geochemical reactions and nutrient fluxes into the estuary.