Impact of Groundwater Flowpaths on Subsurface Denitrification and Nutrient Loading to an Estuary

Groundwater-borne nutrient loads to Indian River Bay, Delaware, contribute to severe eutrophication. Geophysical surveys and measurements of submarine groundwater discharge (SGD) indicate that paleovalley features affect subsurface salinity distributions and discharge patterns. Site-scale variable-density numerical modeling with SEAWAT shows that groundwater flowpaths are influenced by geologic features. Fresh groundwater flowpaths extend offshore within the paleovalley--beneath a low-permeability cap-- resulting in greater mixing between fresh and saline groundwater, complex saltwater circulation patterns, and diffuse fresh discharge offshore. In contrast, away from the paleovalley in an interfluve, focused fresh discharge occurs nearshore. High-resolution groundwater sampling and measurements of biogeochemical parameters in transects across the site characterize the geochemical distribution associated with modeled flowpaths. Denitrification is indicated by quantification of excess N2 gas based on measurements of N2 and Ar. Interpretations are supported by analyses of nitrogen isotope ratios, groundwater age tracers, and long-term monitoring of groundwater recharge and temperatures. Analyses indicate that denitrification occurs along long flowpaths within the paleovalley, resulting in lower nitrate flux to the estuary. Reducing conditions and associated reduced reactants (organic carbon, Fe2+, S2-) that drive denitrification may be supplied by freshwater and marine wetlands within the paleovalley, or by subsurface sediment. In the focused discharge zone in the interfluve, little evidence of denitrification exists and high concentrations of nitrate discharge with fresh groundwater. Results have implications for estimation of groundwater-borne solute fluxes to estuaries and nutrient management, as natural or anthropogenic alterations of coastal hydrology may affect the extent to which nutrients are attenuated prior to discharge.