Porewater Salinity Distribution and Geochemical Characterization Beneath Indian River Bay, Delaware

Eutrophication, caused by excess nutrient inputs related to agricultural and other human activities, is a major environmental problem affecting Indian River Bay, DE. Characterization of nitrogen inputs through submarine groundwater discharge (SGD) to the bays is important as this source may represent as much as one-quarter of the load. Mixing of fresh (oxic) and saline (anoxic) groundwater prior to discharge is hypothesized to cause biogeochemical reactions that affect fluxes of bioavailable nitrogen. Hydrologic forcing operating on temporal and spatial scales is thought to enhance mixing. Low permeability sediments associated with Pleistocene salt marsh and paleochannel fill form a confining unit that allows a fresh water plume to extend several meters from shore, creating complex salinity distributions in the subsurface.  Repeated surveys of salinity, nutrients, and other field parameters were conducted to characterize temporal changes and associations with geologic heterogeneity and evidence of denitrification. Temporal variability of salinity was monitored on time scales ranging from tidal (over which no salinity variations were detected) to seasonal. Offshore electrical resistivity surveys revealed complex subsurface salinity distributions. A variable density model code, SEAWAT, was used to simulate the salinity distribution, groundwater flow, and submarine groundwater discharge fluxes in steady state and transient conditions using a model that incorporates site-specific parameters and a simplified geometry.