THE INFLUENCE OF GROUNDWATER FLOWPATHS AND MIXING ON NUTRIENT FLUXES TO ESTUARIES AND THE OCEAN (Invited Presentation)

Accurate estimation of nutrient fluxes to estuaries and the ocean is essential for designing appropriate management strategies to protect coastal ecosystems. It is complicated, however, by challenges not only in quantifying groundwater fluxes, but also in assessing the impacts of both the origin of discharging water and biogeochemical reactions along subsurface flowpaths. Recent advances in integrating physical and biogeochemical processes is improving understanding of the various pathways for nutrient delivery and mechanisms for attenuation prior to discharge. In Indian River Bay, Delaware, a nitrogen-limited eutrophic estuary, watershed geometry is an important factor affecting fresh groundwater flux, while flowpaths determined by geologic heterogeneity regulate attenuation of nitrate prior to discharge. Ammonium is contributed by recirculating seawater, with fluxes dependent on exchange rate, residence time, and sediment type. In intertidal zones of Delaware beaches, mixing between infiltrating seawater and discharging groundwater promotes reactivity, resulting in denitrification in freshwater-saltwater mixing zones. Measurements and modeling illustrate the impacts of these processes on nutrient fluxes in these and other coastal systems. Consideration of both geological and hydrological influences on biogeochemical reactions in groundwater prior to discharge to coastal waters is critical for estimating and managing groundwater-borne nutrient loads to nearshore marine ecosystems.