NUTRIENT INPUT TO A DEVELOPED COASTAL ESTUARY VIA SUBMARINE GROUNDWATER DISCHARGE

Submarine groundwater discharge is now widely recognized as a significant pathway for transporting contaminants, including nutrients, into the coastal environment. In previous studies, septic effluent from onsite wastewater treatment systems has been implicated as a source of nutrients via submarine groundwater discharge in coastal developments that lack centralized wastewater collection. In order to examine links between onsite wastewater treatment and nutrient discharge at a residential site located on the coast of Mobile Bay, AL, hydrogeological and oceanographic techniques were used to characterize submarine groundwater discharge and nutrient transport during the summer of 2015. Six shallow groundwater monitoring wells were installed around a residential onsite wastewater treatment system to monitor nutrient concentrations (ammonia, nitrate, and reactive phosphate) of the surficial coastal aquifer. Lee-type seepage meters were deployed and monitored for a complete tidal cycle to measure fluxes of submarine groundwater and nutrients to the waters of Mobile Bay. Results reveal elevated nutrient concentrations in both groundwater and submarine groundwater relative to Bay water. Nutrient concentrations in groundwater were highest in wells located in the vicinity of the onsite wastewater treatment system. The dominant form of inorganic nitrogen in both groundwater and submarine groundwater discharge was ammonia, thus indicating incomplete nitrification of ammonia from septic effluent and possible limited soil cation exchange. The average flux of nitrogen to the nearshore environment throughout the tidal cycle was 2.2 x 10^-3 mol m^-2 d^-1 which exceeds many of the previously reported fluxes of nitrogen via submarine groundwater discharge in other Gulf Coast estuaries. Results from this study emphasize the impacts of septic effluent from coastal residential development on coastal water quality, and suggest the need for centralized wastewater treatment or more efficient onsite wastewater treatment systems in coastal communities.