SUBMARINE GROUNDWATER DISCHARGE AND NUTRIENT DYNAMICS IN THE VICINITY OF COASTAL RESIDENCES WITH ONSITE WASTEWATER TREATMENT

Eutrophication driven by anthropogenically-derived nutrients is a growing threat to the environmental resiliency of coastal waters. Efforts are underway to manage nutrients in surface waters; however, recent studies indicate that coastal nutrient input via submarine groundwater discharge may exceed that of surface water discharge. In particular, septic effluent has been implicated as a significant source of submarine nutrient discharge in coastal developments that lack centralized wastewater collection. In order to gain a better understanding of the links between onsite wastewater treatment and coastal water quality at a residential site located on the coast of Mobile Bay, AL, submarine groundwater discharge and nutrient transport were characterized using a combination of hydrogeologic and oceanographic techniques. Six shallow wells (3-m deep) were installed around a residential site with onsite wastewater treatment to monitor groundwater velocity and nutrient concentrations (ammonia, nitrate, and phosphate) of the surficial coastal aquifer. Lee-type seepage meters were deployed to measure fluxes of submarine groundwater and nutrients to the waters of Mobile Bay throughout the tidal cycle. Results indicate elevated nutrient concentrations in both groundwater and submarine groundwater discharge relative to bay water. Nutrient concentrations in groundwater were highest 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 septic effluent and limited soil cation exchange. In addition, the presence of elevated concentrations of reactive phosphorus in groundwater samples suggests reducing conditions in the aquifer that prohibit iron-phosphate complexation prior to submarine discharge. 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 previously reported fluxes of nitrogen via submarine groundwater discharge in other Gulf Coast estuaries. Results from this study highlight the impacts of septic effluent from coastal residential development on coastal water quality, particularly in areas with shallow groundwater.