The transport and ultimate discharge of groundwater into coastal zones may be a significant process in the geochemical and nutrient budgets of many marine nearshore waters. The transport of non-point source pollution from onsite treatment and disposal systems (OSTDS) to shallow groundwater and ultimately surface waters could be an important source of contamination to the marine environment that is frequently ignored by scientist and coastal managers alike. However, when evaluating the impacts from these systems on coastal waters, it is important to consider the natural flux of nutrients from coastal aquifers to surface waters.

Groundwater from a shallow freshwater lens on St. George Island, a barrier island located in the panhandle of Florida, eventually discharges into Apalachicola Bay or the Gulf of Mexico. Nutrient concentrations in groundwaters and surface waters were monitored downfield from three wastewater systems on the island. Natural groundwater nutrient concentrations were obtained from an adjacent uninhabited island. Silicate, which was significantly higher in the drinking water relative to the surficial aquifer on St. George Island, was used as a natural conservative tracer, providing additional insight to the extent of the wastewater plume. Nutrient concentrations were all attenuated rapidly relative to silicate, indicating very little transport of these nutrients to surface waters. Estimates of the groundwater nutrient flux from St. George Island into surface waters were within error of those for the adjacent island, currently not impacted by OSTDS. However, the population density, thus OSTDS density, is a critical concern for these island systems and should be taken into account.