Fate and Transport of Onsite Wastewater Treatment System Derived Nitrogen in Coastal, North Carolina

A two year study (2009-2011) was conducted to assess the impacts that on-site wastewater systems (OSW) have on contributing nitrogen to water resources in coastal North Carolina. More specifically, soil water and shallow groundwater adjacent to two OSW, deeper groundwater from nearby water supply and/or irrigation wells, and estuarine waters were sampled and analyzed for both organic and inorganic nitrogen species. Nested piezometers were installed up and down-gradient of the on-site wastewater systems to allow for sampling groundwater at different depths beneath the water column. Multi-depth, suction lysimeters were installed up and down-gradient of the OSW for soil water sampling and analyses. The levels and quality of groundwater were monitored for 2 years. Various isotopic analyses were used to help quantify nitrogen concentration reduction processes. Results indicated that the concentration of nitrogen in soil water, shallow groundwater beneath and down-gradient from OSW were elevated relative to background wells. Nitrogen concentrations in shallow groundwater decreased with increasing distance from the OSW due to dilution, denitrification, and other processes. Nitrogen concentrations in existing irrigation wells (deeper than the monitoring piezometers) at sites 1 and 2 were similar to background levels. However, shallow groundwater down-gradient of OSW and near the estuary did contain elevated nitrogen concentrations. Above average rainfall was observed during the first year of study, leading to high water tables, while the second year had below average rainfall and deeper water tables. Organic and NH4 were the dominant N species during high water table periods, while NO3 was the dominant nitrogen species during periods of deeper water tables.