Wastewater Nitrogen Contributions to Coastal Plain Watersheds, NC, USA

Abstract

Nitrogen inputs to coastal watersheds have been linked to eutrophication. However, the role that domestic sources of wastewater play in contributing nitrogen to coastal watersheds is not well known in the southeastern USA. In a yearlong study (2011–2012), nitrogen concentrations were compared in watersheds served by septic systems and a centralized sewer system in the Coastal Plain of North Carolina. Surface and groundwater samples from septic systems and sewer watersheds were analyzed for total dissolved nitrogen (TDN), total nitrogen, and nitrogen and oxygen isotopes in nitrate. Groundwater beneath the drainfield and adjacent to streams had median concentrations of TDN at 5.9 and 4.4 mg/L, respectively. Additionally, median groundwater-transported loads of TDN to the stream from septic systems sites (0.6 kg-TDN/year) were significantly greater than sites in sewer watersheds (0.2 kg-TDN/year). Isotopic analyses revealed that effluent from septic systems was the primary source of nitrate in watersheds served by septic systems, while fertilizer and/or soil organic matter were dominant sources of nitrate in sewer watersheds. Nitrogen exported from septic systems contributed to elevated nitrogen concentrations in groundwater and streams throughout the watershed, whereas nitrogen exports from sewers were focused at a single point source and affected surface water concentrations. Based on watershed TDN exports from septic systems minus TDN exports from sewers watersheds, it was estimated that septic systems contributed 1.6 kg TDN/ha/year to watershed exports of TDN. Overall, septic systems and sewers contributed to elevated nitrogen loading and should be considered in nutrient-sensitive watershed management.