Characterization of On-Site Wastewater Nitrogen Plumes in Shallow Coastal Aquifers, North Carolina

On-site wastewater (septic) systems have often been implicated as a source of high nitrogen content in groundwater in Eastern North Carolina. At the national level, the Centers for Disease Control and Prevention (CDC) pointed out the need for enhancing the knowledge of septic systems performance. This paper is part of a study that was designed to address that need. The objectives of this work were to determine if electrical resistivity mapping is an applicable technique for characterizing septic system effluent plumes in sandy soils and to investigate the links between septic system discharge, groundwater quality, and potential contributions to surface waters. Electrical resistivity surveys were completed on two sites that use septic systems in Washington, North Carolina. Plumes of low resistivity groundwater were identified at both sites. Orientation of the plumes matched closely with the direction of groundwater flow away from the septic systems as indicated by groundwater level data from piezometers. Preliminary data of samples taken from each septic tank showed total nitrogen concentrations between 46 and 48 mg/l, mostly in the forms of organic nitrogen and ammonium nitrogen (NH₄-N). Also, preliminary data showed higher concentrations of nitrate nitrogen (NO₃-N) and organic nitrogen in shallow groundwater samples adjacent to septic systems than in background samples. While dissolved nitrogen concentrations typically decreased as the plumes moved further away from the systems, elevated nitrogen concentrations were seen close to the estuary at one site. Future work will include septic tank and groundwater N-isotope sampling and installation of YSI sondes (automated NO₃-N and NH₄-N sensors), to gain more information on N-processing and the fate and transport of dissolved nitrogen in shallow coastal aquifers.