<p>There are over 2 million active onsite wastewater treatment systems (OWTSs) in North Carolina. Effluent discharged from septic tanks to soil contains elevated concentrations of nitrogen, which may pose environmental and public health concerns if the concentrations and mass of nitrogen are not reduced prior to reaching water resources. The treatment efficiency of most OWTSs is not monitored. Information regarding the effectiveness of commonly used OWTSs is important especially in areas where there are high densities of OWTSs and near water bodies that are used for recreation and water supply. Five OWTSs within the Falls Lake Watershed of North Carolina were monitored during this study to determine the effectiveness of the systems in reducing nitrogen loads. The mass of nitrogen in septic tank effluent was reduced by 65% in the vadose zone beneath most OWTSs and further attenuation was observed at one site as the wastewater plume migrated away from the system. The sand filter reduced nitrogen loading by between 41 and 79%. The OWTSs included in this study contributed to elevated concentrations and loads of nitrogen to groundwater. Remediation of the nitrogen contributions to groundwater and surface waters via implementation of permeable reactive barriers, drainageway bioreactors and/or advanced treatment technologies may be needed in some nutrient sensitive areas, particularly in areas with a high density of OWTS usage.</p>