ONSITE WASTEWATER SYSTEM NITROGEN CONTRIBUTIONS TO WATER RESOURCES IN THE PIEDMONT OF NORTH CAROLINA

Domestic onsite wastewater treatment systems (OWS) typically discharge 8 to 12 kg of nitrogen per system to the subsurface environment annually. There are approximately 2 million OWS in operation in North Carolina. If the mass and concentration of nitrogen discharged from these OWS is not reduced before the effluent reaches groundwater and surface waters, then public and environmental health may be threatened. For example, increased nitrogen levels in surface waters can lead to eutrophication, fish kills, and promote the growth of toxic algal blooms. Research regarding OWS nutrient treatment has been focused mostly in the coastal region of North Carolina; however, little research has been conducted in the piedmont, where the geology and soils are different. The objective of this study was to determine the nitrogen treatment efficiency of onsite wastewater systems (OWS) installed in the piedmont region of North Carolina. Five (volunteered) sites with OWS in the piedmont of North Carolina were instrumented with piezometers. Septic tank, groundwater from the piezometers, and surface waters near the sites were sampled for nitrogen concentrations and speciation. Preliminary data show that total dissolved nitrogen concentrations in groundwater beneath the drainfield trenches of conventional style OWS are typically between 5 and 7 mg/L, and the concentrations decrease with increasing distance from the OWS. Effluent sampled from discharging sand filter style systems has contained an average of 10 mg/L dissolved nitrogen. Surface water samples collected down-gradient from the OWS have had lower TDN concentrations than groundwater near the OWS. Isotopic analyses of nitrate in the water samples are being conducted to help determine the sources of nitrate. The data obtained from the study (five sites) should be helpful for policy makers when nutrient management plans for piedmont watersheds are developed.