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

Abstract

Onsite wastewater treatment systems (OWTS) typically process eight to twelve kilograms of nitrogen per system annually. There are approximately two million OWTS in operation in North Carolina. If the mass and concentration of nitrogen discharged from these OWTS are not reduced before the effluent reaches groundwater and/or surface waters, then public and environmental health may be threatened. Increased nitrate-nitrogen (NO3) concentrations in drinking water can cause methemoglobinemia, sometimes called blue baby syndrome. Groundwater transport and discharge of ammonium-nitrogen (NH4) and NO3 to surface waters can lead to algal blooms, eutrophic conditions and fish kills. The objective of this study was to determine the nitrogen treatment efficiency of OWTS installed in the piedmont region of North Carolina. Five (volunteered) sites with OWTS in the piedmont counties of Wake (n = 1) and Durham (n = 4) were instrumented with piezometers (34 total) near and down-gradient from the OWTS. Two OWS were conventional style, two were surface discharge sand filter systems and one was a low-pressure pipe system. Septic tank, groundwater from the piezometers, sand filter effluent, and surface waters down-gradient from the OWTS were sampled for nitrogen concentrations. Data shows that total dissolved nitrogen (TDN) concentrations in groundwater beneath the drainfield trenches of conventional style OWS and in the effluent of surface discharge sand filters were on average 93% and 81% lower than septic tank effluent concentrations, respectively. Therefore, the conventional style OWTS were contributing lower concentrations of TDN to surface waters than the surface discharge sand filters. The findings of this study will be useful regarding management of OWTS derived nutrients in nutrient sensitive watersheds in the Piedmont of North Carolina.