Nutrient Treatment By Onsite Wastewater Systems in Clayey Soils of the North Carolina Piedmont.

Major water resources in the Piedmont of North Carolina including Falls Lake, Jordan Lake, and the Tar River are impaired due to excess nutrient loading. State regulatory agencies have implemented strategies to reduce point and non-point sources of nutrient pollution to the impaired waters. However, the nutrient contributions from onsite wastewater systems (OWS) were not addressed, because there is a lack of research regarding OWS nutrient treatment efficiency, especially in the Piedmont of North Carolina. The goal of this project was to gain a better understanding of the nutrient treatment efficiency of OWS installed in clayey soils of central North Carolina. Four volunteered, residential sites with OWS were studied. The four OWS included two conventional and two single-pass sand filter systems. Septic tanks, groundwater near the OWS, effluent from the sand filters, and adjacent surface waters were sampled each season during 2015 and 5 times overall. Samples were analyzed for total dissolved nitrogen (TDN), phosphate phosphorus (PO₄-P), dissolved organic carbon, chloride, and environmental parameters.  Two component mixing models using septic tank effluent and background groundwater chloride and nutrient concentrations were used to estimate reductions in the mass of OWS derived nutrients. Average TDN and PO₄-P concentrations in groundwater 35 m down-gradient from the two conventional OWS were 98% and 99% lower respectively, than septic tank effluent. Mixing model results showed nutrient mass reductions of 70% (TDN) and 95% (PO₄-P) in groundwater 35 m down-gradient from the OWS. Sand filter effluent concentrations of TDN and PO₄-P were 80% and 90% lower respectively, than septic tank concentrations. The mass of TDN and PO₄-P were reduced by 50 and 83% in the sand filter OWS. Additional treatment of sand filter effluent prior to surface water discharge is suggested to reduce nutrient loading from OWS.