Comparison of Phosphorus Concentrations in Coastal Plain Watersheds Served by Onsite Wastewater Treatment Systems and a Municipal Sewer Treatment System

Abstract

Eutrophication related to excess phosphorus (P) loadings continues to be an important issue for watersheds in North Carolina and other regions. Identifying the contributing sources of P in nutrient-sensitive waters is important for improving water quality. Prior studies have indicated that onsite wastewater treatment systems (OWS) can be a contributing source of P to surface waters, but more information is needed regarding their contribution relative to other wastewater treatment technologies. The goal of this study was to determine if P concentrations in groundwater and surface water were significantly different in a coastal plain watershed served by OWS in comparison to a watershed served by a municipal sewer system (MWS). Groundwater P concentrations were monitored at ten residential sites (five5 OWS and five MWS) once during each season (four times), and stream P concentrations and watershed exports were monitored monthly for 1 year (August 2011–August 2012). Groundwater in the OWS watershed had higher P concentrations than the MWS watershed. Stream P concentrations and P exports were also elevated in the OWS watersheds. However, the OWS were more efficient at reducing P prior to surface water discharge than the wastewater treatment plant that served the MWS watershed. The site-scale and watershed-scale P treatment efficiencies of OWS were between 73 and 99 %, whereas P treatment efficiency for the wastewater treatment plant was 54 %. While the OWS were efficient at reducing P concentrations and loads, OWS were still significant sources of P exports from the studied watershed. Potential contributions of P from OWS should be included in watershed nutrient management strategies along with other known sources such as agriculture and urban runoff if the strategies are to be considered comprehensive.