Paired Watersheds Approach For Evaluating The Influence Of Wastewater Management Strategies On Stream Nutrient Concentrations.

Excess nutrients in North Carolina surface waters have resulted in water quality impairment and aquatic habitat degradation. A better understanding of the watershed-scale influences of various wastewater treatment methods is needed for the development of comprehensive watershed nutrient management regulations. The goals of this study were to determine if significant differences in total nitrogen (TN) and total phosphorus (TP) concentrations and loads were observed in watersheds served by onsite wastewater systems (OWS) in relation to watersheds served by municipal sewer (MWS) and to determine if there were significant differences in nutrient treatment efficiencies between OWS and MWS. Ten sites (5 OWS and 5 MWS) in Pitt County, NC were instrumented with piezometers and sampled seasonally (4 times over 1 year) for groundwater nutrient analysis and physical and chemical characterization of groundwater and wastewater. Influent and effluent samples from the MWS were also collected and analyzed for TN and TP analysis to determine nutrient reduction efficiency. Seven streams (3 OWS and 4 MWS) were monitored monthly for flow, nutrient concentrations, and physical and chemical parameters for one year (August 2011-August 2012). Groundwater and stream TN and TP concentrations and loads in watersheds served by OWS were higher than groundwater and stream nutrient concentrations and loads in MWS watersheds. However, the 4 MWS streams that were monitored did not receive MWS treatment plant effluent. The TN and TP treatment efficiencies for the monitored OWS  were greater than or equal to municipal sewer treatment plant efficiency. Because OWS, like MWS, influence stream TN and TP concentrations and loads, OWS contributions should be considered when nutrient management strategies for watersheds are developed.