A Comparison of Nutrient Contributions By Wastewater Management Approach in the North Carolina Piedmont.

The Falls Lake watershed in the North Carolina Piedmont is impaired because of excess nutrients. Malfunctioning septic systems were listed as a potentially significant source of nutrients to several tributary streams of Falls Lake; but more research was suggested to evaluate the contributions from septic systems. The objective of this study was to determine if surface water and groundwater nitrogen concentrations and exports were elevated in Falls Lake subwatersheds served by septic systems relative to a centralized sewer and a forested subwatershed. Six streams downgradient from septic systems, one stream in a forested area, and one stream served by a sewer system were sampled monthly for total dissolved nitrogen (TDN) and phosphate (P) concentrations and environmental readings from January 2015 until summer 2016. Preliminary results show that septic density plays a critical role in the concentration and masses of nutrients delivered to streams. Median surface water concentrations of TDN and P were elevated in high-density septic watersheds (1.88 mg-N/L; 0.06 mg-P/L) relative to low-density septic watersheds (0.95 mg-N/L; 0.02 mg-P/L), a centralized sewer watershed (0.99 mg-N/L; 0.02 mg-P/L) and a forested watershed (1.00 mg-N/L; 0.03 mg-P/L). Export data did not show significant differences between high-density septic (1.12 kg-N/yr/ha; 0.05 kg-P/yr/ha), low-density septic (1.11 kg-N/yr/ha; 0.02 kg-P/yr/ha), centralized sewer (2.41 kg-N/yr/ha; 0.03 kg-P/yr/ha) or forested watersheds (1.52 kg-N/yr/ha; 0.03 kg-P/yr/ha). This is most likely due to the presence of in-stream natural wetlands downgradient from high-density septic watersheds that may remove significant masses and concentrations of nutrients. Isotopic analysis of δ¹⁵N and δ¹⁸O indicated that nitrate in streams draining watersheds served by septic systems likely came from septic effluent, fertilizer, and/or soil organic matter, while nitrate in streams draining non-septic watersheds likely came from fertilizer and/or soil organic matter. Preliminary data indicate that septic system density influences nutrient concentrations in piedmont streams.