Responses in Baseflow Nitrogen Concentrations to Wastewater Management Approaches in the North Carolina Piedmont

Abstract

Falls Lake is currently classified as impaired because of elevated chlorophyll-a and turbidity levels associated with excess nutrient inputs. Anthropogenic and natural inputs of nutrients, especially nitrogen (N), into surface water bodies have increased over the last century, degrading water quality, and endangering environmental and public health. This research investigated the relationship between baseflow N concentrations and point and non-point sources of wastewater discharges within selected sub-watersheds of Falls Lake. Twenty-eight streams within sub-watersheds of Falls Lake, consisting of 20 septic and 6 sewer dominated wastewater management approaches (WMAs), were sampled monthly over a 1-year period. The Kruskal- Wallis statistical test and pairwise Wilcoxon rank-sum test were used to determine if there were statistically significant relationships between N concentrations and controlling factors. The results of this research show that median nitrate-nitrogen (NO3-N) and total dissolved nitrogen (TDN) concentrations were elevated in septic dominated sub-watersheds, increasing in NO3-N and TDN concentration with increased onsite wastewater treatment systems (OWTS) density. [delta]15N in NO3- isotopic ratios indicated that manure and septic/sewer effluent sources of NO3- were present in surface water in all sub-watersheds regardless of WMA. The baseflow stream nitrate and [delta]15N data in the Falls Lake sub-watersheds suggests that OWTS contribute NO3- to streams in high-density (HD) septic sub-watersheds. Elevated nitrogen concentrations in two sewer sub- watersheds suggest the possibility of legacy nutrients or leaking sewer lines contributing to in- stream NO3-N concentrations. The comparison between geologic setting and in-stream N concentrations showed significant associations between TDN and Triassic Basin septic sub- watersheds. Overall, the results of this study show that OWTS can contribute N to streams and there appears to be an influence based on geologic setting. The information provided by this study will be useful in further research to determine stream remediation approaches and to identify sensitive areas within this watershed and other urban watersheds, based on the methodology and interpreted relationships.