How do malfunctioning onsite wastewater systems influence water quality? Assessing treatment of nutrient and Escherichia coli in central North Carolina.

Abstract

Onsite wastewater systems (OWSs) can be significant sources of nutrients and E. coli to water resources, potentially resulting in water quality degradation especially during times of malfunction. An OWS is considered malfunctioning when septic tank effluent is discharged into drainfield trenches during periods when there is insufficient separation to groundwater (<30 cm), when wastewater upwells to the surface, or when backup of wastewater into the tank or home occurs. Few studies have quantified groundwater and surface water quality downgradient from malfunctioning OWSs. Thus, the goal of this study was to quantify nutrients and Escherichia coli in shallow groundwater and adjacent streams downgradient from malfunctioning OWSs. Three residences served by conventional-style OWSs were instrumented with piezometers near the drainfields for groundwater sample collection. Streams near two of the OWS were also monitored. Sites were sampled 10 times between Feb 2022 and Apr 2023. Samples were analyzed for nutrients, E. coli, and other water quality parameters. Results indicated that groundwater was typically within 30 cm of the trench bottom at 2 sites (Sites 100 and 200), thus these OWSs were malfunctioning for insufficient vertical separation distance. Groundwater beneath the 2 malfunctioning drainfields contained median concentrations of TDN (17.98 mg L^-1), TDP (0.34 mg L^-1), and E. coli (2419.6 MPN 100 mL^-1) that were approximately 19, 34, and 576 times greater than concentrations in background groundwater. Groundwater near a functioning OWS (Site 300) had lower median concentrations of TDN (1.26 mg L^-1), TDP (0.10 mg L^-1), and E. coli (248.1 MPN 100 mL^-1) compared to the malfunctioning sites. Drainfield groundwater regardless of functionality contained elevated pollutant concentrations relative to background groundwater, which had median concentrations of 0.95 mg-N L^-1, 0.01 mg-P L^-1, 4.2 MPN 100 mL^-1 for TDN, TDP, and E. coli, respectively. Streams adjacent to the malfunctioning OWSs also contained elevated concentrations and exports of nutrients and E. coli relative to background. Thus, OWS malfunctions inhibited soil treatment processes contributing to elevated nutrient and E. coli pollution in receiving waters.