Controls on Septic System Wastewater Treatment and Groundwater Quality in Coastal NC

Approximately 60% of coastal NC residences rely on septic systems for wastewater treatment and disposal. Domestic wastewater that is piped to septic systems contains many constituents (bacteria, viruses, nutrients, etc.,) that pose public and environmental health risks if they are not effectively treated. In coastal NC the potential for ground and surface water contamination by septic system seems great, due to sandy (and conductive) soils, shallow water tables, and developments in close proximity to surface waters such as rivers, estuaries, or the ocean. The North Carolina Department of Environment and Natural Resources (NC DENR) regulates septic systems and reports that 1,500 septic systems fail hydraulically (surfacing effluent and/or wastewater back-up in the home) each year, temporarily contributing bacteria, nutrients and other untreated wastewater pollutants to nearby surface waters. Less information is available concerning the pollutant loads from septic systems to shallow groundwaters (that eventually discharge into nearby surface waters) and the effect controlling factors such as soil texture and separation distance have on concentrations of pollutants beneath septic systems.

The study objective was to determine if subsurface treatment failures of septic systems, in relation to state and federal water quality standards are common in coastal NC and to identify areas most vulnerable to shallow groundwater pollution.

Groundwater quality and groundwater levels adjacent to 16 septic systems were monitored for NO3-N, NH4-N, and E.coli concentrations and compared to background groundwater conditions and water quality standards for 15 months. Nitrogen treatment failures (> 10 mg/L NO3-N) were common for systems installed in sand and sandy loam soils, but not sandy clay loams. Bacteria treatment failures (> 126 cfu / 100 mL) were common for all soil types, but were more frequent in coarse textured soils with shallow water tables.