CONTROLS ON PHOSPHORUS TRANSPORT FROM ONSITE WASTEWATER SYSTEMS TO GROUNDWATER AND ADJACENT SURFACE WATERS IN COASTAL NORTH CAROLINA

Eutrophic conditions and water use impairment are common in North Carolina coastal waters because of various point and non-point sources of nutrients. The objectives of these studies were to 1) determine the effectiveness of onsite wastewater systems (OWS) installed in different soils and geologic settings to reduce phosphate concentrations before discharge to groundwater and 2) to assess the fate and transport of OWS derived phosphate at another coastal site. Groundwater phosphate concentrations adjacent to 16 OWS installed in different coastal soils and settings were compared to septic tank effluent and background groundwater concentrations. Mean septic effluent phosphate concentrations were elevated (3.4 to 8.1 mg/L) relative to groundwater adjacent to OWS. Groundwater phosphate concentrations near OWS were highest for systems installed in sand (2.46 ± 2.9 mg/L) followed by sandy loam (0.39 ± 0.45 mg/L) and sandy clay loam soils (0.04 ± 0.03 mg/L). Only OWS in sandy soils had groundwater phosphate concentrations significantly higher than background concentrations. For the second study, piezometers were installed up-gradient of an OWS, between the drainfield trenches, and down-gradient from the system. Groundwater and septic effluent samples were collected for phosphate analysis bi-monthly during 2011. Data indicated that septic effluent concentrations (2.97 ± 0.76 mg/L) were not significantly different than groundwater phosphate concentrations adjacent to the OWS (3.05 ± 0.74 mg/L). Phosphate concentrations decreased with increasing distance from the system. However, phosphate concentrations more than 30 m (the current setback distance requirement) from the systems were elevated relative to background conditions (0.14 ± 0.12 mg/L). OWS installed in sandy, coastal soils with low iron content can contribute significant concentrations of phosphate to groundwater more than 30 m down-gradient from the systems.