Fate and transport of phosphate from an onsite wastewater system in Beaufort County, North Carolina.

The objectives for the study described in this article were to evaluate the fate and transport of onsite wastewater system (OWS)-derived phosphate from a residential system in Beaufort County, North Carolina, and to determine if current OWS setback regulations are sufficient to prevent elevated phosphate discharge to surface waters. Piezometers were installed in nests at different depths adjacent to drain-field trenches and up- and down-gradient of a residential OWS. Groundwater and septic effluent phosphate concentrations, temperature, pH, dissolved oxygen, and electrical conductivity were monitored every two months from February 2011 to October 2011 (five times). The mean groundwater phosphate concentration beneath the OWS (3.05 +/- 0.74 mg/L) was not significantly different than septic effluent (2.97 +/- 0.76 mg/L) and was elevated relative to background groundwater (0.14 +/- 0.12 mg/L). Groundwater phosphate concentrations were inversely related (r2 = .83) to distance from the system. Onsite system setback regulations may have to be increased (>30 m) in some areas to ensure groundwater phosphate concentrations are reduced to background concentrations before discharge to surface waters.