Controls on groundwater nitrogen contributions from on-site wastewater systems in coastal North Carolina

The goal of this study was to evaluate the influence of soil type and separation distance to water table on dissolved inorganic nitrogen concentrations in groundwater adjacent to on-site wastewater systems. Groundwater nitrogen species (NO₃⁻-N and NH₄⁺-N) and groundwater levels adjacent to 16 on-site systems in three different soil groups (group I- sand, group II- coarse loams and group III -sandy clay loams) were monitored for 15 months (January 2007–March 2008) in coastal North Carolina. On-site systems in soil group I had the highest concentrations of dissolved inorganic nitrogen (median of 18.9 mg/L) in groundwater, and most frequently (mean 61%) exceeded 10 mg/L, followed by systems in soil group II (11.0 mg/L, 50%) and soil group III (2.6 mg/L, 9%), respectively. Groundwater NH₄⁺-N concentrations near on-site systems in soil groups I and II that maintained a 60 + cm separation to the seasonal high water table were 4 mg/L lower in relation to systems that had <60 cm separation, but median NO₃⁻-N concentrations were 6.5 mg/L higher. On-site systems in group I and II soils are prone to groundwater nitrogen loading with separation distance often controlling the nitrogen speciation in groundwater near on-site systems.