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.
Keywords: coastal, nitrogen, septic system, soil