Relationship between Groundwater Nitrate Concentration and Density of Onsite Wastewater Treatment Systems: Role of Soil Parent Material and Impact on Pollution Risk

Nitrate (NO3−) pollution from onsite wastewater treatment systems (OWTS) is a concern in coastal communities that rely on groundwater for drinking water because of health risks associated with high levels of NO3− and the potential for eutrophication in areas where ground and surface water are connected. We examined the relationship between OWTS density and groundwater NO3− concentration in glacial soils with different parent material in a coastal watershed in the town of Charlestown, Rhode Island (USA). The area is underlain by glacial till and fluvial deposits, with groundwater as the only source of potable water and OWTS as the only means of treatment. The density of OWTS/ha was not significantly different between glacial fluvial (median=1.0; range, 0.04–4.8; n=207) and till (median=1.3; range, 0.012–3.81; n=160) soil parent material. Nitrate levels (mg N/L) in shallow well samples taken from 2013 to 2022 were significantly higher in till (median=3.7; range 0–15; n=160) than in glacial fluvial (median=2.7, range 0–9.7; n=207) parent material. Groundwater NO3− levels increased linearly with density of OWTS, and the slope (mg N · ha/L · OWTS) and intercept (mg N/L) values for the regression were significantly higher for till (0.99; 2.28) than glacial fluvial (0.66; 1.95) parent material. Regression intercept values >0.5 mg N/L, corresponding to a density of 0 OWTS/ha, point to widespread mixing of contaminated groundwater. Cumulative probability analysis showed that the proportion of samples with NO3− levels corresponding to Extreme pollution risk (>5 mg N/L) was much higher in till than in glacial fluvial parent material at the same OWTS densities. Fewer than 10% of the NO3− values were in the Low risk category (<0.5 mg N/L) regardless of OWTS density or parent material. Our results suggest that OWTS density and soil parent material should be part of the criteria for water management and land use decisions to protect public and environmental health.