GEOSPATIAL ANALYSIS OF DENITRIFYING SEPTIC SYSTEMS AND GROUNDWATER NITRATE CONCENTRATIONS IN JAMESTOWN SHORES, RHODE ISLAND

Abstract

High-density housing areas with onsite wastewater treatment systems (OWTS) and domestic drinking water wells are susceptible to groundwater contamination from nitrate-nitrogen (NO3-N). One solution is to install denitrifying OWTS, which are designed to reduce the effluent nitrogen by approximately half that of conventional OWTS. Geostatistical methods were used to analyze groundwater NO3-N data from the Jamestown Shores neighborhood of Jamestown, Rhode Island to determine if denitrifying OWTS have had an affect on water quality. Temporal trends were analyzed using NO3-N concentrations from sample events approximately 15 years apart, 1996-1997 and 2010-2011, between which a number of denitrifying OWTS were installed. Spatial trends in the effects of housing density, percentage of denitrifying OWTS, and select confounding variables on groundwater NO3-N concentrations were analyzed using directional buffers for groundwater flow and fracture orientation as well as circle buffers. Regionally, groundwater NO3-N did not decrease from 1996-1997 to 2010-2011, likely because of a net increase in houses and septic systems during this time period. Although the statistical significance of each buffer-type varied, groundwater NO3-N was generally found to increase with housing density in the immediate surrounding area and along the fracture orientation and decrease with at least one denitrifying OWTS in the area. Well depth and relative soil permeability (both normalized by housing density) did not have a statistically significant affect on NO3-N concentrations; the sample size for the low permeability soils was too small to statistically analyze, but the NO3-N concentrations were considerably less than for the other soils. Expected groundwater NO3-N concentrations were determined using estimated nitrogen loading from area OWTS, compared with measured concentrations, and a prediction model developed for the effects of increasing percentage of denitrifying OWTS. The expected vs. measured comparison model showed there are some sites with low NO3-N concentrations that do not appear to be affected by the high density of OWTS and some with high NO3-N concentrations above the level predicted by the density of OWTS alone. The prediction model showed that the percentage of denitrifying OWTS needed in the surrounding 400-foot radius to achieve NO3-N concentrations below the action level (5 mg/L) is at least 75% in the highest density areas (3.1 houses/acre) and at least 25% in the average density areas (1.7 houses/acre). The town can use these two models for planning purposes to determine where denitrifying OWTS may be most effective or where confounding variables may have a more significant influence on