Publication Date
12-5-2003
Abstract
The impact of land-use and housing density on ground-water quality has been extensively studied in areas dominated by permeable surficial materials. In New England, however, many residential areas derive their water supply from fractured bedrock aquifers. The relationship between land use, in particular housing density, and water quality in these areas has received little attention and the degree to which existing zoning laws protect the fracturedbedrock ground-water resource is unclear. This study examines a detailed water quality investigation on Conanicut Island, a small fractured bedrock island in Narragansett Bay, Rhode Island. Ground-water samples were collected from 174 domestic wells in areas with housing lot sizes ranging from 1/8 to > 2 acres. Background ground-water chemistry was assessed and wells with evidence of anthropogenic impact were classified where possible according to the source of impact: saltwater intrusion, septic systems, or road salt. Nitrate concentrations ranged from non-detectable to 16 mg/L as N. 81 samples yielded background NO3-N concentrations (below 1 mg/L) and 25 samples yielded significantly elevated NO3-N concentrations (above 5 mg/L). The highest occurrence of elevated nitrate concentrations was found in the Jamestown Shores area where 1/8 to 1/4 acre lots are common and all homes are served by individual septic systems and wells. A statistical analysis of housing lot size and groundwater chemistry was completed to determine whether this association was statistically significant. One acre was identified as a threshold lot size below which degradation of water quality was significant. This lot size criterion suggests that the existing RI Department of Environmental Management 100-foot septic system/well separation distance may be inadequate in this fractured bedrock setting.