Date of Award
Master of Science in Geology
The impact of land use in sand and gravel aquifers has been studied extensively, but the impact on fractured-bedrock aquifers has not. Many water-supply aquifers are in fractured bedrock and their susceptibility to contamination needs to be better understood. This study was done on Northern Conanicut Island, a small fractured bedrock island in Rhode Island. Ground-water samples were collected from 174 domestic wells in areas with housing lot sizes ranging from 1/8 to >2 acres.
Land-use impacts are common from sources such as septic systems, agricultural fertilizers, road salt, and saltwater intrusion. The most common constituents attributable to these sources are nitrate, chloride, sodium, sulfate, and coliform bacteria. The relative ratios of the constituents in leachate from each of the aforementioned sources can be used to identify probable sources of contamination in ground water. Nitrate-nitrogen to chloride ratios were used to identify septic system and fertilizer impacts. Elevated concentrations of sodium and chloride together with location and well depth were used to identify road salt and saltwater contamination. Of the sites with elevated nitrate for which a specific source could be identified, 37 were indicative of septic leachate, 25 of fertilizer, and 22 were from multiple sources. The areas of greatest septic system impact are found in the part of the island with the highest housing density, where lots are smaller than 1 acre. This leads to the belief that there is inadequate dilution taking place between neighboring wells and septic systems. Of the sites with elevated chloride, 22 were found to have a saltwater impact and 5 were contaminated by road salt. The geochemical data suggest that a minimum of 1 acre is required to protect ground-water quality.
Sandorf, Jennifer A., "LAND-USE IMPACTS ON GROUND-WATER QUALITY IN A FRACTURED BEDROCK AQUIFER" (1999). Open Access Master's Theses. Paper 2022.