Use of a Geographic Information System to Assess Risk to Ground-Water Quality at Public-Supply Wells, Cape Cod, Massachusetts

Olimpio, Julio; Flynn, Elizabeth; Tso, Saiping; Steeves, Peter

Methods for assessing the risk to ground-water quality at selected public-supply wells on Cape Cod, Massachusetts, were developed in a 9-month demonstration project in 1987. Geographic information system technology was used to store, manipulate, and analyze information from more than 30 data bases for Cape Cod and the towns of Barnstable and Eastham. Technical, management, and institutional issues identified by the Cape Cod Aquifer Management Project--a 2- year study by the U.S. Environmental Protection Agency, the U.S. Geological Survey, the Massachusetts Department of Environmental Quality Engineering (now the Department of Environmental Protection), and the Cape Cod Planning and Economic Development Commission--(now the Cape Cod Commission) were used to select representative ground-water problems for analysis. Project objectives were to develop methods for assessing risk of ground-water contamination from sources within zones of contribution to water-supply wells that have been designated groundwater- protection areas. The land areas that lie directly above and that recharge zones of the water-table aquifer that contribute water to public-supply wells had previously been selected for use of ground-waterprotection strategies. Three representative factors were chosen for analysis: the siting of future wells to minimize risk from potential contamination sources (Eastham); risk to wells from existing contaminant sources within a zone of contribution to a public-supply well (Barnstable); and assessment of risk to wells from sources within and upgradient to zones of contribution (Capewide). Six assessments were made to demonstrate methods of evaluating water-quality conditions at public-supply wells; these are (1) suitability of potential public water-supply sites, (2) risk to ground-water quality from land-use changes, (3) intertown management of zones of contribution across town boundaries, (4) risk to ground-water quality from underground storage tanks, (5) nitrate concentrations in public-supply well water, and (6) risk to ground-water quality from landfills. The assessments were based on the development of step-by-step approaches that entailed data-overlay and buffering techniques for evaluating each representative water-quality issue. A buffer is a land zone within a specified distance of a point, line, or boundary of an area that is intended to reduce conflict between land uses or between physical features and land uses. Buffering is the act of creating such a zone. Numerous lessons were learned about planning, conducting, and completing a geographic information system-based demonstration project. Data problems included the unavoidable use of cartographically inaccurate base maps with different scales, multiple versions of data bases that did not match, and the need to convert large amounts of nongeographically referenced map and tabular data to digital data bases. Post-project evaluation revealed that too much time was spent on data gathering, too many part-time project staff participated in data compilation and verification, and too little time was allocated for the thorough testing and evaluation of analytical methads and results. The results of this project include the development of an extensive digital data base for regional and local water-quality -related applications, the organization of a large amount of technical information, and the demonstration of overlay methods for evaluating a variety ofpotential contamination sources within an area that supplies water to a pumped supply well. These results are intended to demonstrate the utility of geographic information system technology for improving the efficiency of ground- water-protection planning.