Delineation of Hydrologic Areas of Concern for New Hampshire Water Supply Rivers Using Dye Tracer Studies and Other Methods

摘要

As part of the Drinking Water Source Assessment Program in New Hampshire, the NH Department of Environmental Services (DES) delineated a "hydrologic area of concern" (HAC) for 57 surface sources of public drinking water. The goal of HAC delineation was to focus the inventory of potential contaminant sources in those areas that are likely to have the greatest influence on water quality at the water supply intake. A number of methods were used to delineate the HACs, depending on the type of source (stream or reservoir), size of the watershed, degree of development, presence of dams, and river velocity. For 13 rivers with large watersheds, DES and the U.S. Geological Survey (USGS) designed and conducted dye tracer tests to estimate the travel times and dispersion of soluble contaminants hypothetically released upstream of each water-supply intake. The USGS measured river discharge and injected rhodamine WT dye during annual low and mean flows on each study river. Rivers were sampled downstream of the dye-injection sites to measure the concentrations and arrival times of the dye cloud. Results of the dye tests were used to construct graphs for estimating the travel times and concentrations of soluble contaminants over a range of river discharges and transport distances. Estimated travel times during flow conditions equal to the mean annual flood were extrapolated from these graphs. DES used the estimated distance traveled by a dye cloud peak in 6 hours at mean annual flood to delineate the HAC. Where the 6-hour HAC was too large, DES based the HAC delineation on the presence of large impoundments, or on a 1-mile buffer along each side of the river. The results of the dye tracer tests are also expected to be useful in planning emergency response to a contaminant spill in the studied rivers.