Geophysical and flow-weighted natural-contaminant characterization of three water-supply wells in New Hampshire

Abstract

Three bedrock water-supply systems in New Hampshire were studied, using borehole geophysics and flow-weighted sampling techniques, to determine the sources and distribution of natural contaminants in water entering the boreholes and to assess whether borehole modifications might be used to reduce contaminant levels. Well water in more than 100 community water-supply systems in New Hampshire have natural contaminants, such as arsenic and uranium, above the U.S. Environmental Protection Agency maximum contaminant levels of 10 and 30 micrograms per liter, respectively. The water-system wells were studied to identify fractional contributions of natural contaminants from specific fracture zones. The yields and flow-weighted contaminant levels of such fracture zones were assessed to determine if a modification of the borehole might lead to a reduction in the system’s contaminant levels. The water-supply systems investigated were typical of small community water systems in New Hampshire where a water system may serve 100 connections or less. Each water system consisted of two wells, approximately 300 to 400 feet deep, in generally low-yielding (about 10 gallons per minute or less) crystalline bedrock. The wells were typically operated a few hours per day to fill a storage tank and had tens of feet of drawdown caused by the low well yields. The systems selected had contaminant concentrations slightly above MCL, or a low-level contamination. One of the water systems investigated had low-level (10 to 24 micrograms per liter) arsenic contamination, and two of the water systems had low-level uranium (30 to 40 micrograms per liter) contamination. The contaminant values were blended-water concentrations from the two wells in a system. Each water system had differences in contaminant concentrations between the two wells. In each case, the well with the greater concentration of the two was selected for investigation. In two of the three systems investigated, there was either not enough variation in the borehole contaminant concentration or not enough water-yielding fractures for borehole modifications to be a viable potential remedy to elevated contamination. However, borehole and contaminant conditions in one of the bedrock supply-well systems may be favorable to potential improvement of supplied water by borehole modification where selected fracture zones are sealed off from supplying water to the well.