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Environmental Health - Toxic Substances


U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

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A Conceptual Model for Estimating Regional Ground-Water Velocity in Bedrock of the Mirror Lake Area, Grafton County, New Hampshire


Allen M. Shapiro (U.S. Geological Survey, Reston, Va.), Warren W. Wood (U.S. Geological Survey, Reston, Va.), Eurybiades Busenberg (U.S. Geological Survey, Reston, Va.), Stefan Drenkard (Lamont-Doherty Earth Observatory, Department of Geochemistry, Palisades, N.Y.), L. Niel Plummer (U.S. Geological Survey, Reston, Va.), Thomas Torgersen (University of Connecticut, Department of Marine Sciences, Groton, Conn.), and Peter Schlosser (Lamont-Doherty Earth Observatory, Department of Geochemistry, Palisades, N.Y.)


Estimation of regional ground-water velocities requires a knowledge of the residence time of the ground water and the distance it has traveled. Ground-water residence times are estimated in the Mirror Lake area on the basis of concentrations of chlorofluorocarbons and the parent-daughter isotopes tritium and helium. The distance the ground water has traveled, however, cannot be identified in the Mirror Lake area because there is no single area of recharge to the bedrock and the heterogeneity of hydraulic properties of the bedrock indicate that a regional flow line cannot be conceptualized from successive down-gradient sampling locations. Measurements of alkalinity in water samples collected from fractures in the bedrock are positively correlated with ground-water age. The alkalinity is controlled by the concentration of bicarbonate ions, and carbon isotopes indicate that the bicarbonate concentration is the result of dissolution of calcite in the rock matrix. The diffusion of bicarbonate ions from the rock matrix to fractures is hypothesized as a second indicator of the residence time of ground water in the bedrock. A simple model of ground-water flow and the transport of bicarbonate ions in the bedrock is proposed to investigate the relationsamong ground-water velocity, residence time and alkalinity. The model consists of a single fracture in contact with a rock matrix from which the average ground-water velocity in the bedrock can be estimated.From this simple conceptual model, the length of paths of fluid movement in the bedrock and the average bedrock velocity can be estimated. This conceptual model, however, must be reconciled with other geochemical data that indicates that ground waters of various ages are mixing either naturally or as a result of ground-water sampling.

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