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