U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of
the Technical Meeting Charleston South Carolina March 8-12, 1999--Volume 3
of 3--Subsurface Contamination From Point Sources, Water-Resources
Investigations Report 99-4018C
Analysis of an Open-Hole Aquifer Test in Fractured Crystalline Rock
by Claire R. Tiedeman and Paul A. Hsieh
ABSTRACT
A multiple-well open-hole aquifer test was conducted in fractured crystalline
rock underlying the FSE well field at the Mirror Lake, New Hampshire fractured-rock
hydrology research site. The relation of measured drawdown to distance from
the pumped well is markedly different from that theoretically observed during
radial flow in a homogeneous aquifer. The open-hole aquifer test is analyzed
using a numerical ground-water flow model with the rock represented as vertically
uniform and heterogeneity sim-ulated by two irregularly-shaped high-transmissivity
zones. Model calibration results in a very good match of simulated and measured
drawdowns. In addition, the transmissivity estimates for both the highly-perme-able
and less-permeable parts of the rock are very similar to those estimated by
analysis of an aquifer test conducted with high-permeability fracture clusters
isolated by packers in all wells. This suggests that the open-hole test does
provide useful information about the hydraulic properties of the rock. However,
the het-erogeneity structure of the rock inferred from analysis of the open-hole
test is overly simplified, because it ignores vertical variation in rock properties.
Application of leaky-aquifer and double-porosity analytical models indicates
that while a fair match to some subsets of the FSE wells can be produced, neither
model can simultaneously reproduce drawdowns at all wells, because each assumes
that the hydraulic properties of the pumped aquifer are homogeneous. In the
FSE well field, highly-transmissive fracture clusters equal-ize drawdowns in
wells that intersect the same clusters, while the less-transmissive part of
the rock has a predominant effect on the magnitude of drawdowns because the
fracture clusters are finite in extent. Attempts to simultaneously reproduce
drawdowns at all wells with analytical models fail because the hydraulic parameters
of these models cannot be manipulated so that they mimic the distinct effects
that the high- and low-transmissivity parts of the rock have on drawdown in
the FSE well field.
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