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.