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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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Hydraulic Characteristics of Fractured Bedrock Underlying the FSE Well Field at the Mirror Lake Site, Grafton County, New Hampshire

by

Paul A. Hsieh (U.S. Geological Survey, Menlo Park, Calif.) and Allen M. Shapiro (U.S. Geological Survey, Reston, Va.)

Abstract

At the Mirror Lake site in Grafton County, New Hampshire, 13 wells are drilled in a 120-m (meter) by 80-m area, known as the FSE well field, the characterize the hydraulic and transport properties of the underlying fractured bedrock. Borehole geophysical logs and downhole video camera images show that each well intersects 20 to 60 fractures within the upper 60 m of bedrock. However, single-well hydraulic tests show that only one to three fractures in each well are highly transmissive. Multiple-well hydraulic tests suggest that the few highly transmissive fractures connect with one another locally to form fracture clusters. These highly transmissive fracture clusters, in turn, connected to one another by fractures that are comparatively lower in transmissivity by several orders of magnitude. In such a fractured-bedrock setting, the response to a multiple-well hydraulic test is entirely different from the response in a homogeneous aquifer. These finding s suggest that it is necessary to identify and characterize explicitly the highly transmissive fracture clusters in order to analyze multiple-well hydraulic tests at the FSE well field.

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