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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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Characterizing Bedrock Fractures in Outcrop for Studies of Ground-Water Hydrology: An Example from Mirror Lake, Grafton County, New Hampshire

by

Christopher C. Barton (U.S. Geological Survey, Denver, Colo.)

Abstract

The fractured-rock-hydrology study site at Mirror Lake, New Hampshire provides an opportunity to use the pavement method to characterize bedrock fractures at four highway roadcuts and one glacial pavement adjacent to the bedrock-hydrology test wells at Mirror Lake. The method entails the mapping of the fracture-trace network down to fracture-trace lengths of 1 meter on a detailed geologic base map. These maps, in turn, form the basis for quantifying the scaling, interconnectedness, spatial distribution, and trace-length distribution for the fracture network. The characteristics of each mapped fracture include orientation, roughness, mineralization, aperture, and tectonic features, such as slickensides. The combined fracture data for the five pavement maps made to date indicate the following: Fracture orientation is highly variable with a preferred strike azimuth of approximately 30°E., preferred fracture dips of 7°NW., 50°SE., and 82°SE. Fracture roughness coefficients (RC) range from 0 to 18 RC values, with a mode at 5 to 6. Fracture aperture ranges from a lower cutoff of 0.005 mm to a maximum of 20.6 mm, and the frequency distribution of fractures follows a power-law function with a scaling exponent of -1.5. Fracture-trace length ranges from a lower cutoff of 1 m to a maximum of 24.6 m, and the frequency distribution follows a power-law function with a scaling exponent of -2.4. Fracture connectivity within the network is low compared to that of other sites around the country. Fracture mineralization includes iron-oxide coatings that appear have been biologically precipitated by iron-fixing bacteria in the ground water and Liesegang bands that extend up to about 1 meter into the rock matrix. A paleohydrologic map of the iron-stained fracture traces reveals that ground water flowed in only part of the available network of the open and interconnected fractures.

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