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Application of a geophysical "tool-box" approach to characterization of fractured-rock aquifers -- a case study from Norwalk, Connecticut

John W. Lane, Jr., U.S. Geological Survey, Storrs, Conn.
John H. Williams, U.S. Geological Survey, Troy, N.Y.
Carole D. Johnson, U.S. Geological Survey, Storrs, Conn.
Sister Damien-Marie Savino, United Technologies Corp., Washington, D.C.
F. Peter Haeni, U.S. Geological Survey, Storrs, Conn.

Abstract

The U.S. Geological Survey conducted a geophysical investigation at a site in Norwalk, Connecticut where solvents have contaminated a fractured-rock aquifer. Borehole, borehole-to-borehole, and surface geophysical methods were used to characterize the bedrock fractures, lithologic structure, and transmissive zone hydraulic properties in 11 boreholes and their vicinity. The geophysical methods included conventional logs, borehole imagery, borehole radar, flowmeter, and azimuthal square-array dc resistivity soundings. Integrated interpretation of geophysical logs at borehole and borehole-to-borehole scales indicates that the bedrock foliation strikes northwest, dips northeast, and strikes north-northeast to northeast, dips both southeast and northwest. Although steeply dipping fractures that cross-cut foliation are observed, most fractures are parallel or sub-parallel to foliation. Steeply dipping reflectors observed in the radar reflection data from three boreholes near the main facility building delineate a north-northeast trending feature. Results of radar tomography conducted close to a suspected contaminant source area indicate that a zone of low velocity and high attenuation exists above 50 feet in depth - the region containing the highest density of fractures. Flowmeter logging was used to estimate hydraulic properties in each of the boreholes. Thirty-three transmissive zones were identified in 10 of the boreholes. The vertical separation of the transmissive zones in a borehole typically is 10 to 20 feet.

Open-hole and discrete-zone transmissivity was estimated from flowmeter data acquired under ambient and pumping conditions. The open-hole transmissivity ranges from 2 to 86 feet squared per day (ft ² /d). The estimated transmissivity of individual transmissive zones ranges from 0.5 to 70 ft ² /d. Draw down monitoring in nearby boreholes under pumping conditions identified hydraulic connections along a northeast-southwest trend between boreholes as far as 560 feet apart. The vertical distributions of open fractures can be described by power law functions, which suggest that the fracture network contains transmissive zones consisting of closely spaced fractures surrounded by a less fractured and much less permeable rock mass.

Final copy as submitted to Symposium on the Application of Geophysics to Engineering and Environmental Problems for publication as: Lane, J.W., Jr., Williams, J.H., Johnson, C.D., Savino, Sr. D.-M., and Haeni, F.P., 2001, Application of a geophysical "tool-box" approach to characterization of fractured-rock aquifers -- a case study from Norwalk, Connecticut: in Symposium on the Application of Geophysics to Engineering and Environmental Problems, Denver, Colorado, March 4-7, 2001, Proceedings: Wheat Ridge, Colo., Environmental and Engineering Geophysical Society, CD-ROM.

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