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United States Department of Agriculture

Agricultural Research Service

Title: CHARACTERIZATION OF FRACTURED ROCK AS A BASIS FOR GOUND WATER MODELING

Authors
item Urban James B,
item Gburek, William

Submitted to: American Society of Civil Engineers
Publication Type: Proceedings
Publication Acceptance Date: May 21, 1993
Publication Date: N/A

Interpretive Summary: Not applicable

Technical Abstract: Core drillings at 27 sites within a 7.2-km2 east-central Pennsylvania watershed reveal extensive fracturing to depths of 24 to 30 m. The rock strata are predominately dense shale with minor rock units of sandstone. The areal distribution of the rock fracture zone was defined by surface seismic refraction transects. Surface and borehole seismic data were used to delineate depths of fracture zones which were subsequently pressure injected using a borehole packer to determine hydraulic conductivities. The combined data show the fracture zone to be an interconnected mass of fractures existing as a mantle over the watershed. Intensity of fracturing was greatest in the upper 9 m, intermediate from 9 to 24 m, and decreased to infrequent fracturing at depths of 24 to 30 m. Maximum hydraulic conductivities as determined from packer tests were in the upper 5 to 9 m depth in both shale and sandstone and often exceeded 3 m/day. Hydraulic conductivity in the poorly fractured zone was generally at or near zero; the intermediate zone of fracturing exhibited intermediate hydraulic conductivity values. The MODFLOW ground water model was applied to the watershed using the characterizations of layer depths as basic inputs and calibrated for hydraulic conductivities by comparing water table elevations simulated under steady-state conditions to those observed over the watershed. The seismic and packer test characterizations substantiated by simulation results indicate that the areal distribution of the rock fracture zone at shallow depths may have a substantial impact upon total ground water movement and potential for contaminant delivery to streams.

Last Modified: 10/31/2014
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