USGS Groundwater Information: Hydrogeophysics Branch
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by Carole D. Johnson and John H. Williams
U.S. Geological Survey
Monitoring open-hole water levels during pumping tests remains a common approach to identifying hydraulic connections between boreholes in fractured rock; however, without an understanding of vertical flow in boreholes, this approach can produce unreliable interpretations of fracture connectivity. A combined method of cross-hole flowmeter logging and water-level monitoring proved to be critically important for characterizing hydraulics and fracture network connectivity in a DNAPL-contaminated fractured bedrock aquifer at the former Naval Air Warfare Center in West Trenton, NJ. These results illustrate the importance of borehole flowmeter data for producing more robust interpretations of open-hole pumping test data and more reliable characterization of interconnections between boreholes.
Transient flowmeter logs collected in 68-BR, a 170-ft deep open borehole, while cycling pumping and recovery in a nearby extraction well, were used to characterize hydraulic connections with the extraction well. Open-hole water levels in one borehole responded to pumping, while water levels from 68-BR and six other boreholes showed no measurable response to pumping. Although the open-hole water level in 68-BR remained virtually constant over the duration of the pumping test, a heat-pulse flowmeter log indicated a change in the rate of downward flow from the fracture zone at about 38 ft to the fracture at 94 ft. These results indicate that in 68-BR the less transmissive fracture (at 94 ft) is connected to the pumping well, and the most transmissive fracture zone, which is not directly connected to the pumping well, maintained the ambient water level.
This example demonstrates the inadequacy of relying on open-hole water levels to determine interconnections in fractured-rock aquifers and illustrates the value of flowmeter logging. Cross-hole flowmeter data also were used estimate the transmissivity and storage coefficient of fractures between the observation and pumping boreholes. Use of these analyses in conjunction with single-hole flowmeter testing can help characterize the fracture network in a well field, which is important for detailed characterization and remediation design.
Final copy as submitted to the Geological Society of America for publication as: Johnson, C.D., and Williams, J.H., 2006, Use of cross-hole flowmeter logging to characterize fracture connectivity and fracture flow during a pumping test at the NAWC site, West Trenton, New Jersey [abs.]: Geological Society of America Abstracts with Programs, v. 38, no. 7, p. 528, Paper no. 219-10.
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