USGS Groundwater Information: Hydrogeophysics Branch
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Carole D. Johnson, John H. Williams, and Frederick L. Paillet
Without understanding the flow regime within open bedrock boreholes, sampling programs may not adequately characterize the distribution of contamination in an aquifer. Vertical flow within open boreholes in fractured-bedrock aquifers can cause cross-contamination, mixing, or dilution of water samples. By understanding the flow patterns within the borehole, a sampling strategy can be designed to obtain more meaningful data.
Flowmeter logs collected under ambient and stressed conditions along with fluid and televiewer logs are valuable for identifying ambient vertical flow and the most transmissive fractures that intersect the borehole. Under ambient conditions, flowmeter logs are used to identify the direction and magnitude of vertical flow within boreholes. Under stressed conditions, flowmeter logs are used to identify the transmissive fractures within the borehole by measuring the rate of water entering the borehole at each fracture while simultaneously pumping from or injecting into the top of the borehole at a low rate. Collectively, these data are critically important for assessing the flow regime; determining the location, transmissivity, and hydraulic head of fractures in the borehole; designing well completions to minimize cross contamination; and planning water-quality sampling strategies.
Three case studies demonstrate the importance of determining the flow regime within boreholes. In the first case, flowmeter logging demonstrated that low-flow water sampling would not collect water from the fractures adjacent to the pump in the presence of vertical flow. In the second case, flowmeter logging showed that even under a sample pumping rate of 2 gallons per minute, outflow from the borehole to a fracture of interest was not reversed. In the third case, cross contamination was documented. While the borehole remained open for a few days after drilling, contamination spread from the upper part of the borehole to a previously uncontaminated fracture near the bottom of the borehole. Discrete-interval water-quality sampling indicated that after a year the lower fracture had not returned to background concentrations. These results show that water samples collected from boreholes affected by vertical ambient flow may provide unreliable and potentially misleading measurements of aquifer contamination.
Final copy as submitted to 2004 U.S. EPA/NGWA Fractured Rock Conference for publication as: Johnson, C.D., Williams, J.H., and Paillet, F.L., 2004, Importance of flowmeter logging for aquifer characterization at contaminated bedrock sites, in 2004 U.S. EPA/NGWA Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 13-15, 2004, Portland, Maine. Proceedings: National Ground Water Association, CD-ROM, p. 331.
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