USGS Washington Water Science Center
|U.S. Department of the Interior
U.S. Geological Survey
Release: January 6, 2000
Sue Kahle, Hydrologist
(253) 428-3600, ext. 2616
As an experiment, a U.S. Geological Survey (USGS) scientist at Fort Lewis, Washington, added potassium bromide to ground water that had been contaminated by volatile organic compounds, or VOCs.
The contaminated ground water had been pumped out of the ground and treated; and then after it was dosed with potassium bromide, a chemical tracer, it was returned to the ground-water system that runs under the fort's Logistics Center. The ground-water system has been designated a Superfund cleanup site.
In a small area near the fort's East Gate Disposal Yard, the potassium bromide tracer was monitored for about six weeks to help determine how easily the geological material beneath the water table can transmit water and dissolved contaminants and how rapidly dissolved substances disperse along a flow path. The results of the experiment, which are expected to aid current efforts to clean up the VOC-tainted ground water at Fort Lewis, are presented in a new report from the U.S. Geological Survey.
The experiment's results revealed that the relatively thin layers of coarse permeable material at the experiment site can transmit water and dissolved materials considerably more easily than more traditional tests indicate. The more traditional tests tend to average the transmittal rates for groups of alternating layers of high and low permeability.
The test also revealed a difference between the ground-water flow patterns at the smaller scale--a hundred feet or less--of the experiment's area and the better-known flow direction at the larger scale of thousands of feet under the Logistics Center. "At the smaller scale, the flow patterns turn out to be quite complex, probably in part because of the heterogeneous sediments," said Edmund Prych, the USGS hydrologist who conducted the experiment and wrote the report.
Because of the complexity, Prych said, "interpreting the observed distributions of contaminants and planning remediation schemes at these small scales need to be done with caution."
The USGS conducted the experiment as part of a larger cooperative effort with the U.S. Army and others to develop and apply new technologies to speed up the remediation of the contaminated ground water at Fort Lewis.
The report, A Tracer Test to Estimate Hydraulic Conductivities and Dispersivities of Sediments in the Shallow Aquifer at the East Gate Disposal Yard, Fort Lewis, Washington, by E.A. Prych, is published as U.S. Geological Survey Water-Resources Investigations Report 99-4244. It is available for reading at the U.S. Geological Survey office at 1201 Pacific Avenue, Suite 600, Tacoma, Washington 98402. It can be purchased from the U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225-0286, telephone (303) 202-4610.
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