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Improved Simulation of Contamination in Fractured Rock

Diagram of borehole geophysical setup for tracer-test monitoring. Three boreholes were instrumented with electrodes used to take electrical resistance measurements during the push-pull test, which was performed from a central injection/extraction borehole.
Diagram of borehole geophysical setup for tracer-test monitoring. Three boreholes were instrumented with electrodes used to take electrical resistance measurements during the push-pull test, which was performed from a central injection/extraction borehole.
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Rate-Limited Mass Transfer
In fractured rock the slow contaminant movement is caused by limitations in the rate that contaminants can diffuse out of the less permeable matrix (the host rock) and into the more permeable fractures. The technical term for the movement of contaminants between the two zones of permeability is rate-limited mass transfer. Scientists have observed this phenomenon for many years, and have developed simulation approaches known as bicontinuum or dual-porosity models to simulate how rate-limited mass transfer controls the movement of contaminants.

Electrical geophysical tests measure conductive tracers in both fractures and the rock matrix and thus provide information about bulk concentration and movement of contaminants that conventional methods can't.

U.S. Geological Survey (USGS) scientists use electrical geophysical tests to monitor how rate-limited mass transfer (see text box) controls the movement of contaminants between the host rock (rock matrix) and the more permeable fractures in fractured rock aquifers. The method involves injecting water with an electrical conductivity different from the groundwater (a different salinity, for example) into a well and then pumping it back out of the ground from the same well. The scientists monitor this “push-pull” tracer test by sending electrical current into the ground with two electrodes in a nearby well; the resulting difference in voltage is measured by two neighboring receiving electrodes in the same well. This procedure was repeated for many different combinations of electrodes. The information from the combined geophysics/hydraulic tracer test is used to calculate rate-limited mass transfer coefficients that are based on the realistic field-scale data provided by the push-pull tracer test. The scientists use the coefficients to more accurately simulate the transport of contaminants in fractured rock.

References

Culkin, S.L., Singha, K., and Day-Lewis, F.D., 2008, Implications of rate-limited mass transfer for aquifer storage recovery efficiency: Groundwater, v. 46, no. 4, pp. 591-605, doi:10.1111/j.1745-6584.2008.00435.x.

Day-Lewis, F.D., and Singha, K., 2008, Geoelectrical inference of mass transfer parameters using temporal moments: Water Resources Research, v. 44, W05201, doi:10.1029/2007WR006750.

Singha, K., F. D. Day-Lewis, and Lane, J.W., 2007, Geoelectrical evidence of bicontinuum transport in groundwater: Geophysical Research Letters, v. 34, no. 12, L12401, doi:10.1029/2007GL030019.

Singha, K., Pidlisecky, A., Day-Lewis, F.D., and Gooseff, M.N., 2008, Electrical characterization of non-Fickian transport in groundwater and hyporheic systems: Water Resources Research, v. 44, W00D07, doi:10.1029/2008WR007048.

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Created on Wednesday, June 9, 2010