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Improved Simulation of Contamination in Fractured Rock

USGS scientist preparing equipment for a tracer test monitored with geophysical equipment.
USGS scientist preparing equipment for a tracer test monitored with geophysical equipment, Charleston, S.C.
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U.S. Geological Survey (USGS) scientists and their university colleagues have developed a new study approach, which incorporates data from electrical geophysical tests and tracer experiments, to more accurately simulate the transport of contaminants in fractured-rock aquifers. The approach is designed to identify how dissolved contaminants slowly move out of immobile parts of a highly heterogeneous aquifer (the rock matrix) and into the aquifer's more mobile pathways (fractures and other connected openings). This slow movement results in slow contaminant removal by treatment systems such as pump-and-treat remediation. Some treatment systems have to run for many years, even decades, before all of the contaminants are removed from the subsurface. These conditions typically occur at contamination sites located in fractured-rock aquifers.

In fractured rock, contaminant levels and persistence is affected by the slow movement of contaminants out of the less permeable matrix into more permeable fractures (see diagram). The rock matrix can hold a substantial amount of contaminants, which can act as a long-term source of contamination. Conventional site characterization methods, such as collecting water samples and rock cores for chemical analysis, provide limited knowledge of this contaminant source because pumped samples draw water preferentially from fractures, and expensive rock cores only sample a small portion of the rock. In the new study approach, electrical geophysical methods were used to measure conductive tracers in both fractures and the rock matrix and thus provide information about total concentration and movement of contaminants that conventional methods can't.

Diagram of a mobile and immobile domains in a rock matrix.
Fractured-rock aquifers consist of (1) a mobile domain of connected, permeable pathways, and (2) an immobile domain of low-permeability rock matrix and unconnected or impermeable fractures. Dissolved contaminants are easily transported through the mobile domain; however, negligible transport occurs through the immobile domain. Solutes very slowly move back and forth between the mobile and immobile domains via rate-limited mass transfer.
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This improved data-collection and modeling approach will allow for better simulation of the slow movement of contaminants in the rock matrix. Environmental professionals can use these improved models to design more cost-effective and efficient solutions to problems ranging from pump-and-treat groundwater remediation systems to the implementation of aquifer-storage recovery systems.

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