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Improved Approach for Quantifying Subsurface Contamination

View of a well field at the groundwater contamination site
A saltwater tracer was injected into the shallow aquifer at a uranium-contaminated site in Colorado and monitored for 28 days using a combination of geophysical and well-sampling arrays; the sampling array is marked by the white standpipes adjacent to the central injection tank. Photo credit: John B. Ong, USGS.
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U.S. Geological Survey (USGS) scientists developed an approach that significantly improves the way we measure properties that control contaminant transport and persistence in complex geologic systems such as fractured-rock aquifers. The approach involves monitoring a field tracer experiment using geoelectrical measurements and improved methods for measuring and analyzing the field data.

Contaminants in fractured-rock aquifers reside in the fractures and pore spaces that are interconnected and freely transmit water and contaminants (the mobile portion), as well as in dead-end pore spaces and unfractured portions of the rock that can act as a reservoir of contaminants (the immobile portion) and release them slowly over decades, thereby frustrating remediation efforts.  Contamination in the immobile portion has been difficult to assess in the past because it is largely unseen by conventional sampling, which primarily identifies what is in the aquifer's mobile portion. This conceptual model of aquifers that have mobile and immobile portions is being used more widely even in unconsolidated aquifers (mixed sand, gravel, and clay aquifers) that have immobile portions because of very low permeability zones.

A tracer injection test was conducted using a highly ionic solution, which permitted aquifer monitoring by noninvasive geophysical measurements that mapped changes in geoelectrical distributions within the rock, as well as by conventional water sampling. The geophysical measurements were facilitated by the contrast between the ionic strength of the injected fluid and the aquifer water. The field experiment was conducted at a radionuclide-contaminated site near Naturita, Colorado.

Time lapse measurements of subsurface geoelectrical properties taken twice a day during the course of the experiment enabled the scientists to characterize the properties that control how contaminants are exchanged between the mobile and immobile portions of the aquifer. The measurements were taken using an electrical resistivity array wherein electrodes are placed at various intervals within boreholes in the aquifer.

The data were analyzed using a contaminant-transport modeling approach that enabled both local and large-scale transport properties to be tested. The analysis revealed for the first time the variability in properties affecting very local-scale contaminant exchange with immobile portions of the rock and large-scale contaminant transport.

Improved estimation of the volume of an aquifer's immobile portion will enable more accurate estimates of the amount of a contaminant present at a site, as well as the time it may take to remediate. Detailed characterization of contaminant movement between the mobile and immobile portions of the aquifer should provide improved understanding of the long-term fate of uranium at contaminated sites such as Naturita.

This research was supported by the USGS Toxic Substances Hydrology Program and the U.S. Department of Energy Subsurface Biogeochemical Research Program.

Reference

Briggs, M.A., Day-Lewis, F.D., Ong, J.B.T., Curtis, G.P., and Lane, J.W., 2013, Simultaneous estimation of local-scale and flow path-scale dual-domain mass transfer parameters using geoelectrical monitoring: Water Resources Research, v. 49, no. 9, p. 5615-5630, doi:10.1002/wrcr.20397.

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Created on June 25, 2014