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New technique monitors chromium contamination in groundwater

CHAMPAIGN, Ill. Widely used in electroplating, hexavalent chromium is a suspected carcinogen and a common contaminant in groundwater. Now, scientists have discovered a simple, but effective, method for monitoring this pollutant.

Under certain chemical conditions, hexavalent chromium will convert to trivalent chromium a less toxic form which tends to precipitate out of the groundwater, said Thomas Johnson, a geologist at the University of Illinois. Knowing how fast the reaction is occurring within a contaminant plume would help investigators decide whether natural attenuation is a viable approach at a site, or if active remediation is required.

As reported in the March 15 issue of the journal Science, Johnson and his colleagues graduate student Andre Ellis at the UI and hydrologist Thomas Bullen at the U.S. Geological Survey in Menlo Park, Calif. have developed a means for measuring how fast, and to what extent, hexavalent chromium is changing to trivalent chromium at a given site. Chromium has four stable (non-radioactive) isotopes. By measuring the isotope fractionation in laboratory experiments and in natural waters, the researchers found that lighter isotopes reacted preferentially during the reduction reaction.

This means that the trivalent chromium becomes enriched in lighter isotopes as the reduction proceeds, while the remaining hexavalent chromium becomes enriched in heavier isotopes, Johnson said. By measuring the relative abundances with an isotope-ratio mass spectrometer, we can determine how much reduction has taken place, and then estimate the long-term reduction rate.

The partitioning of the lighter isotopes into the reduction product, trivalent chromium, provides a convenient and effective monitoring technique. As the reduction reaction proceeds, the ratio of heavier to lighter isotopes will change.

A scientist or consultant working on a chromium contamination site can collect a few water samples, analyze them
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Contact: James E. Kloeppel
kloeppel@uiuc.edu
217-244-1073
University of Illinois at Urbana-Champaign
14-Mar-2002


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