"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 today's 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 using our technique, and then determine rather directly how much hexavalent chromium reduction has occurred," Johnson said. "That information can then be used to decide the best course of action -- whether aggressively cleaning up the site or leaving it for nature to run its course."

At some sites, there are naturally occurring reducing agents in the subsurface - such as iron-bearing minerals like magnetite -- that will convert hexavalent chromium to trivalent chromium. If the reaction rate is fast enough, the contaminant can be naturally attenuated. Such an approach is much less expensive and disruptive than active remediation.

At other sites, chemical reducing agents must be injected in the ground to mitigate the pollution, Johnson said. "In either case, we want to provide a technology to monitor whether the reduction is occurring, and if so, to what extent."

Based on their measurements, the researchers suggest that all chromium plating waste, regardless of source or different industrial procedures, has nearly the same initial ratio of heavy to light isotopes as that found in naturally occurring igneous rocks and chromium ores. These findings could simplify the monitoring procedure.

"If all contaminant sources are nearly the same, then detection of hexavalent chromium reduction in groundwater systems would be relatively simple, as the initial ratio would be known and any shift would directly indicate the extent of reduction," Johnson said. "But, if we find that various sources are indeed different, then we can use the unique isotopic ratios as chemical fingerprints to identify specific contaminant plumes."

The National Science Foundation funded the research. - By James E. Kloeppel


[Contact: James E. Kloeppel]

15-Mar-2002