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Wednesday, 31 March 2004

Chromate Contamination at DOE Hanford Site Partly Contained by Natural Reactions with Sediments

John M. Zachara, Calvin C. Ainsworth, Gordon E. Brown, Jr., and Jeffrey G. Catalano

Toxic and carcinogenic chromate (hexavalent chromium as CrO₄^2-) has contaminated the groundwater in Hanford, Washington. At Hanford, hexavalent chromium was used in the industrial process to recover plutonium from irradiated nuclear fuels. The resulting high-level waste corroded its storage tanks and leaked into the desert subsoils. Plumes of contaminated groundwater reaching the Columbia River pose a risk to spawning salmon. Because of the depth and large volume (millions of gallons) of the contaminant plumes, existing abatement technologies are largely ineffective for mitigating the contaminated groundwater at this location. However, recent research indicates that the naturally occurring sediments at the Hanford site can immobilize chromate.

John Zachara (PNNL), Gordon Brown, Jeffrey Catalano (Stanford University) and their colleagues used x-ray absorption spectroscopy at SSRL to safely test contaminated (and highly radioactive) soil samples to determine the relative amounts and chemical forms of the chromate (carcinogenic hexavalent chromium or less toxic trivalent chromium). They found that about 42% of the chromate in the contaminant plumes had become an immobile solid of trivalent chromium due to reacting with ferrous iron-bearing sediments in the aquifer. Unfortunately, more than half of the chromium in the plumes remains as the dangerous hexavalent form (chromate), which moves readily through the subsurface sediments. However, the discovery by Zachara's research group (which is one of many ongoing studies at SSRL looking at contaminated sediments from Hanford) is an important step toward mitigating the problem. Developing technical solutions to such large-scale contaminant problems requires key insights into the form of the contaminant, which is intimately linked to its reactivity. This information allows engineers to assess the hazards posed by such extreme chemical and radioactive materials, as well as to design the most effective, long-term strategies to deal with them.