Nuclear clean-upScientists study how nature cleans uranium from aquifer
A small town in Colorado was the site of uranium ore processing in the 1940s and 1950s, producing yellowcake; when the mills shut down, the mill tailings — a crushed rock byproduct of ore processing — were left behind on the north bank of the river; the tailings were hauled away in the 1990s, but a large amount of uranium that seeped out of the tailings remains as a contaminant in the aquifer and is slowly being released into the Colorado River
Rifle, Colorado, is a small town on the Colorado River, in the heart of the Rocky Mountains. The town is big on natural resources. It attracts hunters, fishermen, hikers, rock climbers. The town’s scenery attracted husband-and-wife environmental artists Christo and Jeanne-Claude as well; they stretched the Valley Curtain, a 200,200-square-foot orange sheet of nylon, across 1,250-foot-wide Rifle Gap. The curtain lasted only 28 hours before another of the area’s natural resources, wind, began to tear it down.
It is the below-ground natural resource — uranium — that draws John Bargar, head of the Stanford Synchrotron Radiation Lightsource’s Molecular Biogeochemical and Interface Sciences group, to the area. “The Old Rifle site” — the focus of Bargar’s research — “was a uranium ore processing site in the ‘40s and ‘50s, producing yellowcake,” for nuclear fuel, Bargar said.
A Stanford Linear Accelerator Center (SLAC) release reports that when the Old Rifle site shut down in 1958, the New Rifle site took its place two miles downstream, with the Colorado River running past both locations. When the mills shut down, the mill tailings — a crushed rock byproduct of ore processing — were left behind on the north bank of the river. The tailings still contained “hexavalent” uranium, whose individual atoms had all given up six of their outermost, or valence, electrons. Valence electrons are extremely important in determining how an atom behaves chemically. One behavior of hexavalent uranium is that it dissolves in water.
While the orange curtain may be long gone and even the tailings were hauled away in the 1990s, a large amount of uranium that seeped out of the tailings remains as a contaminant in the aquifer and is slowly being released into the Colorado River.
How to reduce the amount of hexavalent uranium in the aquifer or lock it in the sediment in an insoluble, inert form before it even reaches groundwater are big questions for the Department of Energy, which is charged with cleanup of both Rifle sites as well as several others under the Uranium Mill Tailings Remedial Action (UMTRA) Project.
That’s where the SSRL Environmental Remediation Program, headed by Bargar, comes in. Since 2009 the SLAC-based collaboration has been a part of the DOE effort to find an answer to uranium contamination in the environment.
The release notes that SLAC researchers study how nature itself handles the contamination in the hopes that nature’s own tools —
