Ewing's Russian colleagues, led by Alexander Novikov of the Russian Academy of Sciences, sampled the groundwater taken from wells up to four kilometers from the scene of original contamination, where radioactivity levels reach roughly 1,000 becquerels (nuclei decaying per second) per liter. Even at that distance, the researchers still measured 0.16 becquerel per liter. Because uranium and plutonium are heavy elements and have low solubility in water, some scientists had expected such contamination to be relatively immobile. Yet, at Mayak, the contamination had spread at least three kilometers in just 55 years. How?
Ewing and his American colleagues used imaging to confirm that the radioactive materials were hitching a ride on colloids--nanoscale particles smaller than one micrometer--specifically, iron oxides present in the groundwater. "These are actual mineral fragments carried in the water," Ewing explains. They are grabbing onto "the uranium and plutonium and carrying it some kilometers away." These iron oxide particles--and other colloids--typically have a negative charge, and the positively charged actinides simply attach to their surfaces electrostatically. And the actinides don't dissolve off the particles, either: "It stays with the solids and travels with them even though the concentrations in solution are low enough that if it was a plutonium solid you would have expected it to dissolve," Ewing notes.
Confirming that actinides can travel on colloids is but a first step. "It further corroborates our understanding of plutonium in the subsurface: it is colloidal, it does move, it's not immobile," says Annie Kersting, a geochemist at Lawrence Livermore National Laboratory. "If you found it there, that's not the endpoint that's just where they put their well. It would be nice to come up with some boundaries on transport concentrations." That work remains critically important for determining how any kind of nuclear repository, such as Yucca Mountain in Nevada, might behave over time as well as for assessing contamination at sites in the U.S. and worldwide.
And iron oxide particles may just be the first transporting colloid that has been clearly identified. Humic acid--an organic complex--and a host of other colloids might serve a similar purpose, as recent research at Rocky Flats in Colorado has shown. "This is a dilemma because it's really difficult; in groundwaters, colloids are ubiquitous," Ewing notes. "It's bad luck that they can be transportation vectors for some of these actinides." In that case, Mayak may serve as a beacon--albeit one of warning--after all. "It's important to look at the geochemistry of the environment to help us in our understanding of what exactly is going to move that plutonium," Kersting adds. "But we need to get away from this idea that plutonium doesn't move, because it does."