Radioactive Waste 214 - Stanford Scientists Discover New Information About The Behavior of Uranium In Soil Around Old Mines
I have often blogged about the problems with disposing of nuclear waste such as spent nuclear fuel from nuclear power plants and toxic mixes of chemicals left over from the manufacture of nuclear weapons such as the contents of the leaking barrels at the U.S. Hanford Nuclear Reservation. There are also many environmental problems at the other end of the nuclear fuel chain related to the activity of mining uranium. Recently, new research has revealed that some assumptions with respect to the chemical behavior of uranium in the soil around old uranium mines do not match current theoretical models.
The Department of Energy's Office of Legacy Management, under the Uranium Mill Tailings Radiation Control Act of 1978, has been remediating 22 sites in Colorado, Wyoming and New Mexico where uranium was mined from the 1940s to the 1970s. Uranium was removed from the sites and the former mines and waste piles were capped more than twenty years ago. Unfortunately, the uranium contamination being detected in groundwater around the old mines is much higher that was projected on the basis of existing scientific models of the chemistry of uranium in the soil.
Since 2014, researchers at the Department of Energy's Stanford Linear AcceleratorCenter at the National Accelerator Laboratory at Stanford University have been working with the DoE Office of Legacy Management on understanding how uranium contamination cycles through the environment at old uranium mines and what makes it so difficult to remove. One of their earlier studies found that uranium was accumulating in low-oxygen sediments near a waste site in Colorado. These sediment deposits contain plant debris and bacterial communities.
The latest findings from the SLAC series of studies indicate that the dominant form of uranium in the sediments is tetravalent uranium which binds to organic matter and clay. This contradicts the earlier understanding that the primary form of uranium in the sediments would be in the form of an insoluble mineral called uraninite. This explains why the uranium kept reappearing in the groundwater around the closed mines.
Different chemical compounds containing uranium vary greatly in how mobile they are in water. While tetravalent uranium is immobile in water, when the water table falls and oxygen penetrates into cavities in the soil that were previously filled with water, tetravalent uranium becomes mobile and can be washed out into the groundwater. If the uranium stayed immobile or was always mobile and flushed out rapidly by ground water, that would remove it as a problem. But with the changing mobility of tetravalent uranium, new surges of uranium contamination keep being generated with the seasonal changes in groundwater levels.
With this new understanding of the behavior of uranium in the soil around old mines, new strategies can be developed to deal with recurrent surges in uranium contamination. Considering the many sites that are being remediated now and that will have to be remediated in the future, this new information will be very useful.
