Engineering technique could help clean up lingering radioactive waste

In the western United States, a toxic relic of World War II and the Cold War remains: radioactive groundwater caused by former uranium ore mining and processing sites. Seven of these sites are found along Colorado’s western slope and engineers at the University of Colorado Boulder are testing out a new technique to clean them up.

Professor Roseanna Neupauer of CU Boulder and Professor David Mays of CU Denver have created a process called Engineered Injection and Extraction, which has shown promising results in removing other contaminants. But uranium, which readily shifts between different oxidation states, has historically proven trickier.

Starting in the 1940s, nuclear weapons development drove uranium mining in the U.S. The process stripped uranium from rock, processed it into a powder and left behind vast piles of radioactive rock, or tailings.  Many mines ceased operations by 1978, but officials didn’t remove the tailings until much later.

“This isn’t necessarily as local as, say, a gas station tank that ruptures in the middle of the city, which has the possibility of endangering water supplies for large amounts of people – these places can be remote,” said Jack Greene, a graduate student in the Department of Civil, Environmental and Architectural Engineering. “But that that’s not to say that it’s not relevant to the people that live near it – uranium can linger for a long time, be transported long distances and be extremely difficult to effectively remediate in the long-term.”

Of the 22 inactive sites from this time regulated under the Uranium Mill Tailings Radiation Control Act of 1978, all but two are in the West. The seven in Colorado are located near the cities and communities of Durango, Grand Junction, Gunnison, Naturita, Rifle, Maybell and Slick Rock.

From 1993 to 1997, the Department of Energy removed 800 thousand cubic yards — equal to a football field-sized 13-story building — of contaminated soil and debris from the Naturita site and disposed of it in a nearby constructed containment cell. By that time, however, large amounts of uranium from the waste piles had leached into the groundwater below and dispersed into a contaminated “plume.”

Removing uranium from groundwater is difficult, costly, and dangerous if pumped to the surface for treatment, so Greene is testing another option: fixing it into a stable, immobile form in the ground.

Uranium in nature occurs in two oxidation states: U(IV) and U(VI). The former, which more common in contaminated plumes, flows with groundwater currents. The latter, on the other hand, is less mobile and sticks to soil or precipitates into solid minerals. When immobile, uranium poses little threat to groundwater quality, since it is out of solution and stays in one place.

“Uranium is interesting — its ‘remediated’ phase isn’t as stable as its mobile forms, which creates a big challenge to develop remediation systems to keep uranium out of groundwater long-term,” said Greene.

The goal is to get uranium to stay in this immobile state. But, studies have shown that the element can quickly remobilize into its more dangerous form, sometimes just months after cleanup. Greene is testing whether a novel engineering technique can provide a solution to this problem.

“We’re checking to see if this process can help decrease the amount of rebound that happens following remediation,” said Greene.

The team of scientists, which also includes Joe Kasprzyk of CU Boulder, Gary Curtis of the U.S. Geological Survey and Ming Ye and Honzhuan Lei of Florida State University, hope their method can be used in future for cleanup of nuclear power and mining sites, and expanded to other heavy metals.

Greene is using numerical models based on the specifics of the uranium-contaminated sites outside of Naturita and Rifle, including soil types, groundwater flow and biogeochemistry. The process injects acetate, a food for native microbes, into the underground contaminant plume. In the same way other organisms use oxygen to “breathe,” the microbes use U(VI) as a final electron acceptor.

By adding electrons to U(VI), the microbes convert U(VI) to U(IV) — the desired immobile uranium phase — effectively removing the contaminant from groundwater. Wells surrounding the plume add and remove water to redirect groundwater flow and spread acetate into the aquifer, which promotes microbial activity throughout the plume. Ideally, under EIE, the element will remain immobile long after remediation.

Greene plans to run tens of thousands of remediation simulations to test how different variables, like water injection and extraction rates and well locations, affect the process. The simulations can help optimize EIE for certain competing objectives, such as cost and remediation efficiency.

Greene will present his findings on EIE’s efficiency for remediation of uranium-contaminated groundwater at the American Geophysical Union’s annual conference in December.

If EIE proves to be efficient in field-based models, the next step would be to test it outside. It might become possible to use the technique to clean up contaminated sites in the next decade.

“This process could be extremely useful to enhance efficiency of uranium-contaminated groundwater remediation for legacy uranium contaminated sites and for private clients currently operating mining and energy operations,” said Greene.

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