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鈥檚 western slope, and engineers at 抖阴旅行射 Boulder are testing out a new technique to clean them up.
Professor Roseanna Neupauer of 抖阴旅行射 Boulder and Professor David Mays of 抖阴旅行射 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鈥檛 remove the tailings until much later.
鈥淭his 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鈥攖hese places can be remote,鈥� said Jack Greene, a graduate student in the .
鈥淏ut that that's not to say that it's not relevant to the people that live near it鈥攗ranium can linger for a long time, be transported long distances and be extremely difficult to effectively remediate in the long term.鈥�
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鈥攅qual to a football field-sized 13-story building鈥攐f 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 鈥減lume.鈥�
Removing uranium from groundwater is difficult, costly听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.
鈥淯ranium is interesting鈥攊ts 鈥榬emediated鈥� phase isn鈥檛 as stable as its mobile forms, which creates a big challenge to develop remediation systems to keep uranium out of groundwater long-term,鈥� 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听solution to this problem.
鈥淲e鈥檙e checking to see if this process can help decrease the amount of rebound that happens following remediation,鈥� said Greene.
The team of scientists, which also includes Joe Kasprzyk of 抖阴旅行射 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 the 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 鈥渂reathe,鈥� the microbes use U(VI) as a final electron acceptor.
By adding electrons to U(VI), the microbes convert U(VI) to U(IV)鈥攖he desired immobile uranium phase鈥攅ffectively 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鈥檚 efficiency for remediation of uranium-contaminated groundwater at the American Geophysical Union鈥檚 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.
鈥淭his 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,鈥� said Greene.听
