In a 2011 Oak Ridge case study, ethanol-fed microbes cut uranium in groundwater from 40-60 mg/L to below the US drinking-water limit

The U.S. Department of Energy’s Integrated Field Research Challenge site in Oak Ridge, Tennessee, has long served as a field laboratory for studying the cleanup of groundwater contaminated by decades of nuclear-weapons production and waste disposal. The site has had uranium contamination as high as 60 milligrams per liter in groundwater and up to 800 milligrams per kilogram in sediments, according to Erick Cardenas, Wei-Min Wu, Mary Beth Leigh and others in a 2008 paper, 'Microbial Communities in Contaminated Sediments Associated with Bioremediation of Uranium to Submicromolar Levels.' For comparison, the U.S. Environmental Protection Agency’s maximum contaminant level for uranium in drinking water is 30 micrograms per liter (0.03 milligrams per liter), the benchmark used to judge progress at the Oak Ridge Integrated Field Research Challenge site.

Weekly ethanol injections stimulate denitrifying, iron-reducing and sulfate-reducing bacteria

Instead of removing the contaminated water for treatment, scientists decided to take advantage of the microorganisms that were present in the sediment. As described in the Cardenas et al. study, groundwater was first conditioned on the surface and then treated with weekly ethanol injections into the subsurface environment. Ethanol served as an electron donor that supported the growth of three types of native bacteria: denitrifiers, iron-reducing bacteria and sulfate-reducing bacteria. Denitrifiers are responsible for the removal of nitrate and creating an anaerobic environment for other processes.


All the mentioned types of bacteria reproduced in great numbers in the treatment zone, while the untreated control well showed no activity of this kind. Nearly two years of weekly ethanol injection caused a decrease in aqueous uranium in the treated zone below the maximum contaminant level set by the EPA for drinking water.

Ethanol converts to Acetate within 24 hours to fuel the Uranium-reducing community

The chemical steps behind this shift are described in Oak Ridge work showing that ethanol injected into the sediment is rapidly converted to acetate within 24 hours, accompanied by nitrate and sulfate reduction. The acetate persists in the sediment for at least a week, serving as a secondary electron donor. According to the Oak Ridge National Laboratory, the two-step process allows ethanol to first reduce nitrate, sulfate, and iron (III) and create conditions favorable for the reduction of uranium, after which the acetate takes over the role of the energy and carbon sources.

A 2011 case study documented Uranium falling from 40 to 60 mg/L to below 0.03 mg/L

This is most comprehensively described in a case study by Weimin Wu et al., ‘Bioreduction and immobilization of uranium in situ: a case study at a US Department of Energy radioactive waste site, Oak Ridge, Tennessee,’ published in Huanjing Kexue Xuebao (Acta Scientiae Circumstantiae) in 2011. By virtue of several years of continual ethanol addition to the same field site, this case study revealed that uranium levels in the groundwater decreased from between 40 and 60 mg/liter to levels lower than 0.03 mg/liter, which is below the EPA’s drinking water standard. The stability of this reduced uranium was also evaluated.

Adding Oxygen or Nitrate back into the treated zone remobilized the Uranium

This case study did not stop at documenting the initial cleanup. Once uranium had been reduced and immobilized in the sediment, the same research team deliberately reintroduced dissolved oxygen and nitrate to the bioreduced zone to test how durable the treatment was. The bioreduced uranium remained stable as long as the zone stayed anaerobic or anoxic, but the addition of oxygen and nitrate caused the uranium to remobilize back into solution. The study also tracked how the microbial community itself shifted in response to these later oxidizing conditions, identifying which organisms were tied to uranium reduction and which were tied to its re-oxidation.

What the combined findings show about long-term Uranium bioremediation

When taken together, both studies follow the same remediation project from two different angles. The 2008 study documents the underlying microbial shift, expansion of denitrifiers, iron reducers and sulfate reducers in response to ethanol and reports that, after roughly two years of intermittent ethanol injection, aqueous uranium in the treatment zone fell below the EPA drinking‑water standard for the first time. The 2011 case study builds on this by showing that sustained ethanol delivery could maintain uranium at concentrations below 0.03 mg/L over a longer period and that this immobilization was reversible when oxygen or nitrate re-entered the treated zone.

Meanwhile, the 2011 case study followed the same process on a larger scale, demonstrating how constant injections of ethanol helped to keep the amount of uranium below 0.03 milligrams per liter while at the same time discovering that this reduction reaction can be reversed when oxygen or nitrate enters the treated area. Overall, the project implemented at the Oak Ridge site can be called one of the best-documented attempts of using microbial communities to reduce the level of contaminated groundwater below the drinking-water standard set by federal authorities.