Cleaning up water in aquifers contaminated with uranium from processing ore for nuclear fuels requires understanding the underground chemistry of how uranium is interconverted between water-soluble U6+ and insoluble U4+. Although scientists had believed that all U4+ in aquifer sediments occurred as UO2, new research shows that a combination of microbial and abiotic reactions also produces other U4+ complexes (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1219198110). One approach to treating uranium-contaminated aquifers is to add a microbial nutrient to create a bacterial bloom that consumes all of the oxygen and creates reducing conditions, said research team leaders John R. Bargar and Noémie Janot of SLAC National Accelerator Laboratory. After using this method on an aquifer in Rifle, Colo., the researchers found that iron sulfide minerals appear to seed on the outside of biomass and reduce U6+ to U4+. The U4+ then may complex with phosphate or other ligands available from nearby biological material. If such ligands are absent, then UO2 may form. The microbes themselves may also reduce U6+ to U4+. The results will change how aquifer behavior is modeled, Bargar said.