Abstract <p>The solubility of uranium dioxide in simulated groundwater from a radioactive waste disposal site, in microbiota solutions, and in solutions of organic substrates for its stimulation was studied. In Russia, surface storage facilities for radioactive waste are subject to conservation. One of its main stages is the remediation and decontamination of adjacent territories and groundwater, including bioremediation technologies. Uranium is the most dangerous radionuclide for all disposal sites, and the stability of its reduced, low-solubility forms plays a crucial role in remediation. This study was aimed at assessing the potential biogeochemical mobility of UO₂ in the upper horizons of groundwater near the Siberian Chemical Combine (SCC) reservoir that underwent bioremediation in 2015. It was found that uranium dioxide is generally quite stable in microaerobic aquatic environments. Its solubility slightly increased in experiments with the stimulation of microbiota development, probably owing to nitrate-dependent oxidation processes and the release of metabolites that can form soluble complexes with uranium. However, in general, the groundwater bioremediation does not pose significant risks of remobilization of reduced uranium species.</p>