Geochemical Characterization of Uranium from Baseline- and Post-Mining Site Conditions at an In-situ Recovery Uranium Mine ABSTRACT: In-situ recovery (ISR) uranium mine restoration is generally based upon a return of the site to baseline conditions. Little or no scientific information is used to justify utilizing baseline conditions for regulatory compliance and the constituents monitored for compliance have not been evaluated to ensure they are proper indicators of restoration. This study examines, for the first time, the pre-existing aquifer parameters, thereby allowing a complete scientific evaluation of the changes that occur during mining, so that specific recommendations can be made on how best to accomplish restoration of those constituents that most impact human and environmental health. The chemical speciation, bioavailability, toxicity, and mobility of U in the subsurface environments are directly affected by reduction and oxidation. U remediation approaches focus on converting and maintaining U in its tetravalent oxidation state because of the formation of sparingly soluble, immobile U(IV) mineral phases, such as uraninite. Therefore it is critical to determine the speciation of U in order to understand the chemistry governing their retention and transport in these environmental systems before effective remediation strategies can be developed to prevent groundwater contamination. We focus on geochemical characterization of U from baseline conditions and compare it to a mined site to help evaluate the impact of mining and the potential for natural attenuation of contaminants. This study attempts to identify the different U species formed in ore bodies under different redox and hydrological gradients, thereby optimizing water management procedures to reduce aquifer impacts and wastewater volumes using traditional wet chemical techniques as well as synchrotron extended x-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopy. This work will advance understanding of the redox processes associated with the reductive sequestration of U from groundwater and will provide regulators with sound scientific evidence for optimizing remediation goals.