Simulation of Groundwater Composition Change Due to Deposition of Uranium Minerals in Dolomite Gravel Fill

A large plume of uranium contaminated groundwater has been formed as a consequence of historical weapons processing and waste disposal at the former U.S. Department of Energy (DOE) S-3 ponds site located at the Y-12 Nationals Security Center in Oak Ridge Tennessee. Measurements of groundwater samples from the site showed much higher pH values and lower concentrations of uranium and aluminum in carbonate gravel fill than in the underlying lower pH saprolite. Field and laboratory studies demonstrate that precipitates with high U content (> 19%) form when acidic U and Al containing groundwater contacts high pH carbonate gravel. Lab morphology and mineralogy analyses reveal that uranyl carbonates and amorphous basalumnite occur as coatings on dolomite gravel and as detached individual precipitates (Phillips et al. 2008). This study employs a new method that integrates the field measurements, the past laboratory analyses, and numerical modeling to determine physical and geochemical processes that are critical to explain the groundwater composition changes. The resulting numerical simulations indicate that significant reductions in aqueous U concentrations occur due to reactions with high pH carbonate gravel. Insights from the numerical modeling on the formation and stability of U precipitates are important to determine U transport properties and the effectiveness of U remediation strategies.