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Another potential source of uranium at the Site may be dissolution of the common uranium <br /> mineral, uraninite, which forms under reduced conditions in circum neutral pH. To test <br /> uraninite solubility controls on uranium concentrations based on oxidation/reduction <br /> dynamics in groundwater at the Site, thermodynamic equilibrium geochemical modeling <br /> was performed using PHREEQC (Parkhurst, 1999) and Geochemist's Workbench®. The <br /> average concentrations for major ions, pH, alkalinity, and uranium was modeled for <br /> monitoring wells at the site to quantify mineral saturation indices of major minerals and <br /> uraninite; redox conditions were quantified by ferrous/ferric iron ratios of groundwater <br /> collected in the field to determine uranium oxidation state. All modeling output files for <br /> monitoring wells are included in Appendix F. A uranium mineral activity diagram <br /> (Pourbaix diagram) was constructed based on thermodynamic constants of uranium <br /> speciation as a function of pe(redox)and pH (Figure 14). Monitoring well data show that <br /> all samples sit outside of the uraninite mineral stability field, indicating that if uraninite <br /> was present, it would dissolve. Uraninite will only form under reducing conditions,and it <br /> has a relatively quick kinetic precipitation/dissolution reaction rate. Uranium species were <br /> nearly all oxidized uranium (+6) in groundwater and would require significant reducing <br /> conditions to reduce uranium to uranium (+4), indicating that uraninite precipitation is not <br /> likely to occur (Figure 14 and Appendix E). Conversely, this also infers that uraninite is <br /> not present and contributing to uranium concentrations in groundwater. As a result, <br /> uranium is likely in the oxidized form [U(+6)], and aqueous concentrations are likely <br /> controlled by sorption/desorption to iron oxyhydroxides and organic carbon compounds in <br /> MW-06 and MW-10. Additionally, based on modeling results, aqueous uranium species <br /> for MW-06 and MW-10 were predominantly UO2(CO3)34- and UO2(CO3)22" (i.e. uranyl— <br /> carbonato complexes with U oxidation state of+6). Coupled with elevated dissolved <br /> calcium concentrations in these monitoring wells, it is likely that these complexes are <br /> actually composed of uranyl--calcium—carbonato complexes, which are electrostatically <br /> neutral and do not adsorb to charged surfaces. This suggests that uranium concentrations <br /> are stable and unlikely to increase or decrease due to their fully oxidized state and <br /> complexation with both carbonate and calcium. Nitrate concentrations are also elevated in <br /> MW-06 and MW-10 causing uranium to be oxidized even under reducing conditions. <br /> Loveland Ready-Mix Concrete 14 December 2019 <br /> 20207_kno.�„_basel „aterqualm= n�. TELESTO <br />