INVESTIGATION OF URANIUM SPECIATION AND MINERALOGY ASSOCIATED WITH URANIUM IN-SITU RECOVERY OPERATIONS

During uranium in-situ recovery (ISR) a lixiviant (i.e., leaching solution) is injected into a subsurface, uranium-bearing sandstone. The composition of the lixiviant is such that uranium is oxidized and mobilized within the confined aquifer. Dissolved uranium is then pumped to the surface for further processing. U.S. NRC regulations require ISR uranium mining facilities to restore groundwaters affected by mining operations. Restoration methods, such as groundwater flush or hydrogen sulfide (H₂S) injection, have the goal of returning the subsurface environment to its original baseline condition to ensure no significant long-term impact on water quality.

Very few examples of geochemical modeling of the groundwater restoration process exist in the open literature. However, the existing models focus on the water quality of the confined aquifer and neglect mineralogy. In this study, the one-dimensional transport functionality of PHREEQC was used to monitor uranium, selenium, and radium speciation and mineralogy across all phases (i.e., baseline, mining, and restoration) of an ISR operation. The model takes into consideration redox, dissolution, precipitation, and sorption reactions; results are compared to published water quality data from the Highland Uranium Project and model limitations are discussed.

Under reducing baseline conditions, the dominant mineralogy includes coffinite, pyrite, hematite, magnetite, and ferroselite. Greater than 99% of the uranium and selenium is present as coffinite and ferroselite, respectively. Conversely, 94% of radium remains in the aqueous phase. As expected under the oxidizing conditions created by the active mining process, uranium oxidizes to U(VI) and a significant fraction precipitates as soddyite. Selenium is oxidized to Se(IV), which remains in the aqueous phase. Restoration of the confined aquifer via groundwater sweep and H₂S injection result in the reduction and precipitation of uranium as uraninite; selenium precipitates as native selenium metal. Therefore, within the constructs of this conceptual model, while the groundwater quality of the ISR site post-remediation may resemble baseline water quality, the mineralogy of the system is different.