QUANTIFYING AQUEOUS URANIUM MOBILITY FROM IN SITU RECOVERY MINING SITES USING ISOTOPE-CONTRAINED REACTIVE TRANSPORT MODELS

Domestic source(s) of uranium (U) ore are critical for low-carbon energy and energy security. U ore production in the USA is primarily by the in situ recovery (ISR) method where local groundwater fortified with oxygen and carbon are used to extract U as soluble U(VI)-CO₃ complexes. The ISR method is economically and environmentally beneficial, producing ore from low-grade deposits with no mine tailings. Despite the advantages of the ISR technique there are significant questions about the long-term ability to restore mined aquifers to their pre-mining condition. Given the concerns about potable water availability in the western USA, there is a renewed focus on understanding the fate and transport of U in and adjacent to ISR mining operations.

We present new elemental concentration and isotopic data from recent studies in the Powder River Basin roll front U deposits. Groundwater samples and 3 sediment cores from the ore zone and down-gradient regions are used to construct reactive transport models of U mobility under pre-mining conditions. Specifically, we focus on U isotopic characterization ((²³⁴U/²³⁸U) and δ²³⁸U) of a roll-front U deposit from Smith Ranch Highland, WY, USA. Our results show substantial variations in δ²³⁸U (up to ~ 2‰) and (²³⁴U/²³⁸U) (ranging from 0.77 to 2.26) in U minerals with depth in each core and amongst the different cores. Vertically, U concentration is correlated with δ²³⁸U; samples with highest U at the center of the mineralized zone exhibit high δ²³⁸U (> 0.0‰), which decreases both upward and downward. The (²³⁴U/²³⁸U) in each core is correlated with δ²³⁸U, with the lowest (²³⁴U/²³⁸U) (<< 1.0) at the center of the mineralized zone.

Preliminary 2-D reactive transport models suggest that: 1) There is significant exchange between the solid phase U and aqueous U. 2) The primary mechanism removing aqueous U from present day groundwater is reductive precipitation. 3) The kinetics of U removal from solution are relatively fast compared to groundwater velocities (~2 m/yr). Overall, our modeling results suggest that the undisturbed groundwater is inefficient at transporting U downgradient and that restoration-remediation strategies that return the ore zone waters close to the pre-mining chemistry should effectively sequester U as precipitated U(IV) minerals inside the aquifer exemption area.