Isotopic and Geochemical Tracers for U(VI) Reduction and U Mobility at an in Situ Recovery U Mine

In situ recovery (ISR) uranium (U) mining mobilizes U in its oxidized hexavalent form (U­(VI)) by oxidative dissolution of U from the roll-front U deposits. Postmining natural attenuation of residual U­(VI) at ISR mines is a potential remediation strategy. Detection and monitoring of naturally occurring reducing subsurface environments are important for successful implementation of this remediation scheme. We used the isotopic tracers ²³⁸U/²³⁵U (δ²³⁸U), ²³⁴U/²³⁸U activity ratio, and ³⁴S/³²S (δ³⁴S), and geochemical measurements of U ore and groundwater collected from 32 wells located within, upgradient, and downgradient of a roll-front U deposit to detect U­(VI) reduction and U mobility at an ISR mining site at Rosita, TX, USA. The δ²³⁸U in Rosita groundwater varies from +0.61‰ to −2.49‰, with a trend toward lower δ²³⁸U in downgradient wells. The concurrent decrease in U­(VI) concentration and δ²³⁸U with an ε of 0.48‰ ± 0.08‰ is indicative of naturally occurring reducing environments conducive to U­(VI) reduction. Additionally, characteristic ²³⁴U/²³⁸U activity ratio and δ³⁴S values may also be used to trace the mobility of the ore zone groundwater after mining has ended. These results support the use of U isotope-based detection of natural attenuation of U­(VI) at Rosita and other similar ISR mining sites.