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

摘要

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 ~(238)U/~235U (δ~(238)U), ~(234)U/~(238)U activity ratio, and ~(34)S/~(32)S (δ~(34)S), 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 δ~(238)U in Rosita groundwater varies from +0.61‰ to -2.49‰, with a trend toward lower δ~(238)U in downgradient wells. The concurrent decrease in U(Ⅵ) concentration and δ~(238)U with an ε of 0.48‰ ± 0.08‰ is indicative of naturally occurring reducing environments conducive to U(Ⅵ) reduction. Additionally, characteristic ~(234)U/~(238)U activity ratio and δ~(34)S 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.