APPLICATIONS OF URANIUM ISOTOPES TO URANIUM IN-SITU RECOVERY SITES

Chemical transformations near the earth’s surface cause significant variability in ²³⁴U/²³⁸U and, only recently measurable, smaller differences in ²³⁸U/²³⁵U. Variability in both these ratios is observed in U ore concentrates from deposits precipitated by low temperature reduction processes. Such deposits, particularly sandstone “roll fronts”, are commonly mined using in situ recovery (ISR). U isotopes can be useful tools in both understanding the formation of roll fronts and in monitoring remediation efforts during and after ISR.

To explore these potential applications, we present ²³⁴U/²³⁸U and ²³⁸U/²³⁵U ratios of water obtained from monitoring wells associated with uranium ore deposits in southwestern Texas. Samples were obtained from: 1) the oxidizing zone located up the hydrologic gradient from the ore body; 2) within the ore zone; and 3) in the reducing zone down the hydrologic gradient from the ore body. Isotopic data is available from both natural and mined ore bodies, and from sites that have been subjected to variable degrees of remediation.

Aqueous weathering of minerals containing U results in preferential leaching of ²³⁴U from a-damaged crystal lattice sites; the result is that low ²³⁴U/²³⁸U is indicative of significant U loss. Conversely, high ²³⁴U/²³⁸U is indicative of U addition from water. Low ²³⁴U/²³⁸U characterizes the zone subjected to ISR and likely reflects intense chemical leaching that occurs during the ISR process. This contrasts with higher, but variable ratios in the reducing zone. The differing ratios in these two zones suggest that ²³⁴U/²³⁸U can be used to trace U subjected to ISR. ²³⁴U/²³⁸U in the oxidizing zone is variable. Here, ²³⁴U/²³⁸U is negatively correlated with U concentration, a situation that suggests that sandstone roll front deposits have high and quantifiable natural U mobility.

Variations in ²³⁸U/²³⁵U have only been recently measured and are generally attributed to reduction and oxidation processes (redox). Data from an unmined roll front deposit strongly support the assertion that this fractionation is linked to redox conditions; ²³⁸U/²³⁵U is highest in the oxidizing zone and lowest in the reducing zone. Low ²³⁸U/²³⁵U, indicative of a strongly reducing environment, is present in all wells down-gradient of the ISR sites.