Geochemical and U-Th isotopic insights on uranium enrichment in reservoir sediments

Uranium (U) geochemistry and its isotopic compositions of reservoir sediments in U mine area were poorly understood. Herein, U and Th isotopic compositions were employed to investigate source apportionment and geochemical behavior of U in 41 reservoir sediments from a U mining area, Guangdong, China. The remarkably high contents of both total U (207.3-1117.7 mg/kg) and acid-leachable U (90.3-638.5 mg/kg) in the sediments exhibit a severe U contamination and mobilization-release risk. The U/Th activity ratios (ARs) indicate that all sediments have been contaminated apparently by U as a result of discharge of U containing wastewater, especially uranium mill tailings (UMT) leachate, while the variations of U/Th ARs are dominated by U geochemical behaviors (mainly redox process and adsorption). The U isotopic compositions (delta 238U) showed a large variance through the sediment profile, varying from - 0.62 to - 0.04 parts per thousand. The relation between delta 238U and acid-leachable U fraction demonstrates that the U isotopic fractionation in sediments can be controlled by bedrock weathering (natural activity), UMT leachate (anthropogenic activity) and subsequent biogeochemical processes. The findings suggest that U-Th isotopes are a powerful tool to better understand U geochemical processes and enrichment mechanism in sediments that were affected by combined sources and driving forces.

Automated summary

The study found that U contamination and mobilization-release risk are high in reservoir sediments in the U mine area, influenced by discharge of U-containing wastewater. Using U/Th activity ratios and U isotopic compositions, the sources and geochemical behaviors of U in sediments can be inferred to be greatly affected by wastewater discharge and natural processes. The results demonstrate that U-Th isotopes are a powerful tool for better understanding U geochemical processes affected by multiple sources.