Evaluating radionuclide mobility in groundwater recharge areas of fractured natural barrier systems using multiple isotopes and microbial indicators.

The distribution of uranium (U) concentrations, which reached up to 322 µg/L, was found to correlate with the pattern of fractures within the natural barrier system (NBS). Analysis of the vertical distribution of dissolved oxygen (DO), dissolved organic carbon (DOC), tritium (H), microbial communities, and HO and SO isotopes revealed insights into oxic water infiltration within the heterogeneous fractured system. Their distribution showed that the average infiltration depth at the KURT site is 200 m, while in external areas with a high frequency of fractures, oxic conditions extended down to 495 m. The SO isotopes suggested the potential for microbial sulfate reduction to play a role in regulating radionuclide mobility in the deep geological system. At approximately 500 m, genera capable of thriving under harsh conditions of low DO and high heavy metal concentrations, such as Novosphingobium, Comamonadaceae_uc, and Desulfuromonas_g2, were identified. These findings indicate hydrogeological variability and microbial adaptation within the deep NBS, highlighting the importance of understanding the deep geological environment for evaluating microbiome performance in regulating toxic radionuclides within repository systems. Overall, this study emphasizes the pivotal role of age tracers, stable isotopes, and microbiome in enhancing the assessment of the long-term stability of fractured granite barriers.