EVALUATING GROUNDWATER RECHARGE AND LONG-TERM STABILITY OF A FRACTURED NATURAL BARRIER SYSTEM USING MULTIPLE ISOTOPES AND MICROBIAL INDICATORS

Natural barrier system (NBS) intends to preserve spent nuclear fuel for geological time scales (~1 million years). This study aims to refine the methodology for evaluating groundwater recharge and infiltration around fractured NBS using multiple environmental tracers. Employing multiple isotopes and microbial tracers, we scrutinized redox conditions within the heterogeneous barrier system. In shallow groundwater, enriched H₂O isotopes and depleted SO₄^2– isotopic values were observed alongside a dominance of aerobic microbes, whereas the opposite trends were seen in deep groundwater. Additionally, deep groundwater in the permeable fracture showed a mixture of aerobic and anaerobic water. This mixing was characterized by a linear mixing line observed in dual isotopic plots of H₂O and SO₄^2– and the co-presence of aerobic and anaerobic bacteria. Below 300 meters, the fractured NBS was devoid of dissolved O₂ and DOC. Around 500 meters, we identified genera such as Novosphingobium, Comamonadaceae, and Desulfuromonas, which are capable of thriving under harsh conditions of low DO and high levels of heavy metals. This highlights the importance of understanding the overall environment in deep geological conditions to thoroughly evaluate the performance of microbiomes in controlling toxic radionuclides within the repository system. This study underscores the pivotal role of age tracers, stable isotopes, and microbial community dynamics in enriching the assessment of the long-term stability of fractured granite barriers.

Acknowledgement

This work was supported by the Institute for Korea Spent Nuclear Fuel (iKSNF) and National Research Foundation of Korea(NRF) grant funded by the Korea government(Ministry of Science and ICT, MSIT) (No.2021M2E1A1085200) and also supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government (MSIT) (No.2021M2E1A1085186).