Study on Geochemical Characteristics Evaluation Through Lithium Isotope Analysis for Long-Term Evolution of Groundwater in High-Level Radioactive Waste Disposal Sites

Understanding the long-term evolution of groundwater in high-level radioactive waste (HLW) disposal sites is crucial for identifying radionuclide migration pathways, assessing environmental impacts, and ensuring long-term stability. This study evaluates the applicability of non-conventional methods, such as metal isotope analysis, in understanding the geochemical long-term evolution of groundwater. Groundwater and rock core samples were collected from boreholes at the Korea Atomic Energy Research Institute’s Underground Research Tunnel (KURT) site. To evaluate the geochemical characteristics and changes in lithium isotope (δ⁷Li) composition in the samples, the lithium isotope analysis was performed alongside the principal component analysis (a traditional method). The extent and intensity of chemical weathering were revealed through comparative analysis of the δ⁷Li content changes in groundwater and rock cores, which could ultimately be interpreted in connection with the groundwater residence time. It was revealed that primary mineral dissolution during the early stages of weathering did not significantly affect the δ⁷Li values in the groundwater but secondary mineral formation resulting from prolonged weathering was a factor in increasing the δ⁷Li values in the groundwater and decreasing the δ⁷Li values in the rock cores. Therefore, the δ⁷Li analysis is believed a useful tool to provide insights into primary mineral dissolution, secondary mineral formation, and subsequent re-dissolution processes driven by water-rock interactions. δ⁷Li analysis could be utilized for understanding the geochemical evolution characteristics of disposal environments and for evaluating the safety of deep geological disposal.

Acknowledgements

This research was supported by the National Research Foundation of Korea(NRF) under the project 'Development of Core Technologies for the Safety of Used Nuclear Fuel Storage and Disposal; NRF-2022M2E1A1052570'.