Integrated Hydraulic Characterization of a Fractured Aquifer at a Natural Analogue Site for Radioactive Waste Repositories in South Korea

Natural analogue studies provide valuable constraints for long-term assessment of deep geological repositories by examining subsurface environments analogous to repository settings. In South Korea, a natural analogue site in Boeun consists of a black slate with two U-rich coaly slate layers. In this study, we conducted an integrated hydraulic characterization of the fractured aquifer to improve understanding of groundwater flow and solute transport, thereby providing a hydrogeological basis for geochemical interpretation and key constraints for subsequent flow–transport modeling. The characterization was based on one borehole and three wells with depth-discrete open intervals. In specific, we integrated borehole logging (e.g., electrical conductivity (EC) profiling, optical borehole imaging (OBI), and flowmeter logging) with hydraulic tests, including slug, pumping, and solute tracer tests. EC profiling reveals depth-stratified groundwater intervals with distinct chemical signatures. OBI indicates pervasive fracturing throughout the borehole, but the fracture aperture varies with depth. Flowmeter logging identifies hydraulically active intervals that closely match the depth where OBI suggests larger apertures, supporting a depth-dependent hydraulic structure consistent with the EC profile. Consistent with the logging results, slug and pumping tests show modest vertical variability in hydraulic conductivity, supporting depth-dependent differences in hydraulic contributions across the borehole intervals. Tracer tests designed based on the integrated logging and test results suggest reduced vertical hydraulic connectivity across the interval separating the two coaly slate layers, implying compartmentalization and weakened connectivity between the stratified groundwater intervals. Based on these findings, we will develop a discrete fracture network-based conceptual model and incorporate it into coupled groundwater flow and transport simulations to enhance the predictive reliability of radionuclide behavior predictions at the natural analogue site.

This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education (RS-2025-25414628) and the Korea government (MSIP) (NRF-2021M2E1A1099413).