The geological disposal of high-level radioactive waste relies on identifying domains with slow groundwater flow through bedrock fractures for suppressing migration of radionuclides released from the waste. In this article, we introduce a method to determine whether groundwater flow through fractures is stagnant by comparing the stable isotope ratios of hydrogen and oxygen in fracture water with those in the surrounding pore water. If the isotope ratios differ, it indicates that water with a different isotopic composition from that of the water originally contained in the bedrock is flowing through the fractures (Fig. 1). Meanwhile, if the isotope ratios are similar, it suggests that groundwater flow through the fractures is minimal or the water is stagnant (Fig. 1). Applying this method to data from the Horonobe area (Fig. 2), we observed groundwater flow along fractures in the shallow domains near the ground surface, while deep domains exhibited slower flow (Fig. 1). Fractures within both shallow and deep zones showed high hydraulic connectivity, indicating that water can intrude from the surface through the fractures. These results indicate that slow groundwater flows occur even in fractures with high hydraulic connectivity.
 The proposed method and findings are instrumental in selecting potential sites for geological disposal and determining the optimal placement of waste materials within repositories.