Dating Old Groundwater with ³⁶Cl and ⁸¹Kr in a Fractured Claystone Formation, Mecsek Mts., Hungary

The age distribution of groundwater retrieved in two boreholes of a claystone formation in the Mecsek Mountains, Hungary, was investigated using age tracers 81Kr and 36Cl, with additional analyses of 4He, 14C, and tritium. The present work aims to search a potential site for a high-level radioactive waste repository. Tracer age results provide significant insights into the temporal evolution of the vertical recharge. Radiogenic 4He concentration increases with depth, and 3He/4He ratios decreasing toward crustal support the hypothesis of an increasing age with depth. Shallow borehole samples reveal a broad age range, including one with an 81Kr model age of approximately 34,000 years, and a 14C model age of 13,400 years. In deeper sections, a 81Kr model age of 625,000 years was broadly consistent with a 36Cl model age of ∼400,000 years, indicating the presence of ancient groundwater. Unexpectedly high 81Kr abundances measured in one borehole indicate underground 81Kr production in a uranium-rich rock formation, adding to uncertainty of 81Kr model ages, whereas the obtained 36Cl ages (∼90,000 to ∼400,000 years) are here considered more reasonable. Noble gas temperatures indicate that recharge occurred during glacial and interglacial periods. This study demonstrates the value of integrating multiple isotopic tracers to address uncertainties in groundwater dating and to provide a robust framework for understanding deep aquifer systems.