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

Abstract

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.