Combining ¹⁴C_DOC and ⁸¹Kr with hydrochemical data to identify recharge processes in the South German Molasse Basin

To face the challenges of ongoing climate change, renewable energy sources are needed to reduce CO₂ emissions to the atmosphere. The South German Molasse Basin (SGMB) contains an important geothermal aquifer in Germany; however, an enhanced understanding of the flow regime at the basin scale remains a fundamental limiting factor in assessing a long-term and sustainable strategy for using this green energy source. Here we report ¹⁴C_DOC and ⁸¹Kr groundwater dating results together with hydrochemical data, stable water isotopes, noble gases and out-diffusion experiments that indicate that the thermal groundwater consists of at least two groundwater sources: a formerly unknown young groundwater component with apparent water ages between 9,800 and 18,700 a and an old groundwater component, which shows an apparent water age of up to approximately 300 ka. The results of the out-diffusion experiments with core material from the Upper Jurassic aquifer (UJA) suggest that diffusion processes between rock-matrix water and more mobile fracture water represent a controlling mechanism for groundwater flow and the observed extended apparent groundwater ages within the UJA. The observed elevated ¹⁴C_DOC activities and the high spatial heterogeneity in groundwater ages close to the southern margin of the SGMB were interpreted as a recharge zone of the system. The combination of the ¹⁴C_DOC and ⁸¹Kr methods shows the potential as a powerful groundwater dating tool for deep aquifers.