14CDOC and 81Kr as groundwater dating tools for identifying recharge processes into the South German Molasse Basin

Theis Winter1, Florian Einsiedl1
1 Lehrstuhl für Hydrogeologie, Technische Universität München

V 11.5 in Verweilzeiterkundung in Grundwasserleitern mittels Tracern und Grundwasseraltern

23.03.2022, 14:45-15:00, HS 1

The Upper Jurassic Aquifer within the South German Molasse Basin (SGMB) is the most important exploration horizon for geothermal energy supply in Bavaria. This aquifer is characterized by a heterogeneous geology which is composed of karstic features and fault-zones. A key to understand the large scale recharge conditions of the SGMB is the use of environmental isotopes.

Analyses for 14CDOC and 81Kr on groundwater from the SGMB were performed to determine the piston-flow ages of the Upper Jurassic Aquifer. In addition, we used water chemistry data and stable water isotopes from the thermal groundwater to bring light on the recharge processes into the basin.

The stable water isotope values range between -10.66 to -11.76 ‰ for δ18O and -85.0 to -86.1 ‰ for δ2H and may indicate infiltration of meteoric water under cold climate conditions. The stable isotope 2D-plot of δ18O vs. δ2H also shows some evidence that mixing of different water components may occur within the basin. 14CDOC- and 81Kr-ages support the interpretation of mixing of at least two groundwater components in the Upper Jurassic Aquifer: A “young” water component from the Pleistocene/Holocene transition (<20.000 years) and an “old” water component with a piston-flow age much larger than 100.000 years.

Major ion concentrations in groundwater are in agreement with the calculated piston-flow ages and increase with increasing water ages but show considerable local differences in their genesis. Based on the new results we can draw new conclusions on the larger scale recharge conditions of the SGMB by combining 14CDOC and 81Kr. Particularly the use of 14CDOC points to the discovery of a formerly unknown water component within the southern part of the basin and also supports the theory of a recharge area in this part.





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