Hydrochemical zoning and chemical evolution of the deep upper jurassic thermal groundwater reservoir using water chemical and environmental isotope data

A comprehensive hydrogeological understanding of the deep Upper Jurassic carbonate aquifer, which represents an important geothermal reservoir in the South German Molasse Basin (SGMB), is crucial for improved and sustainable groundwater resource management. Water chemical data and environmental isotope analyses of δD, δ¹⁸ O and⁸⁷ Sr/⁸⁶ Sr were obtained from groundwater of 24 deep Upper Jurassic geothermal wells and coupled with a few analyses of noble gases (³ He/⁴ He,⁴⁰ Ar/³⁶ Ar) and noble gas infiltration temperatures. Hierarchical cluster analysis revealed three major water types and allowed a hydrochemical zoning of the SGMB, while exploratory factor analyses identified the hydrogeological processes affecting the water chemical composition of the thermal water. Water types 1 and 2 are of Na-[Ca]-HCO₃-Cl type, lowly mineralised and have been recharged under meteoric cold climate conditions. Both water types show⁸⁷ Sr/⁸⁶ Sr signatures, stable water isotopes values and calculated apparent mean residence times, which suggest minor water-rock interaction within a hydraulically active flow system of the Northeastern and Southeastern Central Molasse Basin. This thermal groundwater have been most likely subglacially recharged in the south of the SGMB in close proximity to the Bavarian Alps with a delineated northwards flow direction. Highly mineralised groundwater of water type 3 (Na-Cl-HCO₃ and Na-Cl) occurs in the Eastern Central Molasse Basin. In contrast to water types 1 and 2, this water type shows substantial water-rock interaction with terrestrial sediments and increasing⁴⁰ Ar/³⁶ Ar ratios, which may also imply a hydraulic exchange with fossil formation waters of overlying Tertiary sediments.