water Article Hydrochemical Zoning and Chemical Evolution of the Deep Upper Jurassic Thermal Groundwater Reservoir Using Water Chemical and Environmental Isotope Data Florian Heine * , Kai Zosseder and Florian Einsiedl * Chair of Hydrogeology, Department of Civil, Geo and Environmental Engineering, Technical University of Munich, Arcisstr. 21, 80333 Munich, Germany;
[email protected] * Correspondence: fl
[email protected] (F.H.);
[email protected] (F.E.); Tel.: +49-(89)-289-25833 (F.E.) Abstract: 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, δ18O and 87Sr/86Sr were obtained from groundwater of 24 deep Upper Jurassic geothermal wells and coupled with a few analyses of noble gases (3He/4He, 40Ar/36Ar) 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 87 86 under meteoric cold climate conditions. Both water types show Sr/ Sr signatures, stable water isotopes values and calculated apparent mean residence times, which suggest minor water-rock Citation: Heine, F.; Zosseder, K.; interaction within a hydraulically active flow system of the Northeastern and Southeastern Central Einsiedl, F. Hydrochemical Zoning Molasse Basin. This thermal groundwater have been most likely subglacially recharged in the south and Chemical Evolution of the Deep of the SGMB in close proximity to the Bavarian Alps with a delineated northwards flow direction.