Hydrochemical properties of deep carbonate aquifers in the SW German Molasse basin

Abstract

Background: The Upper Jurassic (Malm) limestone and the Middle Triassic Muschelkalk limestone 18 are the major thermal aquifers in the southwest German alpine foreland. The aquifers 19 are of interest for production of geothermal energy and for balneological purposes. Methods: Hydrochemical data from several hundred wells within two deep limestone aquifers in the Molasse basin of SW Germany have been compiled, examined, validated, and analyzed with the aim to characterize the fluids and to investigate the origin of the fluid properties. Results: The hydrochemical properties of the two aquifers differ in several aspects. The total amounts of dissolved solids (TDS) are much higher within the Upper Muschelkalk aquifer than within the Upper Jurassic. Water composition data reflect the origin and hydrochemical evolution of deep water. Rocks and their minerals control the chemical signature of the water. With increasing depth, the total of dissolved solids increases. In both aquifers, the water evolves to a NaCl-dominated fluid regardless of the aquifer rock. Discussion: The salinity of the aquifers has different sources. In the case of the Upper Muschelkalk, it is linked to deep circulation systems, while the hydrochemical properties in the Upper Jurassic developed due to changing overburden and hydraulic potential.