The hydrochemistry and geothermometry of thermal waters from a deep Jurassic aquifer in Lower Austria–South Moravia region

Potentially important deep hydrogeological structure containing thermal water is in the border area of Austria and the Czech Republic. Thermal water is bound to a deep Jurassic carbonate aquifer (reaching depths above 3 km) underlying the Tertiary sediments of the Alpine-Carpathian Foredeep. Its current use is limited only to spa and recreational purposes serviced by two 1.45 km deep wells. Our study is the first comprehensive study in this area to understand the origin and processes of thermal water formation and evolution. Understanding the processes in aquifers containing thermal water is of fundamental importance both for geothermal potential and for knowledge of possible natural analogues of deep CO₂ storage. The research has shown that thermal water is of dominant Na-Cl type with largely varying TDS (total dissolved solids) values between 0.4 and 56 g/L. Assumed marinogenic origin is confirmed by Cl/Br ratio. The average value of the geothermal gradient is 28.5 °C/km and the observed groundwater temperature ranges from 37 °C to 110 °C, with the highest values found in the deepest part of the aquifer, mainly on the Austrian side. The Jurassic aquifer is divided into two sections: a shallower, less mineralised northwestern section and a deeper, highly mineralised southeastern section, with a relatively impermeable barrier between them. The southeastern section contains mainly fossil seawater of high salinity ranging from 15 to 56 g/l and represents a structure with limited connection to active groundwater flow. In contrast, the thermal water in the northwestern section shows evidence of meteoric water infiltration from NW, as confirmed by TDS concentrations, hydrochemistry, and stable δ²H and δ¹⁸O isotope composition. The most reliable geothermometrical estimations are given by the K-Mg (Giggenbach, 1988) and Na-K (Truesdell 1976; Tonani 1980; Arnórsson 1983) geothermometers with mean reservoir temperature around 95 °C. The average temperature is higher in the water samples from the southeastern section and according to the geothermometrical evaluation, most of the water samples from both aquifer sections are mature chloride waters. Immature waters occur only in the northwestern section, indicating dilution of primary seawater by shallow cold waters of meteoric origin.