TRACING GROUNDWATER SYSTEMS IN UPPER AUSTRIA USING 87Sr/86Sr ISOTOPE RATIOS

The Upper Austrian Molasse Basin is not limited to the use of drinkable groundwater and balneological use of thermal waters. The extraction of crude oil and natural gas, the storage of natural gas and carbon dioxide, and the increasing use of thermal water as an alternative energy source are of high economic importance. In terms of water management, knowledge of the subsurface conditions, the different groundwater horizons, and their interactivity are indispensable. Already existing hydrodynamical models of the Malmian thermal aquifer, however, exhibit uncertainties due to low data density and lack of information. As part of a multi-year project funded by the ÖAW (Austrian Academy of Sciences), the data sets were expanded, among other things, by determining the strontium isotope ratios of selected water samples in Upper Austria. Therefore, concentrations of the most common cations and the strontium isotopic composition of 48 samples from 12 different aquifer horizons, ranging from shallow to artesian groundwater and geothermal and hydrocarbon wells were analyzed by using (MC) ICP-MS.
⁸⁷Sr/⁸⁶Sr ratios range from 0.70841 to 0.72740 and generally correlate with the respective host rock. The hydrostratigraphic horizons show characteristic strontium isotopic signatures, though they cannot be distinctly assigned to a single formation. Given variations in ⁸⁷Sr/⁸⁶Sr ratios can be traced in most instances. Thus, elevated ⁸⁷Sr/⁸⁶Sr ratios > 0.710 of a few samples of the Innviertel Group (0.70915 – 0.71167) can be explained by their sampling location at the crystalline boundary. Higher variability of results within formations, e.g., of the Linz-Melk Formation (0.70928 – 0.71160) in terms of both, ⁸⁷Sr/⁸⁶Sr ratio and strontium concentration suggest mixing processes with waters depleted in ⁸⁷Sr. Equally large variations are shown in the results of waters from Eocene formations (0.70879 - 0.71011). In this case, supported by results of other hydrochemical data, even a multi-component mixing process is assumed. Samples of the Malmian thermal water (0.70971 - 0.71077) exhibit, with one exception, homogenous ⁸⁷Sr/⁸⁶Sr ratios below 0.710.
The results demonstrate the ability of strontium isotope ratio determination to confirm assumptions based on hydrochemical and physical data as well as to provide additional information about the dynamics of (deep) groundwater systems to identify mixing processes within formations.