Dating (81Kr) and 18O-87Sr-isotopes support mixing of deep thermal groundwater in the Austrian Alpine Foreland Basin

The Austrian Alpine Foreland Basin (AAFB) is home to a large population and an important industry location. The subsurface of the AAFB is intensively used since decades for drinking, energy (hydrocarbons and geothermal energy) and balneology purposes. Deeper groundwater systems in the Malmian and in the Oligocene Formations are utilised for geothermal energy and balneology. Groundwaters from shallow sedimentary strata are used for domestic and industrial water supply. In addition, hydrocarbon production is conducted since several decades in this area. To appose these individual interests and to secure a sustainable usage of the resources, the understanding of the basin history, the hydrostratigraphic ages and their interactions are crucial.

Major ion chemistry, δ²H-¹⁸O, and ⁸⁷Sr/⁸⁶-ratios suggest the Malmian thermal waters to be a mixture of meteoric Na-HCO3-waters with depleted δ¹⁸O-values <-11.5‰ with NaCl-brines (δ¹⁸O-values >-4.5‰) and a small fraction of young  CaMg-HCO3-waters.

⁸¹Kr investigations on ten deep groundwater samples (including 1 from Germany) from different hydrostratigraphic units of the Austrian Alpine Foreland Basin (AAFB) imply a differentiated picture of the groundwater residence times.

Exceptional high ⁸¹Kr-model-ages of deep Malmian thermal groundwater samples (around 500’000 years) would suggest extremely low velocities (incl. the cross formation flow) which contradict the existing hydrogeological model concepts of a dynamic thermal water flow in the Malmian reservoir.

Very old deep groundwater portions (> 900’000 years) are visible in an Eocene formation sample (Gallspach) whereas samples from younger strata (Oligocene) exhibit the youngest ⁸¹Kr- model-ages  (< 25’000-240’000 years) [1,2].

The discrepancy between the derived ⁸¹Kr-model-ages of the deep Malmian thermal groundwaters is difficult to reconcile with the most recent numeric thermal-water-model based on a recharge area “Tertiäres Hügelland” (SE of Regensburg) only 100 km northwest of the main users [3]. Possible explanations include diffusion processes in contact areas between the aquifers with the aquicludes or mixing with ⁸¹Kr-free Cenozoic formation waters and the presence of hydrocarbons within the aquifer that could influence the ⁸¹Kr-model-ages.

The very old age of the dominating thermal Na-HCO3-water-component is supported by ³He/⁴He-ratios and ⁴He-contents saturated with a purely crustal helium-ratio and content. The exchange of the Malmian thermal water with its aquifers during his pathway is shown by radiogenic ⁸⁷Sr/⁸⁶Sr-ratios (0.7098-0.7108) and elevated F, Li and Rb-concentrations.

 

• Heidinger, M., F. Eichinger, R. Purtschert, P. Mueller, G. Wirsing, T. Geyer, T. Fritzer, and D. Groß (2019): Altersbestimmung an thermalen Tiefenwässern im Oberjura des Molasse-beckens mittels Krypton-Isotopen. In: Grundwasser, 24, 287-294.
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• Expertengruppe Thermalwasser (2024): Das Thermalwasservorkommen im niederbayerischen -oberösterreichischen Molassebecken: Hydrogeologisches Modell und numerisches Thermal-wassermodell - Kurzbericht. 67 p., Land Oberösterreich. (https://www.landober-oesterreich.gv.at/files/publikationen/w_thermalwasser_bayern_ooe.pdf)