Dating (81Kr) deep thermal groundwaters in the Upper Austrian part of the Alpine Foreland Basin

The Alpine Foreland Basin (AFB) extends from Geneva to Vienna. It is an highly populated area and also hosts important industrial sites. The competition between different areas of utilization, including groundwater, mineral water, geothermal energy, oil and gas as well as underground gas storage increased within the last decades and is still increasing. Therefore, the understanding of the subsurface, the hydrostratigraphic units and their interactions is essential to develop cross-boundary concepts for sustainable management, economic development and security of supply.

The aims of a three-years project, funded by the ÖAW (Austrian Academy of Science), were to delineate and characterise hydrostratigraphic units, to recognise water composition and to determine possible flow pathways within the Upper Austrian part of the AFB. As main data sources, hydrochemistry, stable isotopes, dissolved noble gases of groundwater and isotopic groundwater ages were used to proof and improve hydrogeological concepts. This presentation focuses on the ⁸¹Kr investigations which were carried out within the project and were partly funded by the government.

Determining the age of deep groundwaters which are free of ¹⁴C has been almost impossible for a long time. Improved analytical methods make it now possible to use ⁸¹Kr for age characterisation. Therefore, nine water samples from deep wells representing different hydrostratigraphic units in the Upper Austrian AFB, were used for ⁸¹Kr investigations. These samples include water from Upper Jurassic geothermal reservoirs.

Results imply a differentiated picture of groundwater residence times. ⁸¹Kr model ages of Malmian water samples are uniform which is in line with hydrochemical analyses and stable isotopes of these samples. However, model ages are exceptional high (390000 – 550000 years) which would suggest low flow velocities. This seems to contradict all existing hydrogeological model concepts of a dynamic thermal water flow in Malmian carbonates. The water sample taken from the Eocene (Gallspach) exhibits a very old groundwater portion (> 900000 years), whereas samples from Oligocene strata show the youngest but strongly varying ⁸¹Kr model ages (<25000 – 240000 years). The water sample of Bad Schallerbach is interpreted as a complex mixed system between young and middle-aged deep groundwaters with elevated mineralisation. A contribution from a deeper aquifer is postulated for the water sample in Andorf (240000 years). 

In summary, results of the hydrochemical and stable isotope investigations together with the krypton analyses have shown that connections between the Upper Jurassic thermal water aquifer and younger groundwater systems (Eocene to Oligocene) are obvious. This confirms the hydrogeological model concepts, which assume a discharge of thermal waters east of the Upper Jurassic carbonate rocks into younger strata. Possible factors which may influence the model ages (e.g. diffusion processes, contact with formation waters containing hydrocarbons) were critically discussed on the gained database. However, the discrepancy between the derived ⁸¹Kr model ages of the Malmian thermal waters and the current hydrogeological models could not be resolved yet. Further investigations should focus on recharge areas and therefore on aligning age data and hydraulic models.