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Karst hydrology Untersberg, Salzburg

Initiated by Giorgio Höfer (Geoconsult ZT, Salzburg), two ambitious Master students, Benjamin Bichler and Markus Reischer, supervised by Christoph Spötl, worked in tandem to take a new fresh look at the Untersberg Massif. Located southwest of the city of Salzburg at the Austrian-German border, the Untersberg is a karst area covering about 70 km2 with a central plateau-like zone (11 km2). Hundreds of caves are known in this massif, including the Riesending, currently Germany´s largest and deepest cave with 18.2 km length and over 1 km of vertical extension.
The focus of our work was the karst aquifer of the Untersberg and its poorly understood relationship with the porous aquifer of the adjacent Salzburg Basin. As already demonstrated by tracer experiments between 1967 and 1982 large parts of the karst plateau drain to one major spring in the North, Fürstenbrunn spring, which has been used as drinking water for the inhabitants of Salzburg city since the 16th century. Its discharge varies greatly between 50 L/s and more than 10 m3/s and responds quickly to large rainfall events.
Some 60 springs, wells and streams were sampled on a monthly basis between February 2012 and March 2013 and analysed for various parameters including stable isotopes. These data show that the mean residence time of the water emerging at the Fürstenbrunn spring is 0.4 years. In addition, this water shows a lower intra-annual isotope amplitude compared to other springs in this area. This implies that the water of the Fürstenbrunn spring is well mixed and the rapid discharge increase following strong precipitation events is only a hydraulic response.
Untersberg water locally infiltrates into the Pleistocene gravel aquifer of the adjacent Salzburg Basin (Rositten creek, Fürstenbrunn spring (artificially), and indirectly via the Berchtesgadener Ache), but our results suggest that the main recharge of this aquifer is from precipitation falling on the basin itself. This study found no evidence for deep karst groundwater entering the basin from the Untersberg Massif; an older hypothesis which is also in conflict with tracer observations made along the eastern margin of the Untersberg.

Support for this study was provided by the Austrian Institute of Technology (Stefan Wyhlidal) and the Salzburg AG (Josef Lintschinger, Markus Rußegger), as well as by Geoconsult ZT, Georg Zagler and the University of Innsbruck.