Unusual old water ages of an Alpine karst spring (Central Eastern Alps): Hydrogeology, isotopes (18O/2H, 3H, 3He, 14C) and tracer gas analyses (CFC-11,-12,-113, SF6)

Aim of the study was to delineate an appropriate wide-ranging drinking water protection zone for the tapped main Walchhof spring, which is part of the public water supply of the small city of Radstadt in the region of Salzburg, Austria. The immediate hydrographic catchment area of the spring was geological mapped, various electric conductivity measurements at the river Taurach and its tributaries were carried out to detect potential high mineralised (SO₄) karst water influx and fluorescence tracer tests were performed on the adjacent hydrographic catchment area above the tapped main spring. The Walchhof springs discharge approximate 500 L/s in a complex central-alpine setting within a tectonic window structure built of the Radstadt nappe with permeable carbonate rocks overthrusted by the Schladming-Seckau nappe with mostly non-permeable phyllite rocks. To identify the mean altitude of the catchment area and the Mean Residence Time (MRT) of the spring waters a combination of isotopes ²H/¹⁸O, ³H/³He, ¹³C/¹⁴C and tracer gases (CFC, SF₆) was analysed. The ²H/¹⁸O-isotopes were analysed on weekly samples during 2019. ³H/³He, ¹³C/¹⁴C and (CFC, SF₆) were sampled twice in April and October 2019.  The results indicate a wide-ranging hydrogeological catchment area (max. 90km²) at a mean altitude of 2000 ± 200 m and a mixture of old (10-20 yrs) and very old (several thousand years) waters. However, heavy rainfall and snow melt events can add (< 10%) very young water (MRT: days-weeks) to the tapped main Walchhof spring. The combination of these methods allows to reduce the wide-ranging drinking water protection zone mainly to the immediate hydrographic catchment area.