Timing of an Alpine water cycle unraveled by water isotopes and age-dating tracers (3H, 3He, 14C, CFCs, SF6 and 222Rn), Eastern Alps, Austria

Alpine regions are important as “water towers” in regional water supply of clean groundwaters due to their increased precipitation rates and their unspoiled environment. However, they are often characterised by complex geology structures, covered by down-sliding glacio-fluvial sediments. Groundwater recharge conditions and mean transit times (MTTs) are fundamental components of mountain watershed hydrological systems. Here, we used measurements of stable water isotopes of precipitation, pore water, surface and groundwater. In addition, measurements of environmental age tracers (²²²Rn, CFCs, ³H, ³He, ⁴He and ¹⁴C) were performed to investigate groundwater MTTs from springs in glacio-limnic sediments (<20 m) and deeper wells (>20 m) located along a mountainous hillslope (1,400-800 m) within the Subersach watershed near Sibratsgfäll, Bregenzer Wald, Austria. The near surface spring waters contain ³H and CFCs in excess. The deeper artesian well samples contain ³H and CFCs, in addition to elevated terrigenic ⁴He and low ¹⁴C values, suggesting a mixture of waters characterised by residence times that are modern (<70 years) and pre-modern (>70 years). We show that binary-mixing MTT models with distinct young and old fractions are needed to explain the full suite of environmental tracers, further supporting the importance of groundwater mixing processes.

The vertical unsaturated infiltration in silt/sand dominated glacio-lacustrine sediments were estimated by seasonal variation of ²H/¹⁸O-isotopes in pore-water to be 1-4 m/year approximately. Precipitation in the Flysch dominated area at higher altitudes is transported partly as mountain bloc recharge and ascends into the glacial sediments, indicated by temperatures 2-3° C higher than the mean surface temperature. The MTTs of the shallow groundwater (<20 m) estimated by a combination of isotopes ²H/¹⁸O, ³H/³He, ¹³C/¹⁴C and tracer gases (CFC, SF₆) indicate ages between some months and 4 years. Radon measurements identify springs supplied by very young drainage or surface waters. Deeper (>20 m) artesian wells in the western part are dominated by MTT older than 70 years.

The research project “Understanding of Extreme Climatological Impacts from Hydrogeological 4D Modelling” (EXTRIG) was funded by the Austrian Academy of Sciences.