Spatio-temporal variability of isotopic and chemical tracers in high-elevation water sources, Eastern European Alps

In high-mountain areas, the ratios of stable water isotopes (δ18O, δ2H) and solute concentrations are often used to infer the relative contribution from snow, ice, and rain waters to runoff. Yet, the strong spatial and temporal tracer variability of these hydrological resources is often overlooked. We characterised the water isotopes and chemistry of hydrological resources in two headwater catchments of South Tyrol (Eastern Italian Alps). During the melt season of 2022 and 2023, we collected bi-weekly water samples from precipitation, snow, snowmelt, as well as ice and runoff on the surface of a small glacier and within the debris of two rock glaciers. We found a sharp isotopic seasonality in snowmelt and precipitation, and a tendency of depletion and fractionation with increasing elevation in both resources. End-member mixing models identified permafrost and glacial ice as the dominant components (over rainwater and snowmelt) of the rock glacier and glacier runoff, respectively. Both runoff types had elevated concentrations of sulphate and trace elements (Sr, Ba, Al, Mn, Ni, Co, Fe, Zn, Li, Y). This is attributed to the intense chemical weathering occurring on abundant freshly ground rock particles on the glacier, and to the melt of solute-enriched perennial ice in rock glaciers. While deep water pathways are generally considered as major locations of chemical reactions, we highlight that even the surface runoff of glaciers and rock glaciers can strongly contribute to the export of toxic elements – e.g. Ni – to aquatic ecosystems. This has important implications for drinking water and environmental quality.