Massive mobilization of toxic elements from an intact rock glacier in the central Eastern Alps

In the central Eastern Alps, an increasing number of high-altitude streams draining ice-rich permafrost display high concentrations of toxic solutes, such as Al, F-, Mn, and Ni, that may strongly exceed drinking water limits. To obtain novel insights into the causes for the mobilization of toxic solutes and to assess the environmental hazard, here we present a 2-year dataset (2021, 2022) of monitoring a high alpine stream originating from an intact rock glacier located in eastern Switzerland. The monitoring includes monthly sampling and discharge measurements, as well as continuous tracking of the geogenic fluxes of toxic solutes, using a pressure and conductivity probe. Our monitoring revealed high annual fluxes of up to 10 ta-1 with strong seasonal variations. In particular, the fluxes were highest during the warm summer months and showed strong correlations with hydraulic events such as snowmelt and heavy rainfall. These correlations likely occurred because the mobilization of toxic solutes reflects the last step of a complicated sequence of coupled processes, including (i) the oxidation of sulfides producing sulfuric acid and promoting the dissolution of solutes from the host rock, (ii) temporal storage and long-term enrichment of the dissolved solutes in rock glacier ice, and (iii) their final hydraulic mobilization during climate-change-induced accelerated degradation of rock glaciers. In the studied catchment, the concentrations of toxic solutes strongly exceeded the drinking water limits down to an altitude of 1900 m a.s.l. This depicts a significant hazard for the farmers and their products using the catchment in summer, while the hazard for larger streams in populated areas further downstream is considered limited. Since the fluxes of toxic solutes downstream of rock glaciers likely reflect their final hydraulic mobilization from the solute-enriched rock glacier ice, we hypothesize that flux measurements may serve as a novel environmental tracer to study permafrost degradation.