Rock glaciers in the Central Eastern Alps – How permafrost degradation can cause acid rock drainage, mobilization of toxic elements and formation of basaluminite

Wanner, Christoph; Moradi, Hoda; Ingold, Philipp; Cardenas Bocanegra, Miguel A.; Mercurio, Romano; Furrer, Gerhard (2023). Rock glaciers in the Central Eastern Alps – How permafrost degradation can cause acid rock drainage, mobilization of toxic elements and formation of basaluminite. Global and planetary change, 227, p. 104180. Elsevier Science 10.1016/j.gloplacha.2023.104180

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Naturally occurring acid rock drainage (ARD) appears to be promoted in permafrost environments with pyritebearing host rocks. However, it is poorly understood how the interaction between solid ice, meltwater and pyrite causes the formation of sulfuric acid and the subsequent mobilization of toxic elements. To elucidate the governing processes and to assess the general hazard of ARD in permafrost areas in the context of global warming, we present chemical water analyses for six high-alpine surface waters downstream of intact rock glaciers in the Central Eastern Alps. In addition, we provide a detailed chemical and structural characterization of nanocrystalline Al-hydroxysulfate precipitates forming along the streams and serving as a visual manifestation of ARD. Finally, we show results from column experiments, experimentally simulating the interaction between water and pyrite-bearing paragneisses. Studying ARD on both, the field and laboratory scales, demonstrated that under field-site conditions intact rock glaciers may act as highly efficient chemical reactors, resulting in high
concentrations of toxic elements such as aluminum, nickel, manganese, and fluorine in the sampled high-alpine streams. The most likely reason for the strong mobilization of these elements is their temporal storage and enrichment in the frozen rock glacier core, leading to a quick and focused export in summer when ice melt production rates are high. The analyses of the Al-hydroxysulfate precipitates confirmed the presence of basaluminite (Al4(OH)10(SO4) x 3 H2O), controlling the solubility of aluminum. Geochemical modeling allowed to quantify the inverse behavior of the basaluminite solubility with temperature, resulting in a field-derived standard reaction enthalpy (ΔrH◦) of - 580 kJ mol -1. This value can now be used to determine the solubility of aluminum also at temperatures relevant for permafrost settings (0–5 ◦C). Aerial photographs of the study sites
suggest a strong intensification of ARD in the Central Eastern Alps over the past 20 years, and that the flux of toxic elements mobilized from intact rock glaciers may further increase in the future. Owing to the high abundance of pyrite-bearing rocks, the same may apply to areas downstream of intact rock glaciers worldwide. In conclusion, the water quality downstream of intact rock glaciers with pyrite-bearing rocks must be carefully monitored to assess the future environmental hazard.

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