Water–Rock Interaction and Freeze–Thaw Cycles as Drivers of Acid Rock Drainage Generation by a Rock Glacier in the European Alps

Little is known to date about the processes governing natural acid rock drainage (NARD) generated by rock glaciers. We used paragneiss samples from a catchment with NARD generated by a rock glacier in the Italian Alps for long-term leaching experiments under conditions that are possible within rock glaciers. The findings clearly suggest that at a low acid neutralization capacity of the rock, the dissolution of sulfide minerals, even if they are present in trace amounts, may be the most important process that controls the groundwater acidity at 1 °C, a typical temperature of groundwater discharge from rock glaciers. The acidic conditions increase the solubility and mobility of aquifer lithology-specific trace elements, and concentrations of some heavy metals of geogenic origin (e.g., Mn and Ni) may greatly exceed health standards after a six month interaction of water with paragneisses. Diurnal freeze-thaw cycles were found to be 6-7 times more effective in transformation of coarse rock fragments to fine-grained debris with fresh, reactive mineral surfaces, compared with temperatures above freezing. Cyclic freezing favors an enhanced formation of amorphous silica, a highly effective adsorbent for metal ions, and its redissolution within unfrozen layers of rock glaciers may represent an additional source of trace elements.