Water–Rock\nInteraction and Freeze–Thaw\nCycles as Drivers of Acid Rock Drainage Generation by a Rock Glacier\nin the European Alps

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