Distribution and behavior of arsenic in soils and waters in the vicinity of the former gold-arsenic mine of Salanfe, Western Switzerland

The mine wastes and their environmental impact described in this paper are related to a skarn-type ore deposit, located in the Variscan crystalline basement of the Aiguilles Rouges Massive, Western Switzerland. The main minerals of the deposit are arsenopyrite, löllingite, hematite, scheelite, magnetite, pyroxene, amphibole and dolomite. The ore has been mined for arsenic and gold between 1900 and 1928, producing about 700 t As and 55 kg Au, and leaving mine dumps of several hundred m3, situated on a relatively steep hill above the small Lake Ottans, at an altitude of 2200 m. This work was undertaken to determine the extent of the As-contamination, its environmental impact and to identify actual geochemical processes. Dump materials contain As and sulfur in the 10% range, contaminated soils contain between 50 mg/kg and several % As. Mine and dump percolating waters are weakly mineralized of the Ca–Mg–HCO3–SO4-type and have conductivities between 20 and 170 μS/cm, pH values around 8, and As contents varying from 60 to 4000 μg/l. Waters below the dump zone are of the same type and have slightly lower As-contents: in Lake Ottans, 200 m below the mine area, As is still between 100 and 160 μg/l. Our results indicate that: (1) the contaminated area is limited to about 500 m in radius (1.2 km2) and that its environmental impact is rather limited, except for grazing cattle, (2) below the mine, the As decreases more rapidly in the soils than in the waters, (3) the distribution of the As seems to be determined by predominant alkaline pH in the surface and ground waters and by the distribution of the secondary solid phases (Fe-oxy-hydroxides, clays, carbonate minerals, especially in the lower part). Transport by gravity creep or as suspended particles in water seems to be of secondary importance.