Impatti ambientali delle aree minerarie dismesse

Environmental issues related to abandoned zinc-lead mining operations in the northeastern italian alps

Pavoni E., Colle Fontana M., Cattelan R., Esbrí J.M., Petranich E., Emili A., Higueras P., Covelli S., 2014. Environmental issues related to abandoned zinc-lead mining operations in the northeastern italian alps. Environment Knowledge Week Congress – E2KW2014 - (163-165), Toledo (Spagna), 30-31 October 2014.
The Salafossa mine is located in the eastern Dolomites belonging to the Alpine chain, within the territory of the Veneto region, NE Italy. The mineral body exploited in Salafossa was one of the largest lead-zinc-containing mineral deposits in Europe. The two metals are in the form of sulphides (sphalerite, ZnS and galena, PbS). Mining activity started around 1550 but it was only around 1960 when the richest veins of the minerals were discovered. From the Salafossa mineral deposits, a little more than 11 million tons of tout-venant (ore body), with an average content of 0.9% for Pb and 4.7% Zn, were obtained. In total, the mineral obtained in the mine amounted to 92,000 tons of Pb and 482,000 of Zn. Mining activity continued until 1985, the year of final closure. The first survey (2011) involved sampling of drainage waters (DWs) from both the mine and the Piave River, the major tributary downstream the mine, but also of sediments from the same river and from the bottom of the mine tunnels. The second survey (2014) was designed to recover soils, metallurgical and waste tailings in the area outside the mine, where metal extraction activities were in operation. Very high concentrations of Zn (20.776 mg kg-1), Pb (6.212 mg kg-1), as well as other metals (As, Cd and Tl) were found in the solid samples collected at the bottom of one of the main mine tunnels and close to a deposit of tailings deriving from the ore body (13.283 mg kg-1 Pb and 54.610 mg kg-1 Zn), well above the local “base level" assumed to be the fluvial sediments of the Piave River (2.34 and 115 mg kg-1 of Pb and Zn, respectively). Samples fall between alkaline earth sulphate and alkaline earth bicarbonate waters, except for the Piave River freshwaters which belong to the alkaline earth bicarbonate group. pH values fall within 7 and 8. Zinc was found to be significantly high (up to 1000 µg L-1) in drainage waters, and it is of two orders of magnitude higher than Piave River freshwaters. Conversely, Pb concentrations were below the detection limit with a maximum of 4 µg L-1, probably due to the lower mobility of this element with respect to Zn. Very high concentrations (up to 27 µg L-1) of Tl, a highly toxic element, were found in the DWs in both surveys. Thallium is a minor constituent of sulphides and it is naturally present in the excavated rocks as well as in tailings (63-113 mg kg-1). Since Tl is partially soluble in water (Ksp = 3.6 × 10-4), it can mobilise quite easily, compared to the other metals, moving from enriched solid matrices to drainage waters, with dispersion in the environment and possible accumulation in soils and plants. Metallurgical tailings reach higher levels than in soils for Zn (428,347 mg kg-1) and Pb (46,157 mg kg-1), with significant contents of other metals (As, Cd, Cu, Tl). Tl, especially, shows contents in the range 7 – 789 mg kg-1. Ca is present as a major element in these ore body and the metallurgical tailings (273.176 mg kg-1 on avg.), reflecting the composition of carbonated host rocks. In spite of being an abandoned mine of sulphides, there is no evidence of acid drainage waters in Salafossa at the moment, probably because of the buffering effect produced by carbonate host rocks.