Legacy at abandoned mines: impact of mine wastes on surface waters

摘要

In the Iglesiente district (SW Sardinia, Italy), massive Pb-Zn sulphide deposits, hosted in Lower Cambrian limestone and dolostone, have been exploited intensively from 1870 to 1995. The cessation of mining left large quantities of tailings and mine waste dumps. Among the tailings, the "Red Muds" derive from electrolytic processing to recover zinc. These materials contain high amounts of iron oxides and metals, such as 8.8 percent Zn, 1.1 percent Pb, and 0.04 percent Cd. Seeps from the "Red Muds" show near-neutral pH and extremely high concentrations of sulphate (8000 mg/l), Mg (1400 mg/l), Zn (580 mg/l), Cd (2.5 mg/l), and Pb (0.4 mg/l). Although the flow rates of tailings drainages are usually very low (< 0.1 l/s), the contribution to dissolved concentrations of SO_4, Zn, Cd, and Pb in the Rio San Giorgio, the main stream that drains the mining district, is very high. At present, the weathering of electrolytic and flotation tailings represents a relevant hazard to the aquatic system in the Iglesias valley.