Seasonal contamination of surface waters close to an abandoned Sn-W mine, northeast Portugal

The abandoned Ribeira mine consists of Sn-W quartz veins and is located in Trás-os-Montes, northeast Portugal. The mining activities took place on the northern side of the Viveiros stream valley that crosses the mining area. Viveiros is a tributary stream of the Sabor River, which is about 1 km downstream from the mining site. Tailings and rejected materials were deposited on northern hillside of the valley and are exposed to significant weathering and erosion. The Sn-W quartz veins intruded Silurian quartzites and shales and contain cassiterite, wolframite, sphalerite, scheelite, arsenopyrite, pyrite, chalcopyrite, columbite-(Mn) and bismuthinite. Water samples from the Viveiros stream, Sabor River and tailings seasonal drainages were collected. The water has a sodium bicarbonated to calcium sulphated facies, under maximum tailings influence. The sulphate content is low in these waters (up to 145 mg/L). The pH values are close to neutrality, and the maximum electrical conductivity is 303 μS/cm. The metal and semimetals contents in waters show a seasonal variation characterized by low values in the wet and cold season and high contents in the hot and dry season. The contents of Mn, Cu, Zn and Cd found on the tailings seasonal water drainages are higher than those found in the Viveiros stream and Sabor River in the same season. The highest contents of Cd are associated with the highest contents of Zn. These results point to arsenopyrite and sphalerite dissolution as the principal source of metals and semimetals in these waters. The principal component analysis shows a clear spatial and seasonal distinction between the water collected close to the mine influence in the summer period and the other waters. During this period, the Viveiros stream shows As, Cd and Pb contents (e.g. As 0.29; Cd 0.033 and Pb 0.55 mg/L) higher than those permitted by WHO (Guidelines for drinking-water quality, 4th edn (WEB version), 2011 ) and the Portuguese legislation. Lead, Cd and As are potential threats to the environment due to long-term loadings into the aquatic system. The differences found in the geochemistry of impacted mine water and in its contamination are well expressed if seasonality is considered.