Contamination of surface waters by mining wastes in the Milluni Valley (Cordillera Real, Bolivia) : mineralogical and hydrological influences

This study is one of very few dealing with mining waste contamination in high altitude, tropical-latitude areas exploited during the last century. Geochemical, mineralogical and hydrological characterizations of potentially harmful elements (PHEs) in surface waters and sediments were performed in the Milluni Valley (main reservoir of water supply of La Paz, Bolivia, 4000 m a.s.l.), throughout different seasons during 2002 - 2004 to identify contamination sources and sinks, and contamination control parameters. PHE concentrations greatly exceeded the World Health Organization water guidelines for human consumption. The very acidic conditions, which resulted from the oxidation of sulfide minerals in mining waste, favoured the enrichment of dissolved PHEs (Cd > Zn >> As >> Cu similar to Ni > Pb > Sn) in surface waters downstream from the mine. Stream and Like sediments, mining waste and bedrock showed the highest PHE content in the mining area. With the exception of Fe, the PHEs were derived from specific minerals (Fe, pyrite; Zn, Cd, sphalerite, As, Fe, arsenopyrite, Cu, Fe, chalcopyrite, Pb, galena, Sn, cassiterite), but the mining was responsible for PHEs availability. Most of the PHEs were extremely mobile (As > Fe > Ph > Cd > Zn similar to Cu > Sn) in the mining wastes and the sediments downstream from the mine. pH and oxyhydroxides mainly explained the contrasted availability of Zn (mostly in labile fractions) and As (associated with Fe-oxyhydroxides). Unexpectedly, Pb, Zn, As, and Fe were significantly attenuated by organic matter in acidic lake sediments. Hydrological conditions highly influenced the behaviours of major elements and PHEs. During wet seasons, major elements were diluted by meteoric waters, whereas PHEs increased due to the dissolution of sulfides and unstable tertiary minerals that formed during dry seasons. This is particularly obvious at the beginning of the wet season and contributes to flushes of element transport downstream. The high altitude of the study area compensates for the tropical latitude, rendering the geochemical behaviour of contaminants similar to that of temperate and cold regions. These results might be representative of geochemical processes in ore deposits located in the high Andes plateau, and of their influence on PHE concentrations within the upper Amazon basin. Although mining activities in this region stopped 10 years ago, the impact of mining waste on water quality remains a serious environmental problem