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Environmental behaviour of metals from tailings in shallow rivers: Taxco, central Mexico

Abstract Mining in Mexico has produced important economic benefits since pre-Hispanic times. On the other hand, tailings and rock wastes constitute potential pollution sources in many zones of the country. Taxco, about 150 km south-west of Mexico City, is one of the historical mining zones of Mexico. The environmental behaviour of metals from tailings in a branch of the Taxco river was investigated. Several communities use the Taxco river water along its flow. Samples were taken from tailings, river-waters and sediments. Field determinations included conductivity, pH, Eh, temperature and alkalinity of the water. Mineralogical and chemical analyses were performed in tailings and sediments. Water analyses included main ions, and metals in acidified filtered and unfiltered samples. Quartz, jarosite, pyrite, chalcopyrite, Fe oxihydroxides, gypsum, clay and silica minerals were identified in the tailings. All samples had an acid pH (2·50–4·16) and high concentrations of Pb (up to 2750 mg/kg), Zn (up to 690 mg/kg) and As (up to 3530 mg/kg). Mineralogical and chemical characteristics of tailings indicated occurrence of sulphide oxidation. Higher metal concentrations were found in unfiltered, acidified water samples than in filtered water samples in the year 2000: Fe (up to 21·2 mg/l), Zn (up to 13·4 mg/l) Pb (up to 0·115 mg/l) and As (up to 0·035 mg/l). Zn, Fe and Pb reached much higher concentrations in the dry season than in the rainy season. The Taxco river water does not comply with Mexican drinking water standards. High concentrations of Pb (up to 5280 mg/kg) and Zn (up to 33,500 mg/kg) were measured in sediments. Pb and Zn in sediments accumulated downflow in the dry season. A more uniform distribution was found in the rainy season. Results showed that metals are released from tailings as a result of sulphide oxidation and rain-water erosion. Metals are then transported by the river-water mainly in particulate form and, after settling, increase the concentration of metals in the sediments.