Geochemical distribution of arsenic, cadmium, lead and zinc in river sediments affected by tailings in Zimapan, a historical polymetalic mining zone of Mexico

摘要

In the historical mining zone of Zimapan, Mexico, unprotected tailings deposits are supplying contaminants to the local fluvial system. This research was conducted to assess the environmental hazard of these wastes and river sediments by determining the input, transport and seasonal variability of arsenic, cadmium, lead and zinc, and their speciation by an operationally defined scheme of decreasing lability: Fl, fraction soluble in deionized water; F2, associated to carbonates; F3, oxides and hydroxides of iron; F4, sulfides and organic matter; F5, residual. Higher total concentrations of Cd, Pb and Zn were present in sediments in the dry season regarding the rainy season. In the dry season, As and Pb were principally associated with the more stable F3 and F5 fractions, whereas Cd was in F2 and F5, and Zn in F3, F2 and F5. In the rainy season the association was mainly F3 for As, while F2 and F3 contained most Cd, Pb and Zn. This fractionation indicates that the environmental hazard of Cd, Pb and Zn enhances upon a pH decrease due to their proportion in the carbonatic fraction, and shows a mobility increase during the rainy season.