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Paper No. 7
Presentation Time: 08:30-18:30
ANTHROPOGENIC METAL CONTENT AND SEDIMENTOLOGY OF OVERBANK SEDIMENTS FROM THE MINING DISTRICT OF GUANAJUATO, MEXICO
In many mining districts, metal pollutants from mine tailings have historically been dumped into local streams. The purpose of this study was to determine the concentrations of Pb, Cu, Zn, Mo, Sb, Cs, Cr, Sc, Co, Ni, and Li in eight overbank-sediment profiles from the Guanajuato River, in the Guanajuato mining district and to establish geochemical background values using direct and indirect methods. The background levels calculated using direct method in pristine overbank deposits were similar to or slightly higher than the upper crustal values for Sc, Cr, Co, Ni, Zn and Mo (17.3, 108.7, 18.7, 44.4, 97.2, and 1.1 mg/kg, respectively). In contrast, levels of Cu, Cs and Pb were up to two times higher than crustal values (93.1, 11.2, and 37.5 mg/kg, respectively), and the median concentration of Sb was 8.2 mg/kg, up to 20 times the crustal level. These natural anomalies can be explained by the presence of mineral deposits in the Guanajuato mining district. Ni and Cr were provided by the ultramafic rocks of the Sierra de Guanajuato. The geochemical background established with indirect method of Sc, Cr, Co, Ni and Mo were lower than those found in the natural background. Moreover, the geochemical background established with indirect method of Cu, Zn, Sb, Cs and Pb were higher than those found in the geochemical background established with direct method. Contamination levels calculated by the geoaccumulation index (Igeo) indicated moderate to strong contamination for Cu, Zn, Sb and Pb in more than half of the samples. This contamination may be attributable to historical discharges of mine tailings in the Guanajuato River and the use of Cu during the metallurgical process of amalgamation. Our results provide evidence that anthropogenic metals stored in the fluvial plain of the Guanajuato River represent a major potential source of pollutants to surface and groundwaters downstream from the study area in the Lerma-Santiago basin.