Evaluation of particle dispersal from mining and milling operations using lead isotopic fingerprinting techniques, Rio Pilcomayo Basin, Bolivia

Mining and milling of ores from the Cerro Rico de Potosí precious metal–polymetallic tin deposits of Bolivia have led to severe contamination of water and sediments of the Rio Pilcomayo drainage system. Lead (Pb) isotopic data were used in this study to first document downstream dispersal patterns of Pb contaminated sediment within the channel of the Rio Pilcomayo, and then to determine the relative contribution of Pb from Cerro Rico within alluvial terrace soils that are used for agriculture. The concentration and isotopic composition of Pb within channel bed sediments differed significantly between 2000, 2002, and 2004. These differences presumably reflect changes in the type of ore mined and milled at Cerro Rico, and alterations in dispersal and grain-size dilution mechanisms associated with interannual variations in rainfall and runoff. Within agricultural terrace soils, both Pb concentrations and the percentage of Pb from Cerro Rico: (1) semi-systematically decrease downstream, (2) were found to decrease with terrace height above the channel, and (3) reflect the use of contaminated irrigation water. In upstream reaches (within 30 km of the mills), Pb from mining represents the most significant Pb source, accounting for more than 80% of Pb in the examined agricultural fields. At Sotomayor, located approximately 170 km from the mills, the relative contribution of Pb from Cerro Rico is highly variable between fields, but can be significant, ranging from approximately 15% to 35%. The analysis demonstrates that Pb isotopic ratios can be used to effectively trace contaminated particles through river systems and into adjacent alluvial soils, even where multiple Pb sources exist and Pb concentrations are similar to background values.