TRANSPORT AND STORAGE OF CONTAMINATED MINING DEBRIS BY FLOOD PROCESSES, RIO TUPIZA BASIN, SOUTHERN BOLIVIA

Waste materials from historic mining operations were often disposed of in adjacent river valleys where the debris accumulated to the point of completely or partially blocking the flow of water along the channel. This study examined the downstream transport and storage of contaminated sediment following the erosion of mine tailings impounded within a small river valley at the Abarόa mine site located in southern Bolivia. The erosion, transport and deposition of metal contaminated debris occurred during an intense, but isolated rainstorm event in February, 2003. Topographic surveys following the event indicated that approximately 8,870 metric tons of contaminated mill tailings were transported through a bedrock channel to the Rio Chilco, a tributary to the Rio Tupiza. Paleoflood reconstructions estimated that peak flows within the channel reached 13.5 cms, and were relatively minor in comparison to bankfull flows typical of the Rio Chilco. Trace metal concentrations (including Pb, Zn, Hg, Sb, and As) within the modern channel bed sediments of the bedrock constrained channel semi-systematically decrease downstream of the Abaróa mine until reaching the Rio Chilco, where concentrations abruptly decrease. Metal concentrations within the channel bed sediments are similar to background concentrations within approximately 20 km of the Abaróa mine. However, Pb and Zn concentrations locally increase farther downstream as a result of the influx of mining debris from polymetallic mining operations within the Rio Tatasi basin. There was no evidence that the tailings failure at Abaróa led to overbank flooding and the deposition of contaminated sediments on the floodplains of the Rio Chilco or the Rio Tupiza. Nonetheless, analyses of agricultural soils demonstrate that these materials exhibit metal concentrations exceeding Dutch and Canadian guideline values, presumably as a result of the influx of historic mining debris.