Sources, distribution and storage of heavy metals in the Rio Pilcomayo, Bolivia

The Rio Pilcomayo rises on the Cerro Rico de Potosi precious metal-polymetallic tin deposits of Bolivia, and flows in a southeasterly direction for ca. 600 km to Bolivia's southern border with Argentina. Mining of the Potosi deposits has occurred continuously since 1545, generating large quantities of waste materials in the headwater of the basin. In addition, a tailings dam breach at the Porco mine in 1996 released an estimated 235 000 m(3) of tailings and fluid into the upper reaches of the Rio Pilaya, the largest tributary to the Pilcomayo. Concentrations of As, Sb, Cd, Cu, Pb, Hg, Ag, T and Zn in contemporary channel sediments upstream of the Pilaya confluence are significantly elevated above background values. Elevated levels appear to be associated with pyrite- and other sulphide mineral-bearing tailings materials transported more than 200 km downstream of the Potosi mines. Significant downstream declines in elemental concentrations occur within 15 km, and again between 150 and 200 km, from the mines. The initial decrease in concentrations is due to the rapid dilution of nearly pure tailings effluent released to the river from milling facilities near Potosi. The latter decrease results from a combination of geomorphic processes including the storage of sediment-borne metals within the channel bed and the influx of 'clean' sediment from several large tributaries. Downstream of the Pilaya confluence, concentrations of Cu, Pb, Hg and Zn are only slightly elevated above background values, and Ag, Cd, Sb and Tl cannot be distinguished from background levels. These data suggest that while the Porco tailings spill may have had a significant short-term impact on sediment and water quality along the lower reaches of the Rio Pilcomayo, its longer-term impacts were limited. Metals stored and eroded from alluvial deposits of historical age in upstream reaches appear to be an important source of metals to the river today. An additional, and perhaps mow significant source, is the release of tailings effluent to the river from modem milling operations. The transport of these contaminants downstream of IcIa (203 km from Potosi) appears to be restricted by aggradational processes occurring in the vicinity of Puente Sucre. In addition, downstream of the confluence of the Rio Pilaya, inputs of large amounts of 'clean' sediment have caused dilution of the metal contaminants. Data from other studies where similar geomorphic processes have occurred suggest that the metals in the upper Pilcomayo may eventually be moved downvalley as the aggradational processes are reversed and channel stabilisation occurs. Thus, the most significant impacts of metal contamination may not be realised in downstream areas for decades. (C) 2001 Elsevier Science B.V. All rights reserved.