EFFECTS OF TRIBUTARY INSTABILITY AND SEDIMENT DELIVERY ON HEAVY METAL TRANSPORT IN THE RIO PILCOMAYO, BOLIVIA

The Rio Pilcomayo heads on the Cerro Rico precious metal-polymetallic tin deposits near Potosí, Bolivia in the central Andes. Mining of the Cerro Rico deposits was initiated by the Spanish and has continued without interruption since 1545. Waste materials from the mines and mills were, and continue to be, released directly into head tributaries to the Rio Pilcomayo. This investigation documents the magnitude of heavy metal contamination within alluvial deposits of the Rio Pilcomayo, and assesses the importance of geomorphic processes in the downstream transport and storage of contaminated sediments. Sediment samples collected since 1998 demonstrate that the release of mining and milling wastes to the Rio Pilcomayo has resulted in extensive contamination of water, sediments, and soils of the upper Pilcomayo valley. Concentrations of Cu, Zn, As, Ag, Cd, Hg, Sb, Tl, and Pb in channel bed sediments are particularly elevated, exceeding background values by as much as 1 to 2 orders of magnitude. The trace metal concentrations in channel bed sediments decline rapidly between 140 km and 200 km downstream due to (1) the dilution associated with the influx of sediment from tributaries, and (2) the increased storage of contaminated sediment in the area as a result of long-term aggradation. Preliminary geomorphic data indicate that erosional and depositional processes are spatially out of phase between the tributaries and the mainstem river. Tributaries have experienced significant and ongoing incision to depths as great as 15 meters. Radiocarbon dates indicate that prolonged period of Holocene sediment deposition in tributary valleys began after 9640 years B.P. (based on 6 dates) and that incision began approximately 2,580 years B.P. (based on an average of 3 dates). The influx of sediment from the rapidly degrading tributaries has been transported to the mainstem of the Rio Pilcomayo and is generating aggradation in the middle Pilcomayo. Moreover, these tributary sediments areapparently leading to the storage and dilution of contaminated sediment in the alluvial valley. Models of sediment transport in braided river systems suggest that sediment within the aggradational reach of the Rio Pilcomayo will subsequently be remobilized and transported downstream as a wave of erosion and deposition.