NSF Award
0207439
0207439<br/>Miller<br/><br/><br/> The Rio Pilcomayo heads on the Cerro Rico de Potosi precious metal-polymetallic tin deposits of Bolivia. Mining of the Potosi deposits was initiated by the Spanish in 1545 and has continued without interruption to the present. Our previous studies have demonstrated that the release of mining and milling wastes to the environment during the past 450 years has resulted in extensive contamination of water and sediments of the upper Pilcomayo valley. The primary objective of this investigation is to determine if Pb isotopes can be effectively used in a large river system to quantify the decadal scale transport and storage of contaminated debris from mining operations. Inherent in the study is an analysis of the linkages between geomorphic processes and the long-term dispersal of contaminated materials.<br/><br/>To accomplish the project objectives several tasks will be completed: (1) Pb sources within the upper reaches of Rio Pilcomayo basin will be identified and characterized in terms of their geographical distribution and Pb isotopic signatures. This step will involve the collection and analysis of ore samples from Cerro Rico, the major bedrock units that underlie the watershed, mineralized zones within tributary basins, and channel bed sediments within major tributaries; (2) the alluvial stratigraphy and geomorphology of the tributary basins will be documented and linked to the existing geomorphic-stratigraphic data on the Rio Pilcomayo to (a) refine our understanding of the timing, magnitude, and nature of historic geomorphic events that have occurred along the Rio Pilcomayo, and (b) gain insights into the timing and magnitude of sediment influx to the Rio Pilcomayo from tributaries (both of which are believed to have had an impact on the downstream movement of contaminated sediment); (3) spatial variations in the total concentration of selected metals and Pb isotopic ratios will be documented perpendicular to flow, downstream, and vertically within age-constrained alluvial deposits. Emphasis will be placed on the analysis of 1.5-2 m long cores extracted from contaminated alluvial terraces composed of vertically accreted flood deposits; and (4) sediment mixing models will be used to quantify the relative proportion of Pb in alluvial deposits that is derived from each of the delineated Pb sources.
