LEAD ISOTOPIC FINGERPRINTING OF MINE AND MILL TAILINGS IN AGRICULTURAL SOILS, RIO PILCOMAYO BASIN, BOLIVIA

Mining and milling debris from the Cerro Rico de Potosí precious metal-polymetallic tin deposits of southern Bolivia has been released to the headwaters of the Rio Pilcomayo for the past 450 years, resulting in extensive contamination of water, sediments and soils along the upstream reaches of the river. Previous investigations demonstrated that Pb isotopic data could be used to identify the primary sources of Pb to the aquatic environment, including that derived from the mines at Cerro Rico. This study builds on our earlier work by combining Pb isotopic data with sediment mixing models to quantify: (1) the relative contributions of Pb from mining and milling operations to agricultural soils within small riverine communities located on alluvial terraces along the Rio Pilcomayo, and (2) the primary mechanisms through which these soils are contaminated. The quantity of Pb from the mines parallel geographical patterns in Pb concentrations, decreasing from approximately 70 % of the Pb in soils within 25 km of Cerro Rico to less than 15 % of the Pb approximately 200 km downstream at Sotomayor. Fields located at lower-elevations, immediately adjacent to the river exhibit the largest quantities of Pb from the mines, presumably because these soils were contaminated both by the deposition of contaminated sediments during overbank flooding and by the use of contaminated irrigation waters. Nonetheless, as much as 30 % of the Pb in agricultural soils located within 150 km from the mines and mills was derived from the use of contaminated irrigation waters. Interestingly, soils in fields positioned above known flood levels and which are irrigated by “clean” waters possess some Pb from the mines. This Pb is likely to be derived from the eolian transport and deposition of contaminated alluvial sediments during the dry season.