Transport and Fate along the Rio Pilcomayo
The release of mining and milling wastes to the Rio Pilcomayo from the mines and mills at Cerro Rico have resulted in extensive contamination of water, sediments, and soils of the upper Pilcomayo valley. Aqueous concentrations of As, Sb, Cd, and Pb generally exceed WHO guidelines for safe drinking water downstream to Puente Sucre, and in upstream areas, the guidelines are exceeded by several fold. Concentrations of Cu, Zn, As, Ag, Cd, Hg, Sb, Tl, and Pb in high and low channel bed sediments are also elevated above background values. In fact, in upstream reaches, background values are exceeded by 1 to 2 orders of magnitude. Downstream of Puente Sucre concentrations decrease significantly. Cu, Zn, and Pb are only slight elevated, whereas Ag, Cd, Sb, and Tl cannot be distinguished from background (pre-mining) values. With the exception of Hg, the concentrations of metals within the historic (post-1545) terrace deposits are generally lower than those found in the contemporary channel bed sediments. High Hg concentrations within the terrace deposits are presumably related to the release of Hg from amalgamation mining activities that ceased around 1900.
Rapid declinces in metal concentrations were noted about 150 km downstream of Potosi. These decreases in metal levels have been attributed to a combination of geomorphic and geochemical factors including (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 (Hudson-Edwards et al., 2001). With regards to the latter, it appears that erosional and depositional processes are spatially out of phase along the channel and reflect geomorphic responses to environmental change. The differing responses include upstream incision, mid-reach aggradation (near Puente Sucre), and renewed downstream incision.
Samples were also collected in August of 2000 upstream of Uyuni and analyzed for selected Pb isotopes, in addition to the suite of heavy metals previously analyzed for in 1998. The spatial trends exhibited by the data collected in 2000 are similar to those described above, but the total concentrations were generally higher. The Pb isotopic data were used to identify the primary sources of Pb to the aquatic environment and to determine their relative contributions to pre- and post mining alluvial deposits. The plotting of 206Pb/207Pb and 206Pb/208Pb data from pre-mining terrace deposits generated a linear trend. This linear correlation suggests mixing between the a natural (background) Pb component and Pb from Cerro Rico. Isotopic data collected from modern channel bed sediments indicates that a new source of Pb was introduced to the river from Cerro Rico since mining and milling operations began in 1545 (as expected). It is hypothesized that the change is the result of differences in the isotopic composition of the ore deposits and the rocks on the surface of Cerro Rico that could have been eroded prior to the onset of mining operations. A simple mixing model was used to estimate the fraction of Pb released from mining and milling operations. The modeling results suggest that Pb from mining activities comprises between 30 and 89 % of the Pb in the channel bed materials.
Currently, we are building on our existing data from the Rio Pilcomayo basin to determine if Pb isotopes can be used to separate the influx of Pb from several different sources, and to determine if the isotopes can be used to track heavy metals from the mines into agricultural fields and produce. Specific goals are to: (1) refine our understanding of Pb sources (and their Pb isotopic signatures) within the upper reaches of Rio Pilcomayo basin, (2) document the spatial variations in selected Pb isotopic ratios that exist perpendicular to flow, downstream, and vertically within age constrained alluvial deposits, (3) use sediment mixing models to quantify the proportion of Pb in channel bed and alluvial terrace deposits that is derived from each of the delineated sources, (4) develop a basic understanding of climatic fluctuations, land-use changes, and mining operations within the basin by synthesizing existing climatic datasets and by summarizing historical documents; and (5) combine geomorphic, geochemical and historical data to identify the primary controls on the long-term kinematics of sediment migration, and to conceptually model the transport and storage dynamics of contaminated sediment within the Rio Pilcomayo watershed.
Contamination of Agricultural Farm Fields and Produce
investigation was initiated in 2001 and 2002 to assess the potential human
health affects of metal contamination on four communities (including Mondragon,
Tasapampa, Tuero Chico, and Sotomayor) located along the upper Río
Pilcomayo by examining the potential significance of human exposure pathways
associated with soils, crops, and water (including river, irrigation,
and drinking water supplies). The most significantly contaminated agricultural
soils occurred upstream at Mondragon (close to Potosi) where Cd, Pb, and
Zn concentrations exceed recommended guideline values for agricultural
use. Further downstream the degree of contamination decreases, and metal
concentrations are below Dutch, German and Canadian guideline values.
Metal concentrations in agricultural products from the four communities
were generally below existing guidelines for heavy metal content in commercially-sold
vegetables. Thus, the consumption of contaminated produce does not appear
to represent a significant exposure pathway in most communities. An exception,
however, may be Pb. In carrots, lettuce, and beetroots from Sotomayor
and Tuero Chico, more than 35 % of the samples exceeded recommended guidelines.
Most communities obtain drinking water from sources other than the Río
Pilcomayo. In general, dissolved concentrations of metals in drinking
water from the four studied communities are below the WHO guideline values,
with the exception of Sb, which was high at Tasapampa. The inadvertent
ingestion of contaminated water from irrigation canals and the Río
Pilcomayo represents a potential exposure pathway, but its significance
is thought to be minimal. Given the degree of soil contamination in the
area, perhaps the most significant exposure pathway is the ingestion of
contaminated soil particles, particularly particles attached to, and consumed
with, vegetables. The risks associated with this pathway can be reduced
by thoroughly washing or peeling the vegetables prior to consumption.
Other exposure pathways that are currently under investigation include
the consumption of contaminated meat from livestock and poultry which
drink polluted waters, and the ingestion of contaminated, wind-blown dust.