Strosnider, WHJ; Llanos Lopez, FS; Nairn, RW
Intensive mining and processing of Ag, Sn, Pb and Zn ores
have occurred in various locations within and around the city of Potosi, Bolivia since 1545.
Surface and subsurface waters, stream sediments and soils are contaminated with various ecotoxic
metals in the headwaters of the economically vital, yet highly impacted, upper Rio Pilcomayo
watershed. Previous studies have documented downstream trace metal contamination, however, not
addressed specific sources. The AMD discharges identified in this study help link downstream
pollution to primary origins. The majority of AMD would be considered high-strength due to metal
concentrations and acidity orders of magnitude greater than typical AMD. Discharges from both
operating and abandoned portals as well as tailings-related deposits displayed a high degree of
heterogeneity with total metal concentrations ranging from 0.11 to 7480, <0.022 to 889, <0.0006
to 65.3, <0.001 to 310, 0.12 to 72,100, 0.3 to 402, <0.012 to 34.8, and 0.24 to 19,600 mg/ L of
Al, As, Cd, Cu, Fe, Mn, Pb and Zn, respectively. Net acidity and pH ranged from -10 to 246,000
mg/ L as CaCO3 equivalent and 0.90-6.94 standard units, respectively. Data were gathered during
two sampling events centered around the most extreme periods of the dry and wet seasons of one
water-year. Loadings to local streams were marginally greater for most metals in the wet season.
If observed loadings are historically representative, Cerro Rico AMD has contributed thousands of
tonnes of ecotoxic metals to the upper Rio Pilcomayo over the last five centuries. Metal and
hydrogen ion concentrations in the majority of AMD sampled were several orders of magnitude above
discharge limits set by the Bolivian government, yet no action has historically or contemporarily
been taken.