Water quality impacts of in-stream mine tailings on a headwater tributary of the Rio Pilcomayo, Potosí, Bolivia

Abstract

Rio Tarapaya, a major tributary of the upper Rio Pilcomayo and a crucial water resource in the arid Southeastern Bolivian highlands, has been impacted by in-stream mine tailings and acid rock drainage over the past five centuries, from historic and ongoing mining at Cerro Rico de Potosí. Identification of the effects of in-stream mine tailings on the waters of Rio Tarapaya is essential to better understand the dynamics of these complex systems and propose successful management strategies. A study was performed on river waters and sediments at six sites in a reach of Rio Tarapaya, beginning at the city limits of Potosí and ending just after the confluence with Rio Cayara, a relatively unimpaired river 7.3 km downstream. Concentrations of dissolved metals (such as Fe, Al, Mn, and As) in the Ca–Mg–HCO3–SO4 dominated stream were generally orders of magnitude greater than Bolivian receiving water body criteria and United Nations Food and Agriculture Organization irrigation and livestock drinking water guidelines. Variability of Ca and Mg in water and sediment was controlled by dissolution of gypsum, dolomite and calcite, which was supported by Spearman correlation coefficients. The primary ionic products of in-stream sulfide mineral weathering (SO42−, Fe, Al, Mn, Zn and As), are the dominant ions of stream water and sediment leachate. Results are consistent with PHREEQC modeling of minerals such as gibbsite, melanterite, goethite, calcite and dolomite. In general, the reactivity of bed material increased and stream water quality decreased with increasing distance from Potosí. Metals and metalloids are spiraling downstream via seasonally driven particle transport, sulfide weathering, and salt dissolution coupled with biotic interactions. The reactivity of the millions of tonnes of in-stream tailings in Rio Tarapaya and persistence of contemporary illicit tailings discharges indicates that the upper Rio Pilcomayo will remain severely degraded for the foreseeable future unless substantial remediation is undertaken.