Environmental impacts and metal exposure of aquatic ecosystems in rivers contaminated by small scale gold mining: the Puyango River basin, southern Ecuador

Gold mining in the Portovelo-Zaruma district in southern Ecuador is causing considerable environmental impacts; the most important ones are related to the discharge of cyanide, mercury and metal rich tailings into rivers of t he Puyango catchment area. Cyanide and metal levels in rivers regularly exc eed environmental quality criteria. The contamination impacts biodiversity, with cyanide causing a direct lethal effect on biota close to source and m etal contaminants considerably reducing aquatic biodiversity further downst ream. It is shown that the prevailing neutral or slightly alkaline conditio ns of the rivers ensure that metals are mainly associated with sediment. Ho wever, elevated metal levels in bottom living larvae collected from contami nated sites suggest that these sediment bound metals are readily bioavailab le. Leaching experiments indicate that the relative ease by which metals ar e taken up by larvae is related to the speciation of sediment associated me tals. It is further shown that large amounts of metals, which are bound to suspended sediment under ambient pH conditions, enter the dissolved and dir ectly bioavailable state in more acidic conditions. Metal levels in carnivo rous fish were found to be modestly elevated only, with the exception of me rcury. Mercury levels exceeded 0.5 mg/kg in fish from both contaminated and uncontaminated sites, showing that both methylation and bioaccumulation of mercury are occurring in the Puyango river basin. (C) 2001 Elsevier Scienc e B.V. All rights reserved.