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 the Puyango catchment area. Cyanide and metal levels in
rivers regularly exceed environmental quality criteria. The
contamination impacts biodiversity, with cyanide causing a direct
lethal effect on biota close to source and metal contaminants
considerably reducing aquatic biodiversity further downstream. It is
shown that the prevailing neutral or slightly alkaline conditions of
the rivers ensure that metals are mainly associated with sediment.
However, elevated metal levels in bottom living larvae collected from
contaminated sites suggest that these sediment bound metals are readily
bioavailable. Leaching experiments indicate that the relative ease by
which metals are taken up by larvae is related to the speciation of
sediment associated metals. It is further shown that large amounts of
metals, which are bound to suspended sediment under ambient pH
conditions, enter the dissolved and directly bioavailable state in more
acidic conditions. Metal levels in carnivorous fish were found to be
modestly elevated only, with the exception of mercury. 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 Science B.V.
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