Environmental impacts and metal exposure of aquatic ecosystems in rivers contaminated by small scale gold mining: the Puyango River basin, southern Ecuador
Nh. Tarras-wahlberg et al., Environmental impacts and metal exposure of aquatic ecosystems in rivers contaminated by small scale gold mining: the Puyango River basin, southern Ecuador, SCI TOTAL E, 278(1-3), 2001, pp. 239-261
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.