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Cyanide Contamination of the Puyango-Tumbes River Caused by Artisanal Gold Mining in Portovelo-Zaruma, Ecuador


Purpose of Review

To examine cyanide (CN-) contamination of a large river in southeastern Ecuador that has been severely impacted by the indiscriminate discharge of untreated gold processing effluents.

Recent Findings

Poor environmental stewardship of cyanide (CN-) use by 87 gold processing centers in Portovelo-Zaruma in southern Ecuador to leach residual gold from Hg-contaminated tailings has resulted in high CN- contamination of the Puyango-Tumbes River downstream from the centers.


Free CN- concentrations were high in river surface waters for 50 km below the processing plants, with the highest concentration 9088 times above the CCME standard of 5 μg/L and 1136 times above the 24-h LC50 concentration of 40 μg/L free CN- for some fish species. Due to cyanidation of mercury-contaminated tailings, the formation of Hg-CN complexes is a grave concern, as these complexes have been shown to be highly bioavailable. Preliminary tests conducted in a laboratory using bioassays with Danio rerio sp. and varying concentrations of synthetic Hg(CN)2 salt have shown significant THg bioaccumulation in muscle and kidney fish tissues exposed to concentrations >0.12 mg/L. Furthermore, low MeHg results in fish tissues demonstrated little occurrence of methylation and that the bulk of the total mercury content was in the form of inorganic mercury. Although construction of a communal tailings facility (CTF) in Portovelo is a positive development to reduce riverine pollution, the requirement to truck tailings up to the CTF likely results in inadequate compliance of environmental regulations that are poorly enforced.

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The authors would like to acknowledge the support of the United Nations Industrial Development Organization (UNIDO) for this research (Contract No. 3000018026, Project No./SAP ID 100271). In addition, many thanks to China Gold International Resources Corp. Ltd. for their financial support of part of this research. We would also like to thank INIGEMM in Ecuador for conducting the analyses of the water samples. Finally, thanks to Mr. Danilo Castilho from the Association of Owners of the Processing Plants (APROPLASMIN) for support in the field and Robert Kaplan for his part in the field collections.

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Correspondence to Bruce G. Marshall.

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Marshall, B.G., Veiga, M.M., da Silva, H.A.M. et al. Cyanide Contamination of the Puyango-Tumbes River Caused by Artisanal Gold Mining in Portovelo-Zaruma, Ecuador. Curr Envir Health Rpt 7, 303–310 (2020).

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  • Mercury cyanide
  • Gold processing centers
  • Artisanal miners
  • Tailings management