Mercury isotopic signatures of tailings from artisanal and small-scale gold mining (ASGM) in southwestern Ecuador

Gary Schudel; Robert Kaplan; Rebecca Adler Miserendino; Marcello M Veiga; P Colon Velasquez-López; Jean Remy Davée Guimarães; Bridget A Bergquist

doi:10.1016/j.scitotenv.2019.06.004

Mercury isotopic signatures of tailings from artisanal and small-scale gold mining (ASGM) in southwestern Ecuador

Sci Total Environ. 2019 Oct 10:686:301-310. doi: 10.1016/j.scitotenv.2019.06.004. Epub 2019 Jun 3.

Authors

Gary Schudel¹, Robert Kaplan¹, Rebecca Adler Miserendino², Marcello M Veiga³, P Colon Velasquez-López⁴, Jean Remy Davée Guimarães⁵, Bridget A Bergquist⁶

Affiliations

¹ University of Toronto, Department of Earth Sciences, 22 Russell Street, Toronto M5S 3B1, ON, Canada.
² University of Toronto, Department of Earth Sciences, 22 Russell Street, Toronto M5S 3B1, ON, Canada; Johns Hopkins University Bloomberg School of Public Health, Department of Environmental Health Sciences, 615 N. Wolfe Street, Baltimore 21205-2103, MD, USA.
³ University of British Columbia, Norman B. Keevil Institute of Mining Engineering, Vancouver, BC V6T IZ4, Canada.
⁴ Universidad Tecnica de Machala, Avenida Paquisha Km 5,5 via Pasaje-Machala, Machala, El Oro, Ecuador.
⁵ Universidade Federal do Rio de Janeiro, Inst. de Biofísica Carlos Chagas Filho, Bloco G, CCS, Ilha do Fundão, Rio de Janeiro 21949-902, Brazil.
⁶ University of Toronto, Department of Earth Sciences, 22 Russell Street, Toronto M5S 3B1, ON, Canada. Electronic address: bergquist@es.utoronto.ca.

PMID: 31181517
DOI: 10.1016/j.scitotenv.2019.06.004

Abstract

Artisanal and small-scale gold mining (ASGM) is currently one of the largest anthropogenic sources of mercury (Hg) to the atmosphere and a significant source to downstream terrestrial aquatic systems. Given the potential toxicity of Hg when it is released to aquatic ecosystems, both scientific and regulatory communities have interest in the development of tools and methods for understanding the fate and transport of Hg in the environment. Although Hg isotopes are applied extensively to trace other anthropogenic point sources of Hg in aquatic ecosystems, few studies have used isotopic analyses to investigate the downstream impacts of ASGM. Furthermore, these studies are currently limited by relatively sparse Hg isotopic analyses on ASGM materials. In this study, ASGM samples from Portovelo-Zaruma (Ecuador), representing various stages of the processing of low-grade gold-bearing ores, were analyzed for Hg concentrations and stable isotope ratios. These data were used to assess the isotopic variability of the ASGM endmember and confirm the results of prior isotopic analyses of ASGM materials and downstream sediments from Portovelo-Zaruma. While the Hg concentrations of the ASGM samples varied significantly, isotopic analyses revealed a limited range of signatures that was characterized by relatively heavy mass-dependent fractionation (MDF) with little to no significant mass-independent fractionation (MIF). The signatures of ASGM samples analyzed in this study matched well with previously reported ASGM samples from Portovelo-Zaruma as well as downstream sediments (for up to 120 km). Furthermore, the overall Hg isotopic compositions of potential ASGM endmembers are distinct from typical compositions observed in freshwater ecosystems (e.g., from soil erosion, atmospheric deposition), allowing Hg isotopes to be a powerful tool in tracing downstream Hg contamination from ASGM.

Keywords: Environmental pollution; Mass independent fractionation; Mercury amalgamation; Metal geochemistry; Mine wastes; Stable isotopes.