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

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.

An investigation of mercury sources in the Puyango-Tumbes River: Using stable Hg isotopes to characterize transboundary Hg pollution.

Mercury (Hg) concentrations and stable isotopes along with other trace metals were examined in environmental samples from Ecuador and Peru's shared Puyango-Tumbes River in order to determine the extent to which artisanal- and small-scale gold mining (ASGM) in Portovelo-Zaruma, Ecuador contributes to Hg pollution in the downstream aquatic ecosystem. Prior studies investigated the relationship between ASGM activities and downstream Hg pollution relying primarily on Hg concentration data. In this study, Hg isotopes revealed an isotopically heavy Hg signature with negligible mass independent fractionation (MIF) in downstream sediments, which was consistent with the signature observed in the ASGM source endmember. This signature was traced as far as ∼120 km downstream of Portovelo-Zaruma, demonstrating that Hg stable isotopes can be used as a tool to fingerprint and trace sources of Hg over vast distances in freshwater environments. The success of Hg isotopes as a source tracer in fresh waters is largely due to the particle-reactive nature of Hg. Furthermore, the magnitude and extent of downstream Hg, lead, copper and zinc contamination coupled with the Hg isotopes suggest that it is unlikely that the smaller artisanal-scale activities, which do not use cyanidation, are responsible for the pollution. More likely it is the scale of ores processed and the cyanide leaching, which can release other metals and enhance Hg transport, used during small-scale gold mining that is responsible. Thus, although artisanal- and small-scale gold mining occur in tandem in Portovelo-Zaruma, a distinction should be made between these two activities.