Mercury Pollution in the Amazon: Artisanal Small-Scale Gold Mining or Land-Use Changes?

Artisanal small-scale gold mining (ASGM) is a central economic activity throughout the developing world; however, it can lead to pollution of the environment through the use of Hg to extract the gold. In aquatic ecosystems this Hg can be methylated and bioaccumulate in aquatic food webs leading to high levels of methyl-mercury in the fish, which results in Hg exposure to humans and other organisms that rely on fish as a staple of their diet. Studies conducted in the Amazon show elevated levels of Hg in fish and sediment downstream of ASGM sites. Although most often attributed to Hg used in ASGM, the primary source of elevated Hg concentrations in Amazonian aquatic ecosystems is greatly contested since, until now, there have not been tools to differentiate between Hg from ASGM and Hg from other sources such as soil erosion. To directly assess the contribution of Hg use during ASGM, we applied novel Hg stable isotope analysis to lake sediment cores, river surface sediments, and nearby forest soils from two aquatic ecosystems in Amapá, Brazil, one downstream ASGM and one isolated from ASGM. Samples from the ASGM sites were also analyzed. Isotopic evidence suggests elevated Hg downstream of the ASGM site is dominantly from increased erosion of soils – and not directly from Hg used during gold extraction. Mercury concentrations were analyzed using a combustion-AA system. For isotopic analyses, the Hg from all samples was purified via combustion and trapped in an oxidizing permanganate solution. Mercury isotopes were measured using cold vapor multi-collector inductively coupled plasma mass spectrometry (CV-MC-ICP-MS) at the University of Toronto using sample standard bracketing and a thallium internal standard. As previously published, the river and lake downstream of the ASGM had elevated Hg compared with the upstream river samples and the control lake with no ASGM. For samples associated with ASGM with elevated Hg concentration, such as Hg sampled from a sluice box and from tailing ponds (fresh and old), the Hg is isotopically heavy compared to the upstream and control lake Hg and it is also consistent with Hg from ores (Smith et al., 2004). Tailing pond Hg is isotopically heavier than the fresh Hg (sluice box), suggesting isotopically light Hg is lost in the tailings ponds possibly through reduction and/or evaporation. However the heavy isotopic signatures associated with the ASGM samples is not detected in the lake and in most of the river samples downstream of the ASGM despite these samples having elevated Hg. One river sample, directly adjacent to a mining site, showed a small impact of ASGM in the isotopic composition and had one of the slightly higher Hg concentrations observed in the river. This suggests that the elevated Hg in the lake and most of the river sediments is not directly from Hg use during ASGM. The isotopic compositions observed in the lake and river samples downstream of the ASGM fall within the same range as the upstream river samples and the control lake suggesting they have similar sources (i.e, runoff, atmospheric deposition). Additionally they are similar to the Hg isotopic compositions observed in the forest soils. Instead of Hg use during ASGM, the isotope data suggests that the elevated Hg in the polluted lake is instead from erosion and runoff (assuming atmospheric deposition is the same for both lakes), likely due to land-use change associated with ASGM, agriculture, and cattle production. This study indicates that, through the analysis of stable Hg isotopes, it is possible to trace Hg from ASGM and to assess the extent to which it is impacting local ecosystems and food webs. Accordingly, the findings demonstrate that, in the Amazon, effective Hg mitigation strategies need to address deforestation and land-use practices in addition to ASGM.