LB 19-187 - High levels of mercury in Peruvian Amazon forests from artisanal gold mining

Background/Question/Methods:

A growing threat to tropical forests is artisanal and small-scale gold mining (ASGM), which leads to widespread deforestation and the largest global source of anthropogenic mercury (Hg) emissions. Mercury is a potent neurotoxin that can impact both people and wildlife. However, little is known about the fate and impact of these atmospheric Hg emissions within forests located near ASGM. We asked: (1) Are there patterns in Hg deposition in relation to proximity to ASGM or leaf area index of forest canopies? (2) Is there evidence for Hg bioaccumulation in forest-dwelling resident songbirds near ASGM activity? We collected bulk precipitation, throughfall, litterfall, soil, and songbird feathers in the Madre de Dios region of Peru. Samples were collected across two years from five sites, representing locations both near and far from ASGM activity and containing plant communities of both primary and secondary forests.

Results/Conclusions:

We found that Hg deposition into forests was dependent on both proximity to mining and leaf area index. Intact forests near ASGM had the highest ever reported annual Hg fluxes via throughfall (71 μg Hg m^-2 yr^-1) and among the highest levels of annual Hg fluxes via all pathways (bulk precipitation, throughfall, litterfall; 137 μg Hg m^-2 yr^-1), as well as elevated total Hg and MeHg levels in soils (9100 μg total Hg m^-2). Due to the high Hg inputs and storage coupled with Hg methylation rates similar to those found in temperate forests, we found that resident songbird Hg concentrations in these forests were 2-12 times higher than those in remote forests (distant from ASGM) and sometimes exceeded thresholds known to impact reproductive success. While the fate of Hg within these forests is concerning, these results suggest the importance of forests for sequestering Hg. If the forests are burned or cleared, Hg would likely enter nearby aquatic ecosystems. These results raise important questions about the impact of Hg pollution on terrestrial ecosystems as well as for indigenous communities and wildlife that depend on them.