Over 100,000 acres of the Peruvian Amazon have been deforested in the past thirty-five years, predominantly due to artisanal and small-scale gold mining (ASGM). Although conservation areas have been established to protect this global biodiversity hotspot, atmospheric deposition of ASGM-derived contaminants is prevalent both inside and outside conservation area boundaries. During ASGM, mercury (Hg) is used to separate gold from sediment. Once Hg is burned off – representing the largest source of anthropogenic Hg emissions globally – it is transported atmospherically and deposited onto the landscape by: (1) dissolving in rainfall, (2) sorbing to atmospheric particles that are intercepted by leaves, or (3) entering leaves during foliar gas exchange. In this study, we quantified Hg deposition from these pathways and Hg storage in soils, along with how land cover impacted these processes. We collected samples from two intact forests far from mining, one intact forest near mining, and two disturbed forests near mining. At each site, we collected rainfall in open areas, rainfall under canopy cover (throughfall), and canopy foliage to determine total Hg (THg) inputs and soil samples to determine Hg storage. We also measured methyl Hg concentrations (MeHg, the bioavailable form of Hg) and leaf area index at all sites.
Results/Conclusions
Surprisingly, we found that conservation areas with intact forests received the highest inputs of Hg deposition and stored the highest amount of Hg. Hg concentrations in throughfall were highest in the intact protected forest near mining and lower at all other sites (throughfall THg range: 7.9-60 ng/L). Similarly, the highest Hg concentration in foliage was from trees near mining (foliage THg range: 19-39 ng/g). Total Hg concentrations in surficial soils reflected depositional patterns, with elevated concentrations at forested sites near mining (THg range: 0.027-580 μg/g). Hg was deposited in the bioavailable form of MeHg predominantly at one of the forested sites far from mining and in the protected forested site near mining. Collectively, these results show that Hg is preferentially scavenged by intact forested areas, stored within these forested soils, and present as MeHg. It is thus possible that these forested conservation areas, which are intended to protect the Amazon rainforest, are areas with an elevated risk of Hg exposure.