Gold mining in the Peruvian Amazon increases mercury bioavailability through artificial lake expansion

Jacqueline Gerson, University of California, Berkeley, Berkeley, CA, Simon Topp, Xiao Yang and Tamlin M. Pavelsky, Geological Sciences, University of North Carolina at Chapel Hill, Chapel Hill, NC, Claudia Vega and Luis E Fernandez, CINCIA, Peru, John Gardner, University of Pittsburgh, Pittsburgh, PA, Emily Bernhardt, Biology, Duke University, Durham, NC

Background/Question/Methods
One of the most immediate threats to the Peruvian Amazon – a global biodiversity hotspot – is illegal artisanal and small-scale gold mining (ASGM), which results in widespread land cover change. In ASGM, forests are cleared, rivers are dredged, and mining ponds are created. Gold is isolated using mercury (Hg), a potent neurotoxin, which then enters the aquatic ecosystem via atmospheric deposition or direct dumping. While previous studies have shown widespread deforestation and Hg contamination from ASGM, little is known about how ASGM changes the amount of lotic and lentic environments and how Hg loading and methylation differs across these environments. In this study, we sought to understand how ASGM changes the extent of lotic (riverine) and lentic (lake) environments and how Hg loading and net methylation differ across these environments. We analyzed remote sensing imagery of the Peruvian Amazon over the past 35 years and collected water samples from a 200-km reach of the Madre de Dios River, its tributaries, and surrounding oxbow lakes and mining ponds in areas both upstream and downstream of ASGM activity.
Results/Conclusions
We found that areas near ASGM have more than doubled the surficial extent of lentic systems (oxbow lakes and mining ponds) in the past 20 years, while no change in the extent of lentic systems has occurred in more remote areas. Additionally, though the total loading of Hg into aquatic ecosystems is highest in the river downstream of ASGM, both oxbow lakes and mining ponds have increased Hg methylation compared to the river. These results suggest that lentic ecosystems, which are increasing in abundance, are hotspots for Hg methylation, with potential implications for top predators and indigenous communities that depend on fish from these aquatic ecosystems.