Lake-dominated landscapes produced by artisanal gold mining amplify the bioavailability of mercury

Abstract:

Artisanal and small-scale gold mining (ASGM) is the largest global source of anthropogenic mercury emissions and a local driver of landscape change. This is exemplified by previous work showing widespread deforestation and mercury contamination associated with ASGM. However, little is known about how ASGM activity changes the extent of lake environments that effectively convert deposited mercury into the bioavailable form of methylmercury. We extracted surface water extent from remote sensing data for the Peruvian Amazon over the past 34 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 upstream and downstream of ASGM activity. Land cover analysis revealed that lake area extent has increased since 2000, primarily through the proliferation of mining ponds. This increase in ponded waters (‘lentification’) is most extreme within heavily mined watersheds, which show increases of 670% compared to only 20% in less impacted watersheds. Simultaneously, field samples revealed that mercury is converted to its more toxic methylated form at rates 5-7 times greater in lakes than in rivers within the region. Together, these results suggest that the synergistic ‘lentification’ and Hg loading associated with ASGM are promoting Hg bioavailability and exposure risk for the people and wildlife that depend upon these rivers and lakes. Examination of additional ASGM hotspots globally indicates this is a widespread phenomena.