The proliferation of pits and ponds created in recent years by miners dredging for small deposits of alluvial gold in Peru's Amazon has dramatically altered the landscape and increased the risk of mercury exposure for indigenous communities and wildlife, a new study shows. The study, published in Science Advances Nov. 27, found a 670% increase in the extent of ponds across the landscape in heavily mined watersheds since 1985. These formerly forested landscapes are now increasingly dotted by these small lakes, which provide low-oxygen conditions in which submerged mercury—a toxic leftover from the mining process—can be converted by microbial activity into an even more toxic form of the element, called methylmercury, at net rates 5-to-7 times greater than in rivers. The miners use mercury, a potent neurotoxin, to separate ore from soil and sediments, often without adequate safety precautions to protect themselves or the environment. Some of the mercury used by the miners is burned off into the air or spilled into nearby rivers, creating far-reaching environmental and human health risks. Artisanal and small-scale gold mining (ASGM) is now believed to be the largest global source of anthropogenic mercury emissions.

Health impacts can include tremors, muscle weakness, vision and hearing impairments, and loss of coordination and balance. In severe cases, it can lead to birth defects or death.

To conduct the study, the scientists collected water and sediment samples at sites upstream and downstream of artisanal gold mining sites along the Rio Madre de Dios, its tributaries, surrounding lakes, and mining ponds during the dry season in July and August of 2019. They measured each sample for total mercury content and for the proportion of that mercury that was in the more toxic form of methylmercury. By combining these measurements with more than three decades of high-resolution satellite data from the region, they were able to determine the extent of artificial ponding and mercury contamination at each site, and identify causal links.

"You can clearly see that the increase in artificial lakes and ponds in heavily mined areas accelerated after 2008, when gold prices dramatically increased along with mining activity," said study co-author Simon Topp of the the University of North Carolina at Chapel Hill. By contrast, the total surface area of ponds in areas without intensive mining increased by an average of only 20% over the entire study period. "We expect that this trend, and the environmental and human health risks it causes, will continue as long as gold prices remain high and artisanal small-scale gold mining is a profitable activity," Topp said.

Condensed from EurekaAlert!