Mining ponds amplify the risks of mercury in the Peruvian Amazon

The proliferation of pits and ponds created in recent years by miners digging small deposits of alluvial gold in the Peruvian Amazon has dramatically altered the landscape and increased the risk of mercury exposure for indigenous communities and wildlife, a new study shows.

“In heavily mined river basins, there has been a 670% increase in the extent of ponds in the landscape since 1985. These ponds are almost entirely man-made lakes created as thousands of former mine shafts fill with rain and groundwater over time.” said Simon Topp, a doctoral student in geological sciences at the University of North Carolina at Chapel Hill, who co-led the study.

Landscapes once dominated by forests are now increasingly dotted with these small lakes, which, according to the study, provide poor oxygen conditions in which submerged mercury, a toxic residue from the gold mining process, can be converted by microbial activity in an even more toxic one. form of the element, called methylmercury, at net rates 5 to 7 times higher than in rivers.

“Methylmercury poses particularly high risks to humans and large predators because it bioaccumulates in body tissue as it moves along the food chain. This is of particular concern given the high biodiversity and large number of indigenous peoples living in the Peruvian Amazon. “said Jacqueline Gerson, a PhD student in ecology at Duke University who also co-led the study.

These increased risks likely also occur in other places where unregulated artisanal small-scale gold mining takes place, including Asia, sub-Saharan Africa and other parts of South America, he said.

Topp, Gerson, and their colleagues published their peer-reviewed study on November 27 in Advances in science.

Artisanal gold miners use mercury, a powerful neurotoxin, to separate their gold ore from soil and sediment, often without adequate safety precautions to protect themselves or the environment.

Mercury poisoning can cause a wide range of health impacts, including tremors, muscle weakness, impaired vision and hearing, and loss of coordination and balance. In severe cases, it can lead to birth defects or death.

Some of the mercury used by miners is burned into the air or poured into nearby rivers, creating far-reaching environmental and human health risks that have been well documented in previous studies. The new study is the first to document how mining has altered the landscape and simultaneously amplified the risks of mercury poisoning through the creation of ponds and the microbial transformation of mercury into methylmercury that occurs there.

To conduct the study, scientists collected water and sediment samples from sites upstream and downstream of artisanal gold mining sites along Peru’s Madre de Dios River, its tributaries, surrounding lakes and mining ponds during the dry season in July and August 2019. measured each sample for the total mercury content and for the proportion of that mercury that was in the most 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 stagnation and mercury contamination at each site and identify causal links.

“It can clearly be seen that the rise of man-made lakes and ponds in heavily mined areas accelerated after 2008, when gold prices rose dramatically along with mining,” Topp said. In contrast, the total area of ​​ponds in areas without heavy mining activity increased on average by only 20% during the entire study period.

“We expect this trend, and the environmental and human health risks it causes, to continue as long as gold prices remain high and small-scale artisanal gold mining is a profitable business,” he said.