At first glance, the Amazon rainforest of Peru’s Los Amigos Conservation Concession might seem like a pristine wilderness. Brightly colored birds flit through the jungle. A dense canopy of trees echoes with the cries of howler monkeys. Jaguars pad quietly through the shadows. Giant otters swim in Cocha Lobo Lake. But the forest is hiding a toxic secret: It is tainted by mercury at levels as high as those found in industrial regions in China, according to new research.
The mercury is the product of hundreds of illegal, small-scale gold mines, and is leaving its poisonous fingerprint in forest wildlife. “These forests … are receiving an enormous load of mercury, and the mercury is indeed entering into the food web,” says biogeochemist Jackie Gerson, a postdoctoral researcher at the University of California, Berkeley, who led the research as a Ph.D. student at Duke University. The new study, the first to describe such effects anywhere in the world, is another strand in the growing web of evidence that connects mining to mercury pollution in rivers, fish, and forests.
Gold mining has recently outstripped coal burning as the world’s single largest source of airborne mercury pollution, annually releasing as much as 1000 tons of the potent brain and reproductive poison into the atmosphere. Using mercury to extract gold is a miner’s dream: The cheap, liquid metal, when mixed with a slurry of water and raw ore, binds with the precious gold. Miners then heat the globs of mercury and gold until the mercury burns off, floating away as a vapor.
In Peru, the technology has helped small-scale miners transform more than 100,000 hectares of jungle along the remote Madre de Dios River into a moonscape: The water-filled pits and mounds of rocks are visible from the International Space Station. Scientists have traced some of the mercury into nearby ponds and rivers, where it has contaminated fish eaten by miners and Indigenous peoples. But it wasn’t clear where the rest of the mercury was going. Previous tests found surprisingly low levels in the soils of within deforested sites in the Madre de Dios region, says Luis Fernandez, a tropical ecologist and executive director of Wake Forest University’s Center for Amazonian Scientific Innovation, which was involved in the new study.
To follow the mercury, Gerson and fellow scientists visited two mine sites with previously logged forests, two jungle areas at least 50 kilometers from the mining, and the Los Amigos forest, which sits near a mining hot spot. They collected rainwater, soil, and leaves from the forest floor and from trees.
Samples from the Los Amigos forest stood out. Mercury had soaked into the jungle there at 15 times the rate of nearby deforested areas. “At first I didn’t believe how high some of the numbers were,” Gerson says. The levels—137 micrograms per square meter of soil each year—were higher than those in any forests tested near coal power plants in Europe and North America, and on par with industrial cities in China like coal-reliant Chongqing, the researchers write today in Nature Communications.
The findings suggest the jungle trees act like a kind of mercury sponge. Leaves, which are coated in mercury-tainted dust, also absorb gaseous mercury as they take in air. The metal eventually finds its way to the forest floor as leaves fall or rain washes off the dust. After capturing the water that dripped from the forest canopy, researchers found that more than twice as much mercury washed down with the rain at Los Amigos than at any other survey site. Whereas mercury concentrations were highest in tree leaves at the two mining sites, the total amount of mercury was higher in the Los Amigos forest, thanks to the greater abundance of trees.
The results suggest forests can buffer some of mercury’s ill effects, by stashing it in leaves and soil. People and wildlife aren’t generally at risk from this locked-away mercury, Gerson says. “You could walk through the forest, you could swim in the water, you could bury yourself in the leaves and you’re not going to get mercury toxicity from doing that.” That ability to sequester mercury is “another reason … for keeping tropical forests standing,” Fernandez says.
Still, the airborne form of mercury can become very dangerous when it seeps into water and sediments and is turned into a different form, methylmercury, by waterborne bacteria. From these bacteria, the mercury moves into the tissues of organisms higher on the food chain, including fish.
The researchers found sobering signs that methylmercury is making its way into forest creatures. Tests in three species of songbirds in Los Amigos found they had mercury levels between two and 12 times higher than similar species captured in a forest far from the mining. In seven out of 10 black-spotted bare-eyes (Phlegopsis nigromaculata), the levels of mercury were high enough to impair their ability to reproduce.
“[This] shows that it is entering the food web,” says Emily Bernhardt, a Duke University biogeochemist who supervised Gerson. “There’s a lot more to do to see what that means. This is just scratching the surface of that question.”