According to the Center for Amazonian Scientific Innovation, Canada’s NGO Artisanal Gold Council has registered that some 181 tonnes of mercury are released into the environment every year in the Madre de Dios region.

Scientists consider the Madre de Dios River area one of the most understudied tropical biodiversity hotspots in the world

Such a massive, cumulative amount of pollutants is showing its effects now. “Significant impacts to rivers were observed as a result of the artisanal scale gold mining, with as much as 10 times normal suspended sediment concentrations—a measurement of how many sediment particles are in the water,” the Dartmouth scientists said in their report.

According to their study, though higher sediment concentrations were observed year-round, they were most pronounced during the dry season, when undisrupted rivers in this region of southeastern Peru generally run clear, with low sediment concentrations as compared to the wet season.

“Most artisanal-scale gold mining in Madre de Dios is conducted during the dry season, as the heavy machinery that is required to clear the land for operations cannot be used during the wet season. As a result, the mining activity is triggering a reversal of natural seasonal cycles, turning clear water muddy and disrupting the hydrological activity of the riverine ecosystems,” the experts wrote.

How they did it

To reach these conclusions, the Dartmouth scholars used satellite imagery from 1984 to 2018 at 32 river reaches in the Madre de Dios River watershed, which is located near the largest tributary to the Amazon, the Madeira River. This effort meant analyzing over 15,500 samples from 3,200+ NASA LandSat images, including images of the same sites through time.

Based on the spectral reflectance properties of rivers and sediment, the estimated amount of sediment was calculated using algorithms developed for the study, and changes were tracked over the 34-year study period.

The findings demonstrated that 16 of 18 stretches of rivers near artisanal-scale gold mining areas had higher levels of suspended sediment concentrations. By comparison, for the 14 sites studied that had not been affected by mining, only five were found to have higher levels of sediment over time. The increase in sediment in rivers downstream of the artisanal scale gold mining areas was found to be related to the area of upstream land being mined.

“Though these mining operations are called ‘artisanal,’ they are occurring on a widespread scale, and our data shows that sediment introduced into the rivers of the Madre de Dios region is profoundly changing important natural systems,” Evan N. Dethier, the lead author of the study said in a media statement. “Similar to deforestation for agriculture elsewhere in the Amazon, these mining operations are rapidly destroying pristine rainforest. However, we have found that gold mining has a far greater impact on river water quality than deforestation for agriculture, due to intensive sediment disruption directly in the rivers themselves, and through the use of mercury in the mining process.”

A previous study by the Center for Amazonian Scientific Innovation had also found that, between 1985 and 2017, illegal mining caused the loss of almost 100,000 hectares of forest in Madre de Dios. This finding was later corroborated during a flyover mission carried out by the Amazon and National Surveillance Centre, which is run by the Peruvian Air Force.