"Mining boom across the tropics degrading river systems", 27 August 2023

"A recent study published in the journal Nature found that gold and mineral mining in and near rivers across the tropics is degrading waterways in 49 countries.

The paper explains that river mining often involves intensive excavation, which results in deforestation and increased erosion. Much of the excavated material is released to rivers, disrupting aquatic life in ecosystems nearby and downstream. This inorganic sediment, particles of clay, silt, and sand, is carried by rivers as “suspended sediment,” transmitting the environmental effects of mining downstream.

Prior research has reported that such suspended sediment also carries toxins such as mercury used in river mining processes, which further affects water quality and can be detrimental to human health and the environment.

“For hundreds, if not perhaps, thousands of years, mining has been taking place in the tropics but never on the scale as we’ve seen over the past two decades,” Evan Dethier, who worked on the study while he was a postdoctoral researcher at Dartmouth College, said in a media statement. “The degradation of rivers from gold and river mining throughout the tropics is a global crisis.”...

The data shows that more than 35,000 kilometres of tropical rivers are affected by gold and mineral mining around the world. This is equivalent to 6% of the 500,000 kilometres of tropical rivers that run across the planet. Furthermore, mining has caused suspended sediment concentrations to double in 80% of the 173 rivers represented in the study, relative to pre-mining levels.

“These tropical rivers go from running clear either throughout the year or at least through part of it, to either being choked with sediment or muddy year-round,” Dethier said. “We found that almost every single one of these mining areas had suspended sediment transmitted downstream, on average, at least 150 to 200 kilometres from the mining site itself but as much as 1,200 kilometres downstream.”

There are 30 countries that have both active river mining operations and large tropical rivers that are more than 50 meters wide. The researchers found that in those countries, on average, 23% of the length of their large rivers is affected by mining. In some countries, more than 40% of the total length of those large rivers is altered by mining, including French Guiana (57%), Guyana (48%), Cote d’Ivoire and Senegal (40%)...

Since mining began, they found that two-thirds of the rivers represented in the study exceeded the turbidity guidelines for protecting fish on 90% of the days or more, meaning the cloudiness of the rivers was higher than recommended.

“When rivers and streams experience high levels of suspended sediment, fish are unable to see their prey or predators and their gills may become choked with sediment and damaged, which can lead to disease or even mortality,” Lutz noted...

The co-authors call on policymakers to work with stakeholders to help mitigate the environmental and social impacts that mining is having on tropical rivers given that it’s likely to continue into the foreseeable future."