Illegal mining has muddied tropical rivers worldwide

David A. Lutz

Combing through tens of thousands of river images by hand, LCLUC PI David Lutz and team pinpointed where sediment loads begin to rise along each watercourse. They cross-checked those locations with high-resolution imagery from private satellite companies and scoured news and social media for any clues to the cause of the increased muddiness. Some was due to oil palm plantations, but mining was by far the dominant cause. In all, 381 sites showed a surge in muddiness because of mining. The survey grew out of work by Bowdoin College geomorphologist Evan Dethier in the Peruvian Amazon. He and others have studied the growth of mining in Madre de Dios, a province bordering Brazil, sparked by a spike in gold’s price during the 2008 financial crisis. The region’s miners use small-scale techniques not unlike those in 19th century gold rushes. They dredge sediments from the beds and banks of the Amazon tributaries, then add mercury, a cheap and toxic liquid metal, to the watery slurry. It selectively binds to several precious metals, including gold, creating heavier nuggets that fall out of the slurry. After the nuggets are collected, the sediment “tailings” are dumped back into the river.

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Tuesday, February 28, 2023