This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The bright orange plumes of water scientists first noticed billowing across a river in Alaska's Brooks Range in 2018 were even more acidic and metallic than water leached from metal mines, despite their pristine surroundings. That's according to a new University of California, Davis, study of six affected watersheds.
The study, published in the journal AGU Advances, found that these metals can travel far and become more toxic as they go. Metal concentrations were about 70 times higher as far as 60 miles (97 kilometers) downstream from where they entered the river. "This is very acidic, metallic water," said senior author Brett Poulin, a UC Davis professor of environmental toxicology.
"The good news is the river's mainstem did not become more acidic. It's buffering the pH, indicating there is some resiliency in this system. But there are metals that increased downstream, and those can be transported very long distances." Poulin and colleagues previously attributed the rusty-colored water to thawing permafrost, an increasingly common occurrence as the climate warms.
Water trickles into the ground, encountering minerals long trapped in the previously frozen soil. This new interaction releases iron, sulfuric acid and other metals into nearby streams, turning them orange and threatening water quality for plants, wildlife and people. "This was a process that was not forecast, predicted or included in any assessment of how the Arctic will change under a warmer climate, but we're seeing it in rivers across the Arctic," Poulin said.
"Since our 2024 paper, 'rusting rivers' have been documented in boreal and Arctic Canada, and other parts of the world that have permafrost." For their new paper, the authors expanded the research to six affected watersheds, all within national parks and preserves in northwest Alaska's Brooks Range. They spent the summers of 2022 to 2024 collecting water samples from the watersheds' mainstems, tributaries and hillside seeps to quantify the effects of acid rock drainage on river chemistry. They used long-term water quality records to trace the event's onset across the region to 2019—which coincided with Alaska's hottest summer to date, followed by a snowy winter.
The analysis showed abrupt, large spikes in sulfate and zinc concentrations that year, followed by a steady decline and recovery phase. "When you have more snow, it insulates the soil, preventing refreezing," said first author Taylor Evinger, a Ph.D. candidate in Poulin's lab at UC Davis. "We think water went deeper into the soil because of soil thaw, interacted with these minerals, and all of these watersheds were triggered around the same time." Previous studies have documented more than 200 rivers and streams affected by this phenomenon.
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