Scientists have identified the two biggest reasons that once-pristine rivers across the Arctic are growing cloudy with toxic orange iron particles that smother insects and suffocate fish.
A new study published in Communications Earth & Environment builds on earlier research documenting widespread contamination in Alaska's Brooks Range. As the climate warms, a layer of Arctic soil that had been frozen for millennia has begun to thaw. Previous studies suggested that thawing permafrost was the ultimate cause of the damage. This new study proves that beyond a shadow of a doubt.
The new study also reveals two distinct ways in which thawing soil is rusting rivers and helps scientists predict where the damage is likely to spread next.
To investigate the rusting, the research team studied a wide regional view of a vast mountain region, then zoomed in on a specific river system, followed by an even closer look at a single creek. This deep-dive allowed them to connect big patterns to specific, on-the-ground processes.
"At middle, more heavily forested elevations, there isn't much going on. But at the higher and lower elevations we could see distinctly different phenomena," said Roman Dial, math and biology professor emeritus at Alaska Pacific University and first author of the study.
At higher elevations, the problem starts in rocky ground containing pyrite, also known as fool's gold. For many years, because the ground was frozen, water and air didn't affect the pyrite. Warming and thawing have set into motion a process called acid rock drainage, often associated with mining. However, here it is happening away from mines across a vast, natural, and formerly unspoiled landscape.
"When pyrite meets water, it comes apart. It breaks down into iron and sulfur, creating sulfuric acid as well as sulfate and other toxic metals," said Tim Lyons, UC Riverside biogeochemist and paper co-corresponding author. "When the iron-rich water mixes with more oxygen, the iron turns into rust-like particles that color the water and stain the bottom sediments orange."
At lower elevations, the story is different. Here the landscape is covered with wetlands that are changing shape and expanding downward as permafrost melts. In these soggy places, the soils are low in oxygen. So, microbes (mostly bacteria) "breathe" iron instead of oxygen.
"When we breathe, oxygen goes in and gets converted to the carbon dioxide that we exhale," Dial said. "Similarly, microbes are consuming iron in the lowland soils and converting it into a water-soluble form that seeps into streams and results in rusting as it meets oxygenated surface water."
Unlike the acid rock drainage, the lowland microbes aren't directly producing acid or sulfate. That difference is one of the clues researchers used to tell the two processes apart.
Taken together, both help explain why orange waters are appearing across such large regions that now stretch across northern Alaska, closely tracking areas where permafrost is thawing.
The study also reveals a delayed effect that could help predict future contamination. Each summer, the active, top layer of soil thaws to its deepest point before refreezing in winter. Iron released during one thaw can become trapped, then flushed into rivers the following year.
By analyzing long-term ground temperature data alongside stream chemistry, the team found that this lag can be used to anticipate increases in metal levels.
"That means we can use ground temperatures to help predict water quality in the future," said University of Alaska ecologist and paper co-author Paddy Sullivan, who first noticed the dramatic river changes that looked "like sewage" in 2019 while conducting fieldwork in the region.
Mines typically control the waters near them to minimize pollution. For this study, the team partnered with scientists at the Red Dog zinc mine, who have long-term temperature records from boreholes drilled deeply into the earth and from chemistry sampling in stream water. By linking those underground measurements with changes in stream chemistry, the researchers were able to directly connect thawing permafrost to the rusting rivers.
The ecological consequences of the rusting are severe. Fine iron particles can stay suspended in water for more than 100 kilometers, clouding rivers, smothering algae, disrupting insect populations, and clogging fish gills.
In Alaska and neighboring Canada, the team suspects these changes are affecting salmon, which rely on clean gravel beds for spawning and on algae-based food webs during early life stages.
The findings also suggest the problem will expand globally. Similar conditions involving melting permafrost and metal-rich rocks exist in northern Canada, the Andes, and the Alps, for example.
"It's already happening in Russia, and will keep happening anywhere you have the right geology and warming temperatures," Lyons said. "It started as a canary in a coal mine in the Brooks Range, but now those canaries are chirping all over."
Unlike pollution from mining, which can sometimes be contained, this process is diffuse and difficult to manage. The researchers note that this is happening even in the most remote natural areas of the United States. "You'd think if any ecosystem could hide from the effects of warming and big human footprints, it'd be this one. But it's not so," Lyons said. "There is no safe place."
However, the ability to predict where contamination may occur could help identify and protect critical habitats. Researchers say this is especially important for communities dependent on these waters, and the fish that live in them, for food and cultural practices.
"There's no fixing this once it starts," Lyons said. "But we can give people downstream a heads up and work hard to protect the places that are still safe and less vulnerable to the rusting."
