A study of six drainage basins found acidic, metal-rich springs whose effects are felt more than 100 kilometers away. The main rivers moderate acidity, but not always metal concentrations
By Avi Blizovsky
In remote parts of Alaska, where there are no active mines, researchers have begun to see orange water flowing into rivers that were once clear. The color resembles rust, but the important question is not just what you see in the photos: What is happening to the composition of the water, and how far does the effect last? A study published in the journal AGU Advances monitored six drainage basins in the Brooks Range of northwest Alaska. It found that in some of the springs that feed the rivers, the water is acidic and very rich in metals, and that some of the metals are carried dozens of kilometers downstream.
The phenomenon is related to the thawing of permafrost, ground that has remained frozen for years. When the ground thaws, water can seep deep into the ground and reach previously isolated sulfur minerals. The contact with water and oxygen encourages chemical reactions that produce acid and release iron and other metals. Iron compounds give the tributaries their orange hue. This is acid drainage from rocks: a process that is chemically similar to acid drainage from mines, although in the locations studied, a mine is not required for it to occur.
What did the researchers find in the field?
A team from the University of California, Davis, in collaboration with the U.S. Geological Survey and the U.S. National Park Service, modeled major rivers, tributaries, and springs on slopes in six drainage basins in the summers of 2022–2024. Nine of the springs sampled had different chemical compositions; several were very acidic, with pH values below 4, along with high concentrations of iron, zinc, nickel, cadmium, and other metals. The researchers compared the springs and found that some were more acidic and richer in metals than water sources tested in nearby mining areas.
It is important to distinguish between the source and the river into which it flows. The water in the main rivers tested was largely successful in neutralizing the acidity, so it should not be concluded that the extreme pH value measured at the source characterizes the entire river. In contrast, concentrations of certain metals in the water and particles increased after the entry of the damaged tributaries. At the various measurement points, an increase of 3 to 21 times was recorded compared to the water above the entry point. Along the Salmon River, the team recorded concentrations of certain substances that were up to 70 times higher, even about 103 kilometers downstream. This is a finding for certain substances and measurement points, and not a uniform description of all the water along the river.
A sharp change, followed by signs of recovery
Long-term water quality records in two of the drainage basins indicated a spike in sulfate and zinc around 2019–2020. The timing coincides with Alaska’s unusually warm summer of 2019 and the snowy winter that followed. A thick layer of snow may have insulated the ground and delayed its refreezing; the researchers suggest that deeper thaw opened up pathways for water to flow to the sulfur minerals. The overlap in time supports this explanation, but it doesn’t allow us to determine precisely what each factor contributed in each drainage basin.
The monitoring also found a gradual decline in sulfate and zinc concentrations in the years following the spike. That doesn't mean the problem has gone away: Hundreds of streams and rivers in the area have already been identified as affected or suspected of being affected, and the new study shows that the metals can travel long distances. However, the data show that the process doesn't necessarily progress at the same rate every year, and that repeated measurements over time are important.
What is known about the risk?
The researchers emphasize that so far, this study has not documented any proven harm to humans or animals from the acid drainage at the remote sites tested. They also do not report any mass fish kills or known damage to local drinking water. The concentrations of the metals and the distance they travel warrant further examination of potential impacts on freshwater systems and downstream animal communities.
The story of the orange rivers adds another dimension to the consequences of Arctic warming. Thawing permafrost is also known for its greenhouse gas emissions and soil instability; here, the impact on the chemistry of flowing water is revealed. Precisely because the acidity in the main river may seem normal, testing of metals and small tributaries is needed to understand what has really changed.
Questions and Answers
Why does the water turn orange?
The thawing of the ground allows water to reach the minerals deep below. Chemical reactions release iron, among other things, and its compounds color some of the springs and tributaries a rusty hue.
Are all rivers as acidic as mine water?
No. Extreme acidity was measured in some of the springs. In this study, the main rivers moderated the acidity, although concentrations of certain metals increased downstream.
Has it been proven that residents or fish have been harmed?
Not in this study. The researchers found a change in water quality, but further studies are needed to determine the extent of the impact on animals and drinking water supplies.
The scientific article
Source: University of California, Davis Announcement, September 24, 2026
More on the subject on the science website
- Greenland and Antarctica lost 11.3 trillion tons of ice
- Thawing the ice freeze could accelerate global warming
- Climate scientists are investigating the acceleration of methane greenhouse gas emissions in the Arctic
- Arctic Alaska is burning like it hasn't in 3,000 years
- Climate disruptions, extreme weather events and global connections: the Arctic is crying out
