Professor Matt Lindsay samples metal precipitates below acidic seepages that discolour stream water in central Yukon.Elliott Skierszkan/Supplied
In days gone by, fortune seekers were lured to central Yukon by stories of rivers that ran with gold.
Now researchers exploring the same region have come across a new and different phenomenon: rivers that run brilliant orange with rust.
In some cases, streams have become so toxic they can kill surrounding vegetation and leave acid burns on the landscape that are visible in satellite imagery.
“We’ve seen dozens of these, and they’ve typically started occurring within the last 10 years,” said Elliott Skierszkan, an environmental geochemist at Carleton University and lead author of a study that documents the effect.
The abrupt changes to the chemistry of some Yukon streams can be attributed to the loss of permafrost throughout the region, according to the study, published Thursday in the journal Science.
With the climate now warming well above historic levels, groundwater is reaching layers of rock and soil that had been frozen for thousands of years. Sulphide-rich minerals that are characteristic of the region are leaching out.
PhD Candidate Andras Szeitz examines iron-oxide rich rock in Central Yukon.Andras Szeitz/Supplied
Once dissolved, those minerals produce sulphuric acid, which causes pH levels to plummet while metals are released in high concentrations. Iron in the water reacts with oxygen and precipitates out as rust.
Dr. Skierszkan said the effect is dramatic and reminiscent of the run-off from mining operations in similar rock. But the loss of permafrost is more widespread than any mining site.
He added that when the team began gathering data in 2019, members hoped they might be able to secure funding for 10 or 20 years to see if there was any discernible change in water quality because of permafrost loss. Instead, they saw changes within two to three years, exceeding anything they were expecting.
“It just got everyone in gear to make sure that we’re collecting more samples to really understand where this is happening and how it’s happening,” Dr. Skierszkan said.
Dozens of acidic seepages with toxic metals concentrations have emerged in formerly pristine streams in Tombstone Territorial Park, Yukon, because of thawing permafrost.Lolita Hughes/Yukon Parks
Data for the study were gathered through the Tombstone Waters Observatory, a facility set up by McMaster University to monitor several streams that flow from the mountains of Tombstone Territorial Park and across the Dempster Highway, which runs from Dawson City, Yukon, to Inuvik in the Northwest Territories. The streams ultimately drain into the Peel and Ogilvie Rivers, which are part of the Mackenzie Basin, or into the Yukon River. What they have in common is that the water flows across a region of sulphide-rich sedimentary rock that originally formed on the seafloor in low-oxygen conditions.
Once they were on land, sulphides near the surface would have leached out over a period of time. But the retreat of permafrost has allowed water to mobilize minerals that lie deeper.
The result has implications for water quality, particularly for Indigenous community members who collect drinking water in the park, or recreational hikers who camp by alpine lakes that can become contaminated by the acid and metals seeping up from the ground. Farther downstream, there is the potential that fisheries may be impacted.
With a warming climate, groundwater is reaching layers of rock and soil that were once frozen for thousands of years.Matt Lindsay/Supplied
Mara De La Rosa, a spokesperson with Yukon’s Department of Environment, said the territory has been collaborating with research partners and supporting efforts to monitor the changes.
“This collaboration helps ensure that potential risks to water quality, ecosystems and communities are identified early and considered in decision-making,” she said in a statement.
The new study connects the stark changes seen at the region’s headwaters with more subtle shifts observed in the territory’s major rivers, where water quality measurements have a longer history. This includes rising concentrations of sulphate, a chemical byproduct of the reactions happening farther upstream.
Between 2022 and 2024, acidic seepages linked to permafrost thaw have released toxic metals.Andras Szeitz/Supplied
And there is evidence that the phenomenon is emerging on a large scale, well beyond the territory’s borders.
Patrick Sullivan, who directs the Environment and Natural Resource Institute at the University of Alaska Anchorage, said the Yukon study echoes findings he and others reported in 2024 in Alaska’s Brooks Range, which has a similar geological history.
Dr. Sullivan, who was not involved in the Yukon study, noted that the Canadian team added an interesting new element in their work by estimating how much carbon dioxide the acidic water produces once it encounters carbon-bearing rocks that are also present in the area.
The results suggest that the release of sulphide minerals, triggered by permafrost thaw, could be having a growing influence on the global carbon cycle.
Some streams have become so toxic they can kill surrounding vegetation and leave acid burns on the landscape.Elliott Skierszkan/Supplied
He also noted that rising sulphate concentrations in Yukon’s large rivers were observed well before anyone noticed the dead vegetation and turbid orange water emanating from sites of sulphide release.
“This highlights the importance of long-term government sponsored monitoring efforts for early detecting and even forecasting of major changes in the environment,” Dr. Sullivan said.