Alaska, September 29: Rivers and streams in Alaska’s remote Brooks Range are turning orange as thawing permafrost alters how water moves through the landscape and exposes mineral rich rocks to increased weathering, according to a new study published in AGU Advances.
Scientists first drew attention to the unusual orange colour in 2018, when previously clear waterways began developing rust coloured sections. The colour is produced when iron released from rocks oxidises and forms orange deposits. Researchers say, however that the visible change is only one part of a larger shift in water chemistry.
The study examined six watersheds between 2022 and 2024, collecting samples from rivers, tributaries and hillside seeps. Some of the acidic seeps had a median pH of 3.2, compared with 8.3 in upstream mainstem rivers.
The researchers also found substantially higher concentrations of metals and sulfate in the acidic seeps. Across 23 measured metals, the median combined concentration was about 451 milligrams per litre in the seeps, compared with approximately 0.1 milligrams per litre in upstream mainstem waters.
Some samples contained levels of zinc, nickel and cadmium comparable to, or higher than, concentrations reported from mine affected waters in the region. The researchers stress that the phenomenon is not caused by abandoned mines. Instead, it is a form of natural acid rock drainage, in which minerals in the underlying geology react with water.
Permafrost connection
As permafrost thaws, water can penetrate deeper into the ground and reach sulfide-bearing minerals that were previously less exposed to flowing water. Weathering of these minerals can release acidity, sulfate and metals into streams.
The chemical changes are not necessarily confined to the small tributaries where they originate. In the Salmon River, researchers detected elevated sulfate, iron, zinc, nickel and cadmium about 97 kilometres downstream from an affected drainage point. Some concentrations were up to 70 times higher than upstream measurements.
Long-term monitoring also showed sharp increases in sulfate and zinc in some watersheds between 2019 and 2020, with individual increases of as much as 263%. The researchers linked the timing to unusually warm conditions and heavy snowfall while noting that concentrations later declined in some areas.
The findings indicate that the chemistry of these drainage systems can fluctuate rather than increase continuously.
Scientists say the affected waterways are important habitats for fish and other aquatic organisms and are also used by Arctic communities. Continued changes in water chemistry could therefore have implications for freshwater ecosystems.
The researchers are continuing to monitor the rivers to understand how permafrost thaw and changing water pathways may influence the release and downstream movement of minerals in Alaska’s Arctic landscapes.