Rusting of Rivers in Alaska’s Brooks Range Likely Resulting from Iron Oxidation Thought to be Caused by Thawing Permafrost Exposing Bedrock and/or Thawing Wetlands Activating Iron-Rich Soil Bacteria

   Rivers in Alaska and the Yukon are turning orange due to iron oxidation. The source of this chemical alteration that is very similar to the formation of acid mine drainage is the thawing of permafrost and/or wetlands. That thawing is caused ultimately by global warming which has been significantly amplified in the Arctic as the following graphic shows. This is known as Arctic Acceleration.

Melting permafrost is thought to have several potential dangers, the biggest perhaps being the widespread release of methane that has been sequestered in the frozen ground and that if released can result in a feedback mechanism that could theoretically become a runaway feedback where that methane heats up the atmosphere even more and makes melting and methane release even more widespread. The rusting of rivers due to iron oxidation, however, was apparently not predicted.

     The issue is particularly problematic in Alaska’s Brooks Range which extends 700 miles from Alaska into Canada’s Yukon Territory. At least 75 rivers and streams have rusted in the region just over the last 5-10 years. The discoloration of the water is due to oxidizing iron and sulfuric acid, which means high concentrations of heavy metals are precipitating in the rivers. The oxidation of minerals in the soil may also be lowering the pH of the water, increasing its acidity. pH readings as low as 2.5 have been recorded which makes the issue equivalent to acid mine drainage from coal mines and some minerals mines. The level of dissolved metals as measured by electrical conductivity was roughly equivalent to that of industrial wastewater. The water is undrinkable by environmental standards. Many of the rivers before that had alkaline chemistry with pH above 7. There are obvious dangers to plant life, aquatic life, and fish in the rivers as toxic metals leach out and the rivers become strongly acidic.  

     In 2022 the USGS and National Park Service began an investigation to map out the rusting rivers' extents and to better understand and quantify the effects. Integration of geology, hydrology, and permafrost degradation are all involved in these chemical changes. They are also studying the biological effects. We know that acid mine drainage causes massive negative effects on aquatic organisms, and this is likely to be very similar in effect. There are some people who rely on the local rivers for fish that will also likely be affected. Some of the rivers are known for spawning salmon runs downstream. Researchers think that the phenomenon is also likely to occur in other places in the Canadian and Russian Arctic regions.

     There are two prevailing theories for the phenomenon. One is that the thawing permafrost is exposing bedrock resulting in the release of iron and sulfuric acid into the streams. The other is that soil bacteria, activated by thawing wetlands, are producing the soluble iron. The acidic rivers, like acid mine drainage, also carry high sediment loads due to the precipitating metals. This creates murky water that makes it harder for fish and other aquatic creatures to find food. Iron and aluminum dissolved in the water can also accumulate on fish gills, affecting respiration. As explained below, both ideas may be contributing.

     Kobuk Valley National Park has warmed by 2.4 degrees Celsius (4.32 degrees Fahrenheit) just since 2006. Researchers think about 40% of the permafrost within the park has been thawed. Biodiversity has already been severely restricted in some of these rivers and if the phenomenon continues it seems likely that many will become devoid of life like acid mine drainage infested rivers and streams. Researchers found evidence of thawing permafrost throughout the region with the unmistakable rotting vegetable smell where microbes digest the matter and emit CO2 and methane.

     The thaw of permafrost under a wetland leads to the chemical reduction of oxidized iron that becomes soluble in water. Then, when the reduced iron particles after transport via groundwater come into contact with sufficiently oxygenated waters, they can re-oxidize and precipitate out as rust. That is the basis of the thawing wetlands theory. It is supported by findings of characteristic gray soil under reducing conditions sampled below the once frozen ground.

     The very low pH water can cause acidic “burning” of vegetation which is depicted in some of the images below. The exposed bedrock or acid-rock drainage theory better explains the high acidity. Acid-rock drainage is known to occur when streams weather the sulfide rock associated with ore deposits. The phenomenon in the Brooks Range area had actually been occurring at much lower levels well into the past but has obviously been vastly accelerated by the thawing. The acid-rock theory assumes that the thawing permafrost is allowing oxygenated water to access pyrite-rich shale for the first time in thousands of years, forming sulfuric acid and oxidizing the reduced iron that would normally precipitate out as rust, to continue downstream. The region’s rocks also contain alkaline limestones that neutralize the acid which causes precipitation of the iron. Timothy Lyons, a geochemist at the University of California, Irvine said “It's like a one-two punch. You have the shaley rocks with pyrite that source the acid and the iron, and then the limestones neutralize that acid and cause the iron to come out of solution.” Researchers think other dangerous metals including copper, zinc, cadmium, lead, and even arsenic could be leaching out.

     The whole problem may continue and expand as more permafrost is melted. Beavers have also moved north, making more summer wetlands in the area, supporting the wetlands theory. It now seems likely that both processes, acid-rock drainage and soil bacteria are contributing to the increased soluble iron content of the waters.  

     Oddly perhaps, the natural rusting of the rivers has affected the remediation of waters from The Red Dog Creek mine, one of the world’s largest zinc mines. The mine is well downstream of the affected waters, but those upstream waters had been affecting the remediation system of waters exiting the mine. The area of the mine was already affected by acid-rock leaching so when remediation began the waters actually became cleaner than they had been before the mine was built. The water became less acidic, but the total dissolved solids (TDS) increased due to the sulfates and calcium hydroxide the mine was adding to the water to remove metals. Dissolved metals from upstream due to the thawing permafrost made TDS increase in the waters downstream that the remediation project was halted for a year while the mining company constructed a $19 million reverse osmosis plant to clean up the wastewater. The bottom line is perhaps that the permafrost has become a bigger polluter than the mine. According to the Scientific American article: “Lime is often dumped into tailings ponds at old mines to buffer acid, but you can't “lime” an entire mountain stream, just as you can't refreeze the ground around it. Perhaps the only real hope is that once all the permafrost has thawed and all the iron has rusted, these wild rivers will be able to flush out the contamination and restore themselves, although that would take decades at least.”

References:

75 Alaskan Rivers Turn Bright Orange, Tainting Water Supply, Endangering Humans and Wildlife; Scientists Investigate. Marissa Papanek. Knews. December 20, 2023. 75 Alaskan Rivers Turn Bright Orange, Tainting Water Supply, Endangering Humans and Wildlife; Scientists Investigate (msn.com)

Why Are Alaska’s Rivers Turning Orange? Alec Luhn. Scientific American. January 1, 2024. Why Are Alaska's Rivers Turning Orange? | Scientific American

The Rusting of Arctic Rivers: Freshwater Ecosystems Respond to Rapidly Uptaking Metals. US Geological Survey. Alaska Science Center. November 6, 2023. The Rusting of Arctic Rivers: Freshwater Ecosystems Respond to Rapidly Uptaking Metals | U.S. Geological Survey (usgs.gov)