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PNAS Study Links Permafrost Thaw to Toxic, Orange Rivers in Alaska’s Brooks Range

Researchers report metal levels above EPA aquatic toxicity thresholds with few viable options for remediation.

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“This is what acid mine drainage looks like,” Tim Lyons, a biogeochemist at UC Riverside, told the university in a September 2025 article. “But here, there’s no mine.”
The scientists’ observations at the watershed suggest the potential for “for widespread future stream degradation with permafrost thaw further north and elsewhere at high latitudes,” they wrote. 

Overview

  • The peer‑reviewed analysis centered on the Salmon River confirms that thaw‑driven oxidation of sulfide‑rich rocks produces sulfuric acid that mobilizes metals into the water.
  • Sampling from August 2022 to August 2023 found dissolved metals, including iron, aluminum, and cadmium, at concentrations exceeding EPA benchmarks for aquatic life.
  • Chemical & Engineering News reports most EPA‑regulated metals were elevated along the main stem, with arsenic, lead, and selenium generally below safe‑limit thresholds.
  • Scientists say the rusty discoloration and chemistry resemble acid mine drainage despite the absence of mining, and similar changes are already appearing across many Arctic streams.
  • Current fish tissue levels are not deemed hazardous to humans, yet researchers warn of risks to fish, insect larvae, and salmon spawning habitat, with the process effectively irreversible until permafrost recovers.