Oral Presentation Society for Freshwater Science 2026 Annual Meeting

Acid Rock Drainage in a Warming Arctic: Abrupt Onset and Persistent Downstream Effects in Rivers Across Alaska’s Brooks Range (134637)

Taylor G Evinger 1 , Jonathan A O'Donnell 2 , Michael P Carey 3 , Joshua C Koch 3 , Carson A Baughman 3 , Kenneth R Hill 4 , Rebecca Frei 1 , Brett A Poulin 1
  1. University of California, Davis, Davis, CA, United States
  2. Arctic Network, National Park Service, Anchorage, Alaska, United States
  3. Alaska Science Center, U.S. Geological Survey, Anchorage, Alaska, United States
  4. Arctic and Central Alaska Networks, National Park Service, Fairbanks, Alaska, United States

Streams and rivers in Alaska’s Arctic provide essential ecological and societal benefits but are increasingly vulnerable to degradation from permafrost thaw and the release of toxic elements. Here, we evaluated the water chemistry of six watersheds across Alaska’s Brooks Range (2022-2024) to quantify the effect of thaw-driven acid rock drainage on river chemistry. As a control, we sampled pristine mainstem rivers absent of major point-source acid rock drainage inputs and found low metal concentrations (median; sum of 23 metals: 99 µg/L) in conjunction with alkaline pH (pH: 8.3). In contrast, point source acid rock drainage from hillside seeps (locations of acidic groundwater upwelling on hillsides) exhibited low pH (pH: 3.2), high sulfate (SO42-: 3,025 mg/L), and higher major and trace element concentrations compared to alkaline streams (sum: 451 mg/L), consistent with metal-rich sulfide mineral sources. Similarly, tributaries impaired by seeps exhibited lower pH (pH: 6.9), higher sulfate (SO42-: 390 mg/L), and elevated metals (sum: 9.2 mg/L) than rivers from control, unimpaired watersheds. Across the six watersheds, point-source acid rock drainage inputs from seeps and tributaries increased mainstem concentrations of SO42- up to 962% and many other elements (Fe, Al, Mn, Zn, Ni, Co, Cd, Cu, and rare earth elements (REE)). Further, a temporal analysis (2015-2024) of SO42- concentration revealed an abrupt increase of 174-263% in 2019 and 2020 across two watersheds following record warm and snowy years. This timing is consistent with the onset of orange staining in multiple watersheds identified by remote sensing observations. In the Salmon River watershed, impaired tributaries significantly increased filtered and particulate constituent concentrations (SO42-, Fe, Zn, Ni, and Cd) that remained elevated >100 km downstream, highlighting the potential for long-range transport. The low pH and high metal concentration of seeps and tributaries impaired by seeps observed across watersheds reveal the high potential for acid generation and metal release across watersheds spanning the western Brooks Range. Collectively, these results document a high potential for acid rock drainage to affect mainstem river chemistry across the western Brooks Range that persists downstream, with implications for biogeochemical cycles and ecological and human health.