Oral Presentation Society for Freshwater Science 2026 Annual Meeting

Arctic rusting streams and rivers show spatial and temporal variability in dissolved metal concentrations from acid rock drainage (134537)

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

Streams and rivers in Arctic Alaska are turning orange due to acid rock drainage that mobilizes metals from sulfide minerals below thawing permafrost. These “rusting” streams are often characterized by lower pH and elevated sulfate (SO42-) and metal concentrations (e.g., iron (Fe), aluminum (Al), and zinc (Zn)) with potential consequences for fisheries, biodiversity, and human health. However, the spatial and temporal variability of chemical signatures from acid rock drainage, and the relative contributions of point versus diffuse sources remain poorly understood. Here, we performed repeat synoptic samplings of hillside seeps (n=4), tributaries (n=6), and mainstem sites (n=7) in the Nakolikurok Creek watershed (817 km2) in the Brooks Range, Alaska, USA, during June, July, and twice in August 2024 to understand how acid rock drainage chemical signatures varied spatially and temporally. Across the whole watershed, the pH ranged from 2.8—8.5, and was lower in the seeps and impaired tributaries compared to mainstem sites (Kruskal-Wallis p<0.001). Seeps and impaired tributaries showed distinct chemistry from unimpaired tributary and mainstem sites. Hillside seep chemistry was highly temporally variable; for example, SO42- concentrations in a single seep varied from 13,700 mg/L in July, to 5960 mg/L in August, and climbed to 13,200 mg/L the following week. A subwatershed leverage (relative chemical influence) analysis indicated that tributaries (impaired and unimpaired combined) contributed -13-6465% of dissolved (filter-passing) Fe and -8-151% total Fe to the outlet of Nakolikurok Creek, suggesting that some tributaries dilute Fe while others disproportionately enrich downstream concentrations. Tributaries sampled June-August showed variable spatial stability (temporal persistence of spatial patterns) in both dissolved and total Fe (40% stable, 40% unstable), Al (66% unstable) and Zn (33% unstable), likely due to episodic inputs from impaired tributaries. Mainstem sites were predominantly moderately stable. Sulfate, a conservative tracer of acid rock drainage, was strongly related to continuous specific conductance (R2=0.96), with specific conductivity and sulfate concentration peaking in late July. These findings demonstrate that acid rock drainage influences Arctic river chemistry through highly heterogeneous and dynamic source contributions, complicating efforts to predict downstream metal exposure under continued permafrost thaw.