Abrupt onset and persistent downstream effects of acid rock drainage on rivers across northwest Arctic Alaska
Arctic streams and rivers are increasingly vulnerable to degradation from permafrost thaw that releases toxic elements via natural acid rock drainage. However, the spatial and temporal evolution of acid rock drainage on river chemistry across diverse mineral settings remain poorly constrained. Here, water chemistry in six watersheds across Alaska's Brooks Range (2022–2024) quantified the effects of acid rock drainage on river chemistry. Point-source acid rock drainage inputs (seeps and impaired tributaries) exhibited lower pH (median; pH: 3.2 and 6.9, respectively), higher sulfate (SO 4 2− : 3,025 and 390 mg L −1 , respectively), and elevated major and trace element concentrations (sum of 23 metals: 451 and 9.2 mg L −1 , respectively), compared to upstream unimpaired mainstem rivers (pH = 8.3, SO 4 2− = 68 mg L −1 ; metals = 0.1 mg L −1 ). Across watersheds, these inputs increased mainstem concentrations of major and trace elements (e.g., Fe, Al, Mn, Zn, Ni, Cd, and rare earth elements) and SO 4 2− ; however, mainstem river pH remained stable due to sufficient buffering. Within the Salmon River, concentrations of filter-passing and particulate fractions of constituents (SO 4 2− , Fe, Zn, Ni, and Cd) were elevated ≤70-fold at >100 km downstream of acidic inputs, documenting long-range transport. Temporal analyses of SO 4 2− and Zn concentration (2015–2024) revealed abrupt increases (≤263%) in two watersheds between 2019 and 2020, aligning with antecedent record warm and snowy winter conditions. Collectively, our results document the abrupt onset of acid rock drainage across Arctic rivers with potential for long-range downstream transport and implications for biogeochemical cycles and ecological and human health.