LAUSR

We monitored physical–chemical conditions in the North Fork of Clear Creek in Colorado (USA) before, during, and after the start of remediation (lime treatment) to remove metals from two major inputs of acid mine drainage (AMD) water. In addition, we analyzed historical monitoring data that extended back more than two decades. Concentration–discharge (C–D) and load–discharge (L–D) plots accounted for discharge dependence in concentrations and loads of metals, major ions, and other water chemistry parameters. Total and dissolved concentrations, and loads of the metals decreased after remediation began, with the largest decreases usually during low stream flow. However, postremediation concentrations and loads remained slightly to considerably higher than reference, probably because of unidentified groundwater seeps and/or small surface flows. Dissolved Cu concentrations decreased much less than total Cu concentrations, because the percentage of total Cu in the dissolved phase increased considerably as particulate Fe (PFe) concentration decreased. We conclude that 1) water chemistry can change to a new steady state or pseudo‐steady state relatively quickly after major AMD inputs to a stream are remediated; 2) elevated flows during snowmelt and rainfall periods can mobilize additional amounts of major ions and metals, resulting in in‐stream concentrations that are manifestations of both dilution and mobilization; 3) although lime treatment of AMD‐related waters can decrease metal concentrations, it does not decrease elevated concentrations of major ions that might impair sensitive stream invertebrates; 4) although Fe is toxic to aquatic organisms, PFe adsorbs other metals and thereby provides protection against their toxicity; and 5) use of C–D and L–D plots and element ratios can indicate the presence of unidentified AMD inputs to a stream. Environ Toxicol Chem 2023;42:495–511. © 2022 SETAC