Cu isotopes resolve acid mine drainage sources and treatment pathways at the argo tunnel water treatment plant

AMD poses severe environmental hazards, releasing toxic metals into ecosystems and adversely impacting water quality. This study leverages Cu isotopes as environmental tracers to identify AMD sources and decode treatment mechanisms at the Argo Tunnel Water Treatment Plant (ATWTP) in Idaho Springs, Colorado. The δ65Cu isotopic signatures from distinct sources (Argo Tunnel, Virginia Canyon, and Big Five Tunnel) effectively differentiate the Cu contributions from each source. Mass balance modeling reveals that Mn-oxides accounted for 54% of Cu retention, and Mn-oxide adsorption is the dominant mechanism for Cu removal at the ATWTP, although Fe-oxides are more abundant. These findings suggest that adjusting Mn concentrations in influent waters could enhance Cu removal, forming the basis of the proposed CLEAR PATH strategy (Chemical Lime Enhancements for Advanced Remediation and Purification At Treatment Hubs) to optimize AMD treatment. This research also demonstrates the utility of Cu isotopes for tracing AMD sources and refining treatment processes, offering actionable insights for improving environmental remediation in mining- impacted regions. This work provides the first plant-wide copper-isotope mass balance ever reported for a full-scale AMD treatment plant and the first operational quantification of Cu partitioning between co-precipitating Fe- and Mn-oxides. • δ 65 Cu traced distinct AMD sources even after mixing at plant intake. • Adsorption identified as the main driver of Cu isotope fractionation in lime-based AMD treatment. • Plant-wide δ 65 Cu mass balance confirmed full Cu accounting in ATWTP. • Mn-oxides retained more Cu than Fe-oxides (54% vs. 46%) despite lower abundance. • CLEAR PATH strategy: Mn management improves Cu removal efficiency.