Physical–Chemical Recovery of a Montane Stream After Remediation of Acid Mine Drainage: Timing and Extent After Turning off the Tap.

This article focuses on the physical–chemical monitoring and analysis of remediation effects on acid mine drainage (AMD) in the North Fork of Clear Creek (NFCC), Colorado, USA. Following the diversion and lime treatment of two major AMD inputs via the North Clear Creek Water Treatment Plant (NCCWTP) starting in 2017, concentrations and loads of metals such as Al, Cd, Cu, Fe, Mn, Ni, and Zn decreased substantially, particularly during low stream flows, though many postremediation metal levels remained elevated compared to upstream reference conditions. The study highlights that elevated stream flows can mobilize additional metals and major ions, complicating dilution effects, and that lime treatment reduces metal concentrations but does not significantly lower major ion concentrations, which may continue to impair sensitive aquatic organisms. Additionally, particulate iron (PFe) plays a dual role by adsorbing toxic metals and reducing their dissolved, bioavailable forms, while itself being toxic at high concentrations. The authors conclude that rapid shifts to new steady-state water chemistry can occur after major AMD inputs are treated, but residual unidentified sources, such as groundwater seeps, may sustain elevated metal and ion levels despite remediation efforts.