Integrated assessment of chemical and biological recovery after diversion and treatment of acid mine drainage in a Rocky Mountain stream

Responses of stream ecosystems to gradual reductions in metal loading following remediation or restoration activities have been well documented in the literature. However, much less is known about how these systems respond to the immediate or more rapid elimination of metal inputs. Construction of a water treatment plant on the North Fork of Clear Creek (NFCC; CO, USA), a US Environmental Protection Agency Superfund site, captured, diverted, and treated the two major point-source inputs of acid mine drainage (AMD) and provided an opportunity to investigate immediate improvements in water quality. We conducted a 9-year study that included intensive within- and among-year monitoring of receiving-stream chemistry and benthic communities before and after construction of the treatment plant. Results showed a 64%–86% decrease in metal concentrations within months at the most contaminated sites. Benthic communities responded with increased abundance and diversity, but downstream stations remained impaired relative to reference conditions, with significantly lower taxonomic richness represented by a few dominant taxa (i.e., Baetis sp., Hydropsyche sp., Simulium sp., Orthocladiinae). Elevated metal concentrations from apparent residual sources, and relatively high conductivity from contributing major ions not removed during the treatment process, are likely limiting downstream recovery. Our study demonstrates that direct AMD treatment can rapidly improve water quality and benefit aquatic life, but effectiveness is limited, in part, to the extent that inputs of metals are captured and treated. Consideration should also be given to the effects of elevated major ion concentrations from the treated effluent not removed during the lime treatment process. Continued chemical and biological monitoring will be needed to quantify the NFCC recovery trajectory and to inform future remediation strategies.