Integrated Assessment of Chemical and Biological Recovery After Diversion and Treatment of Acid Mine Drainage in a Rocky Mountain Stream

Abstract

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. Environ Toxicol Chem 2023;42:512–524. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

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

“…Only for Ni were both the post‐treatment total and dissolved concentrations relatively close to the reference concentrations (38% and 20% higher than at the reference site, respectively). Therefore, most of the posttreatment metal concentrations downstream of the reference site remained elevated and might have continued to adversely affect sensitive aquatic organisms (see Kotalik et al, 2022). Like the elevated conductivity, these elevated metal concentrations might have resulted from nondiverted AMD sources and other nonidentified inputs, plus possible release from the streambed.…”

“…Although these results demonstrate frequent nonachievement of remediation goals for some metals on the NFCC and Clear Creek, the extent of impairment of sensitive aquatic organisms by metals might be even higher because the Superfund remediation goals were designed to protect brown trout, which have high sensitivity to Cd (USEPA, 2016) but only intermediate sensitivity to Cu, Mn, and Zn (DeForest & Van Genderen, 2012; Stubblefield & Hockett, 2000; USEPA, 2007). For example, benthic macroinvertebrate communities downstream of the reference site on the NFCC and at the Main site on Clear Creek continue to be impaired relative to the reference site, probably as a result of continued elevated concentrations of metals and/or major ions (Kotalik et al, 2022).…”

“…In addition, high water velocities that occur during runoff scoured metal‐rich floc from the stream substrate in the spring of 2017. Because deposition of Fe‐oxyhydroxide flocs can impair stream invertebrates (Kotalik et al, 2019), this improvement of the physical habitat probably contributed to postremediation increases in the numbers and richness of benthic taxa in the NFCC (see Kotalik et al, 2022). However, thin, dark reddish, armored coatings still persist on streambed cobble and boulders.…”

“…Except for possible precipitation of Mg-hydroxides and/ or Mg-carbonates at high pH in the lime reactor, lime treatment does not remove major ions. Therefore, the continued elevated concentrations of major ions might have important implications for continued impairment of sensitive aquatic organisms in receiving waters, even when metal concentrations have decreased to nontoxic concentrations (Kotalik et al, 2022). In addition, Fe appears to be a second double-edged sword in the water column.…”

“…We present an analysis of concentrations and loads of metals, major geochemical ions, and routine water chemistry parameters that reveals the rapidity with which some physical-chemical conditions improved but others did not change after remediation. A parallel analysis of biological communities in the NFCC before, during, and after remediation has been reported by Kotalik et al (2021), and a synthesis of the biological and chemical interactions is presented in Kotalik et al (2022).…”