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

Abstract

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.

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

“…Spatial and temporal variation in concentrations of major ions and conductivity at BTP, Gage, and Main suggest that these patterns were associated with the water treatment facility. In contrast to patterns observed at station BNT after mining‐related effluents were diverted, conductivity and concentrations of major ions downstream of the treatment facility either changed little or increased after remediation (Meyer et al, 2022 ). We hypothesize that in addition to metals, elevated concentrations of major ions impeded full recovery.…”

“…The inverse relationship between Cu and Fe is toxicologically important, because 1) mesocosm experiments have shown that Cu and Fe are responsible for much of the toxicity to benthic organisms relative to their aquatic life criterion values (Clements et al, 2019 ), and 2) their concentrations in surface waters are regulated in the United States and most other jurisdictions around the world. In the NFCC, the percentage of Cu that is dissolved is strongly and inversely correlated with the particulate Fe concentration (Meyer et al, 2022 ). This relationship in part explains why the concentration of dissolved Cu at Gage decreased by only 20% after treatment whereas the total Cu concentration decreased by 67% (Meyer et al, 2022 ).…”

“…The limited improvement in benthic communities was especially notable at BNT and BTP, the most heavily impacted stations. The modest recovery of NFCC 2 years after remediation was unexpected given that metal concentrations rapidly decreased soon after the NCCWTP became operational (Meyer et al, 2022 ). This result was particularly surprising at BNT because of its proximity to the upstream reference station and because discharges from the two major AMD sources (Gregory Incline and National Tunnel) were completely diverted around this station.…”

“…In particular, heptageniid mayflies, which are among the most sensitive organisms to major ions (Clements & Kotalik, 2016 ; Cormier et al, 2013 ), recovered little after remediation. In addition to these high average conductivity values, seasonal increases in conductivity during periods of low stream discharge (e.g., fall samples) at all NFCC downstream sites often exceeded 700 µS/cm (Meyer et al, 2022 ). These results suggest that despite significant decreases in metal concentrations after remediation, other water quality changes associated with the water treatment facility affected benthic communities.…”

“…Effects of station, season (spring, summer, fall), and remediation (before vs. after) on benthic macroinvertebrate communities were analyzed using three‐way analysis of variance (ANOVA; using PROC general linear model [GLM; SAS Institute]). Before statistical analyses, benthic and water chemistry data were averaged for BTP‐1 and BTP‐2 as a combined BTP station because pretreatment data were not available for BTP‐1 and because these two stations have very similar water chemistry characteristics (Meyer et al, 2022 ). To generate BTP station averages, benthic abundances among taxa were averaged for every paired BTP‐1 and BTP‐2 sampling date, and the composite water chemistry values assigned to BTP‐1 and BTP‐2 for each respective benthic sampling date were averaged.…”