Microbial populations associated with the Cu/Zn Kam Kotia Mine after geosynthetic clay liner cover system installation

The largest liability associated with the metal-mining industry derives from acid mine drainage (AMD), caused by accelerated chemical and/or biological oxidation of sulfidic minerals within waste rock and mill tailings. AMD is characterized by acidic water, containing high concentrations of sulfate, iron, and associated metal(loid)s. At the abandoned Cu/Zn Kam Kotia Mine (ON, Canada), installation of a geosynthetic clay liner (GCL) cover system was completed in 2008. The goal of this study was to characterize the microbiology of the GCL cover system and the underlying tailings to assess the performance of the cover for mitigating AMD generation. Tailings beneath the GCL were characterized by circumneutral pore water (pH 5.5 to 7.3), high sulfur and low carbon contents. Total genomic DNA was extracted from each layer of the tailings impoundment, and was analyzed using a high-throughput amplicon sequencing of the 16S rRNA gene. The lowest richness was ob-served in the tailings, which present an adverse environment for many microbial species. β-diversity was investigated by non-metric multidimensional scaling based on Bray-Curtis dissimilarity matrices. While each layer of the cover system clustered closely together, lower microbiota similarities were observed in the tailings. Acidophilic iron-oxidizing (aIOM), acidophilic sulfur-oxidizing (aSOM), sulfate-reducing (SRM), and aerobic heterotrophic microorganisms were enumerated using the cultivation-dependent most probable number (MPN) technique, with heterotrophs being the most numerous group of viable microorgan-isms across the depth profile. Acidithiobacillus (At.) ferrooxidans was identified as the most abundant aIOM/aSOM in the tailings. Both low relative abundances and low viable counts of oxidizers in the covered tailings suggest significant reduction of sulfide oxidation rates at the Kam Kotia mine site, due to the presence of the GCL.