The influence of bacteria on the stability, speciation and mobility of arsenic in contaminated sediments at Terra mine, N.W.T., Canada
Drysdale, Jessica Ann
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Terra mine is an abandoned copper and silver mine in the Northwest Territories, Canada, from which mine tailings were deposited into Ho-Hum Lake, adjacent to the mine’s processing plant. The tailings contain elevated levels of arsenic (As), resulting in As levels exceeding Canadian sediment and water quality guidelines in the lake, and in downstream wetland water and sediment. This field and laboratory study focuses on the microbial ecology, and the reduction and oxidation of As, iron (Fe) and sulphur (S), in the wetland downstream from Ho-Hum Lake. This wetland is proposed as a passive remediation system for removal and storage of As. Using microcosm experiments, the stability of As-bearing sediments was compared in the upper, middle and lowestmost areas of the wetland over a 42-day period. Fresh sediments and sediments amended with a 10 mM acetate solution, both mixed with water, were compared. While no significant geochemical differences were found between acetate-amended and unamended microcosms, formation of inorganic As-S species was higher in amended microcosms, suggesting that micro-organisms were more active in the system because they were not carbon-limited. Formation of methylated-As species increased over time in all samples, including abiotic controls. Bacterial sulphate reduction occurred during the first 10 days of the experiment, perhaps resulting in precipitation of sulphide minerals. X-ray adsorption near edge spectroscopy was used to assess solid-state speciation of As in the sediments and indicated that pre-microcosm sediments from all sites showed high proportions of As(III)-S and As(III)-O speciation. Post-microcosm sediments revealed a 13% increase in the proportion of As(V)-O species, whereas abiotic controls showed only an 8% increase. DNA sequencing in post-microcosm sediments identified As, Fe and S reducing bacteria, and the geochemical patterns of As, Fe and S in the microcosms indicate the bacteria are likely active in the system. Microbial diversity and solid-state speciation of As in the sediments were assessed at varying depths at the microcosm sites, but correlation analysis revealed no significant relationship between As speciation and microbial diversity. A positive correlation between diversity and depth, and a negative relationship between As concentration and diversity, were found, perhaps indicating decreasing contamination with depth in the wetland.