BIOGEOCHEMISTRY OF ARSENIC IN A MICROCOSM EXPERIMENT USING SEDIMENTS FROM AN AS-CONTAMINATED WETLAND ADJACENT TO MINE TAILINGS

Terra Mine is an abandoned copper and silver mine in the Northwest Territories, Canada, at which mine tailings were disposed of into an existing lake adjacent to the mine’s processing plant. The mine tailings contain elevated levels of arsenic (As) resulting in As levels exceeding Canadian sediment and water quality guidelines. High levels of As are found in Ho-Hum lake, as well as in the downstream wetland water and sediments. The reduction and oxidation of As by As-reducing and As-oxidizing bacteria in the wetland water and sediments downstream of Ho-Hum lake are the subject of a field and laboratory study as the wetland has been proposed as a passive remediation system for the removal and storage of As.

Microcosm experiments were performed over a 42-day period, comparing three locations (upper, middle and lower sites) within the wetland to assess the overall stability of As-bearing minerals. The formation of dissolved methylated-As and thioarsenic species was tracked over time as these species in dissolved form may be an important consideration in determining the toxicity of As. The reduction of As(V) to the more mobile and potentially harmful dissolved As(III) was also investigated, as it can be catalyzed by As-utilizing bacteria. The microcosms compared fresh sediments and sediments amended with a 10 mM acetate solution; however, no significant difference was found between the acetate-amended and non-amended microcosms. The dissolved methylated and thiomethylated As species increased over time at all three locations from 3% or less of the total As on day 1 to 8%, 25% and 35% of the total As in the upper, middle and lower sites, respectively. Samples taken at the end of the experiment were analyzed by X-ray absorption spectroscopy to identify changes in the As oxidation state. Results indicate that there was an average 18% increase in the proportion of As(V) over the course of the experiment. Terminal restriction fragment length polymorphism accompanied by DNA sequencing of bacteria in pre- and post-microcosm sediments will help identify the microbial ecology at various locations within the wetland and in the microcosms. It is expected that As-utilizing, as well as sulfate- and iron-reducing, bacteria will be identified and contribute to understanding As cycling in the wetland and the role of bacteria in this cycle.