Characterization of Arsenic Hosting Phases in Subaqueous Tailings at Terra Mine, Northwest Territories, Canada
Over the lifetime of operations at Terra Mine, an abandoned silver-copper mine in the Northwest Territories, an estimated 500,000 tons of tailings containing 540,000kg of arsenic (As) were deposited into Ho-Hum Lake. Arsenic is being released from the tailings to the overlying surface water, resulting in dissolved As concentrations in the surface water of Ho-Hum Lake of 50-80µg/L, exceeding the 5µg/L Canadian Council of Ministers of the Environment (CCME) guideline for the protection of aquatic life. The elevated As concentrations in the surface waters of Ho-Hum Lake present a concern for the local vegetation and aquatic life and the local indigenous communities. The objective of this study is to characterize solid and aqueous phases of As present in Ho-Hum lake subaqueous tailings and porewaters and identify the most significant geochemical processes driving the behaviour of As hosting phases. Surface water, pore water and lake bottom sediments were collected and analyzed. Water chemistry results were analyzed using speciation modeling to model mineral stability and aqueous speciation. Solid phase As speciation and mineral association was determined using synchrotron-based bulk X-ray absorption near-edge spectroscopy, scanning electron microscopy and electron microprobe analyses. Results indicate that tailings cover the entire bottom of Ho-Hum Lake. Primary As phases are stable wherever they are under permanent water cover. Where surface water is shallow near the shore of Ho-Hum lake, available sulphate and biological activity allows for the precipitation of secondary As sulphides. Where positive Eh conditions are observed in subaqueous tailings, As adsorbed to Fe (oxy)hydroxides are stable. Under slightly acidic to neutral pH and reducing conditions, Fe (oxy)hydroxide phases are undergoing reductive dissolution. Where porewater conditions are alkaline and Eh is negative, As adsorbed to Fe (oxy)hydroxides are stable but are sensitive to changes in pH, temperature and organic matter. The proposed remediation plan of lowering the lake level and expanding the outlet wetland would likely destabilize primary As phases near the lake shore and negatively affect the stability of Fe (oxy)hydroxides under negative Eh and alkaline conditions. Leaving Ho-Hum lake as-is would likely result in less As entering the local ecosystem in the long term.