Hydrogeochemical Evaluation and Impact of Remediation Design on Arsenic Mobility at Historical Gold Mine Sites
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Historical gold mine tailings at several sites in Nova Scotia, Canada are publicly accessible and may pose a threat to human and ecosystem health because of high arsenic (As) concentrations in the tailings (max 25 wt.%) and associated pore waters (up to 100 mg/L). Two of these sites, Montague and Goldenville, are under consideration for remediation. Similar tailings sites have been cleaned up by covering the mine wastes with soil. However, the tailings at Montague and Goldenville have been weathering for at least 70 years, leading to a wide range of As-bearing secondary minerals which may dissolve under a soil cover, releasing As to local waters. The challenge of remediating these heterogeneous tailings lies in the different Eh-pH niches in which iron arsenates (oxidizing, acidic), calcium-iron-arsenates (oxidizing, alkaline), and sulfides (reducing) are stable. The main objectives of this study were to: 1) characterize pre-remediation geochemical controls on As mobility in subsurface tailings; 2) establish hydrogeological influences on As mobility; and 3) identify geochemical changes that result when a low organic soil cover is applied to the tailings. Pore water measurements were combined with bulk chemistry, scanning electron microscopy, and synchrotron micro-X-ray diffraction analyses, which were used to characterize the mineralogical composition of the tailings. Groundwater and surface water flow regimes throughout the tailings were defined through the use of piezometers and hydraulic conductivity measurements. Laboratory leaching experiments were used to assess the effects of a soil cover on the tailings. Variable weathering conditions over time have resulted in a continuum of saturation and redox environments and a range of As hosts in the tailings. In some areas, tailings pore waters are mixing with stream waters leading to As transport beyond the tailings. Applying a low organic soil cover does not induce reducing conditions in the tailings or cause dissolved As concentrations to increase compared to field pore water concentrations. This type of soil cover is effective in slowing sulfide mineral oxidation while maintaining stable conditions for secondary As-phases. The results of this research can be used to inform remediation decisions and guide ongoing environmental management of historical gold mine sites.