EVALUATION OF A LOW-ORGANIC SOIL COVER FOR REMEDIATION OF ARSENIC-RICH GOLD MINE TAILINGS

During mineral processing, tailings are generated as a waste product and have historically been disposed of as a sediment-water slurry into local wetlands and streams. In Nova Scotia, over 3,000,000 tonnes of tailings were generated from gold mines in operation between the 1860s and 1940s. Since then, the tailings have been variably weathering for over 70 years, leading to the natural attenuation of As in multiple secondary mineral phases and the development of four tailings end members with distinct geochemistry and mineralogy. We evaluated As mobility from these end members under a shallow 30 cm thick, low-organic soil cover in a laboratory leaching experiment. Each column replicated the vertical stratigraphy of one of the four major tailings types (hardpan tailings, oxic tailings, wetland tailings, and high-Ca tailings) found at the Montague and Goldenville tailings sites. To simulate field conditions, two years’ worth of synthetic rainwater was added progressively to each column and two dry seasons were incorporated into the leaching protocol. Chemical analysis of the column leachate showed that the hardpan tailings released acidic, As-rich waters under the cover but this acidity was buffered by surrounding oxic tailings. Oxic and high-Ca tailings released As continuously throughout the experiment. Leachate from the wetland tailings became acidic after the first dry season, but As concentrations decreased over time. Dilution was not a significant process controlling element concentrations despite higher water:rock ratios and leach rates as compared to typical field conditions. Instead, mineral precipitation and adsorption limited As concentrations in leachate, though not below drinking water guidelines. This study shows that the use of a shallow, low-organic content soil cover does not prevent an influx of oxygen to the tailings or create reducing conditions that would reductively dissolve As-bearing phases (e.g. scorodite). However, during dry seasons this type of cover will not prevent the release of As to local waters from sulfide oxidation. Due to the complex mineralogy of these tailings, a soil cover may represent the best choice for keeping current As-bearing phases stable while also slowing As-bearing sulfide mineral oxidation reactions.