Investigating processes driving copper, chromium, and sulphur chemistry changes across space and time in a boreal Canadian gold mine tailings impoundment

A long history of mining has resulted in hundreds of tailings cells in Canada that are orphaned or abandoned. Often the only feasible restoration strategies for such tailings, due to financial constraints, are facilitation of natural revegetation or surface amendment followed by direct planting into tailings. Small changes in substrate conditions, therefore, can have large impacts on revegetation success. This research focuses on a 2.6 km² gold mine tailings impoundment in northern Ontario, Canada. Over three years, samples were collected from multiple locations and depths across the tailings cell and directly from the processing plant. Elements of interest include S, Cu, and Cr. Metal concentrations were measured using ICP-MS and changes in elemental speciation were measured using synchrotron x-ray absorption spectroscopy. Cr is unchanged along sampling gradients, while S and Cu exhibit great variation in their chemical state along sampling gradients. Results surprisingly show that trace organic carbon from gold processing has a strong effect on Cu speciation, and we further discuss the efficacy of correlating proxy measures of chemical states (e.g. S redox state, pH, conductivity, and carbon content) to Cu and Cr speciation. The results from this research provide insights into how chemical characteristics, including elemental speciation, can vary across time and spatial scales in mine tailings impoundments and what processes may be driving these changes. Future work should consider the described processes when designing sampling methodologies and restoration strategies of similar tailings impoundments.