Characterizing and Modelling Tailings Hydrogeochemistry Under a Composite Cover at the Abandoned Kam Kotia Mine, ON

Abstract

A principal environmental concern associated with mining is the weathering of mine waste in the presence of oxygen and water that generates acid mine drainage (AMD) concentrated in toxic elements. Installation of composite covers on mine waste is a remedial option to prevent the generation of AMD. Consisting of layers of materials of contrasting grain sizes, composite covers can inhibit oxygen and water ingress. Despite extensive studies on the design of composite covers, there is limited information available on the field performance of composite covers on legacy tailings. This thesis investigates the hydrogeochemistry of a legacy tailings impoundment remediated with a five-layer composite cover in 2008 and evaluates the long-term field performance of the cover. To investigate the hydrogeology and gas transport processes in the cover and tailings, groundwater flow, vadose zone hydrology, stable water isotopes, and pore-gas concentrations were monitored over multiple years, and numerical modelling of the variably saturated flow and calculations of gas flux were conducted. To characterize the geochemistry and mineralogy of the tailings under the composite cover, pore-water and groundwater sampling and geochemical analysis, selective chemical extractions, optical and electron microscopic study of thin sections, and synchrotron-based X-ray absorption spectroscopy experiments were conducted. To quantitatively assess the hydrogeochemical processes in the tailings from deposition to remediation by a composite cover, 1-D reactive transport simulations were implemented using the multi-component reactive transport code MIN3P. Characterization and modelling of the cover hydrogeology and gas transport indicate that during the summer, fall, and spring, the capillary barrier effect of the cover functioned well. The clay maintained saturation except for temporary desiccation during summer droughts. Potential local cover defects enabled oxygen advection into the tailings at one out of three monitoring locations. A seasonal change in the hydrology of the cover system occurred in the spring when snowmelt infiltrated deep into the tailings and reduced both oxygen diffusion and advection via cover defects, resulting in depleted pore-gas oxygen concentrations. Overall, the cover was effective in reducing infiltration and oxygen flux into the tailings. Mineralogical studies suggest that due to the high pyrite content (up to 74 wt%), sulfide oxidation depleted pyrites to a depth of 0.1-0.25 m. Aqueous geochemical results suggest that at one monitoring location, the cover reduced AMD generation and improved pore-water quality. The presence of a peat layer rich in organic carbon below the tailings facilitated sulfate reduction, albeit limited in capacity. In contrast, at another monitoring location, potential cover imperfections facilitated continued sulfide oxidation, resulting in low pore-water pH and elevated aqueous concentrations of sulfate, Fe, Zn, Cu, and As. Crystalline oxyhydroxides are a major sink for trace elements and may release them through desorption, acidic dissolution, and potential reductive dissolution under strongly reducing conditions. The complex geochemical processes indicate that sulfate, As, Zn, and Mn may remain long-term contaminants of concern. 1-D reactive transport simulations that incorporated transient infiltration, post-cover hydrogeochemical changes, a dynamic temperature regime, sulfide-mineral oxidation, acid neutralization, secondary-mineral precipitation, and sorption processes resulted in good agreement with the field results. Simulated temporal changes in reaction rates and geochemical parameters indicate that where intact, the cover lessened sulfide oxidation by both oxygen and Fe(III) and improved pore-water quality over time. Sulfate, Zn, and As persisted regardless of cover performance, whereas Cu and Al were the most sensitive to cover imperfections. The combined results from hydrogeochemical characterization and modelling suggest that even though the composite cover was effective in controlling infiltration and oxygen transport a decade after placement, complex geochemical processes in legacy tailings resulted in the persistence of deteriorated water quality and several elements may remain of long-term concern. This study can provide insight into cover design and mine remediation.