Coordinated landform and multi-layer gas barrier cover system design for uranium tailings

Abstract

During the late 1950s to early 1980s, historic operations at a former uranium mine and mill site in Canada resulted in the deposition of about 5 million tonnes of uranium tailings into two tailings management areas (TMAs). In the 1980s, the site was rehabilitated to the standards of the day, which included placement of a simple cover system consisting of a single 0.3–0.5 m thick layer of sand and gravel over the tailings. The authors, as part of a project team, evaluated the performance of the past decommissioning efforts relative to current regulatory standards and recommended that the TMA cover system be upgraded to better control surface erosion, reduce the emission of radon gas, and limit water infiltration into the tailings as part of the progressive rehabilitation of the former mine site. A multi-layer cover including a sand-gravel-bentonite (SGB) layer was selected to upgrade the soil cover of the TMAs to limit migration of radon gas from the tailings. This type of diffusion barrier cover system incorporates a ‘performance’ layer of sand and gravel amended with bentonite and a ‘protective’ layer of sand and gravel above the performance layer. Performance, in terms of reducing radon diffusion, is attained through sustained saturation of the performance layer by providing water retention in the protective layer. This type of cover system has typically been installed on relatively flat grades because infiltration of precipitation into the cover is a contributing factor to successful cover performance. A parallel objective of the rehabilitation of the TMAs is to provide a passive closure landform that requires limited maintenance and monitoring over the longer term. A robust landform and drainage system has been designed to control the flow of water and eliminate ponding on the TMAs with the objective of maintaining long-term physical stability while reducing maintenance and monitoring requirements. Through improved landform/drainage design, the following are expected to be achieved: (1) long-term physical stability of cover surface, (2) reduced infiltration into the tailings and loading of contaminants to the downstream environment, and (3) containment of clean runoff from the TMAs within an excavated aggregate/infiltration area with conveyance of flow to the subsurface (i.e. no surface discharge). The landform design for the TMAs includes surface slopes of up to 10H:1V. Upgrading of the cover system commenced in 2015 and was completed on TMA-2 in 2017. Performance monitoring of the completed upgraded cover has been ongoing since 2017, including measurement of saturation levels, suction pressures and temperature within the cover, radon flux from the surface of the cover and visual inspection of the cover surface for signs of erosion and slope instability. Monitoring results have indicated that design objectives are being met or exceeded and the performance of the cover, in terms of internal cover saturation levels, is being achieved on slopes of up to 10H:1V. These results demonstrate that landform/drainage design that allows for shedding of clean surface waters can be achieved while maintaining adequate saturation levels within the diffusion barrier cover.