Authors: Meiers, G; Kenyon, Z; Butsavage, P; Pernito, M

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Meiers, G, Kenyon, Z, Butsavage, P & Pernito, M 2023, 'The Ridgeway TMF cover system after 20 years of atmospheric forcing, what we knew then and now', in B Abbasi, J Parshley, A Fourie & M Tibbett (eds), Mine Closure 2023: Proceedings of the 16th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth,

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Dry cover systems are of particular interest in mitigating potential closure risks associated with tailings dam structures at closure. The Rio Tinto, Kennecott Ridgeway Mining Company (Ridgeway) tailings management facility (TMF), which was reclaimed with a dry cover system, is a unique case study spanning over twenty years of closure performance. The lined Ridgeway TMF contains approximately 60 million tonnes of potentially acid generating tailings (PAG). Mining operations ceased in November 1999 and the TMF was reclaimed with a hydraulically placed saprolite clay water store-and-release cover system. Placement of the cover system was initiated in January 2000 and completed in May 2000. A stated key performance objective of the cover system is to maintain a high degree of saturation in the cover and tailings, thereby limiting oxygen ingress and providing geochemical stability to the potentially acid generating tailings. Vegetation on the cover is primarily Bermuda grasses, millet, and sericea lespedeza. The vegetation was mowed (vegetation management) to maintain the grassland; however, this practice was discontinued in later years allowing for woody brush species to colonize the landform. In 2022 vegetation management was implemented to return the cover to the original grassland vegetation. The cover system design was supported by water balance numerical simulations to evaluate performance under varying climatic conditions. In 2007 a field response numerical simulation model was developed to gain further insight into the predicted performance of the cover system. Simulated oxygen ingress was predicted to be less than ~0.2 mol/m2/yr. A cover system assessment program was implemented in 2022, to obtain multiple lines of evidence to support the performance of the cover system following 22-years of atmospheric forcing. The assessment program included an in-situ sampling and characterization program to assess vegetation characteristics and geotechnical and geochemical properties of the tailings and cover material. Numerical simulations were completed to evaluate climate change and mitigation measures for addressing potential performance risks. The oxygen ingress rate through the cover system was estimated to be Low or 1 to 5 mol/m2/yr. The simulated results suggest that vegetation management could reduce the rate of oxygen ingress. Climate change poses a potential risk to long-term performance where vegetation management may not provide adequate mitigation. The degree of saturation in the tailings and cover material in October 2022 ranged from 68% to 100%, which would support the estimated oxygen ingress. This paper presents the conceptual model of closure performance based on an information review and results of the in-situ sampling and characterization program as well as the results of the preliminary soil-plant-atmosphere numerical simulation models.

Keywords: Tailings management facility, tailings storage facility, dry cover system, oxygen ingress, vegetation, metal leaching and acid rock drainage

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