Influence des facteurs climatiques sur la performance de la nappe phréatique surélevée combinée à un recouvrement monocouche

On a mine site, one of the largest and potentially significant environmental issues is the tailings management facility. Tailings, from the ore processing process, may still contain a large quantity of sulphides. The chemical reaction of these sulphides with water and oxygen is responsible for acid mine drainage (AMD). It is characterized by low pH and high concentrations of metals and sulphates, which are harmful to the environment if not controlled. In order to limit the generation of AMD, it is important from the beginning of the mining operation, to develop a reclamation plan for the tailings management facility. Reclamation methods are intended to remove one of the three components of sulphide oxidation: water, oxygen or sulphides. In wet climates, the reclamation methods used favour the exclusion of oxygen, such as the method of the elevated water table combined with a monolayer cover. This technique allows the water level to be maintained at an optimal level allowing the tailings to remain saturated or close to saturation, thus limiting the oxidation of sulphides and the generation of DMA. This technique is inherently dependent of precipitations, which will largely be responsible for water level variations within the tailings facility. The expected climate change by 2100 will result in a change in precipitation patterns. In Quebec, significant changes in annual average temperatures and precipitation and an increase in the frequency and intensity of extreme events are expected. Increased precipitations would have a positive impact on the reclamation scenario, while periods of drought could make it ineffective in stopping the generation of AMD. The objective of this study is to investigate the long-term performance of the elevated water table method combined with a monolayer cover in the context of climate change. This reclamation method is to be implemented at the tailings facility of the Doyon-Westwood mine site (Cadillac, Québec). In order to test this method, field cells were constructed on the site. Hydrological and geochemical monitoring was conducted from 2015 to 2018. The materials used for cell construction were characterized in the laboratory in 2015 during cell construction to determine saturated hydraulic conductivity and the water retention curve of the materials. New tests were conducted as part of this study to confirm the results obtained in 2015. A 1D numerical model was built using the SEEP/W software to represent the cells. The model was calibrated using meteorological data from the Rouyn-Noranda station and modifying the hydrological properties of the materials to reproduce the data of volumetric water content obtained during cell monitoring in the field. The Doyon tailings ponds #2 was then numerically simulated using the average Doyon tailings thickness and a monolayer cover of one-metre thickness made with desulfurized Westwood tailings. In order to integrate climate change, several climate models according to two radiative scenarios (RCP4.5 and RCP8.5) were used. A total of 18 climate simulations were obtained. As part of this study, three climate scenarios from these simulations were used, including one representing the worst-case scenario for the reclamation method with a small increase in precipitation combined with a significant increase in temperatures. Climate parameters from the three selected climate scenarios were integrated into the numerical models (field cells and ponds #2) to simulate the behaviour evolution of the reclamation method from 2020 to 2100. The various numerical simulations carried out demonstrated the main impact of parameters such as water table level and material properties. The thickness of the monolayer cover can also be optimised to improve the behaviour of the reclamation method. When the design parameters of the reclamation method mentioned above are optimized, saturation of the Doyon tailing which reflects the good performance of the method is ensured even in the case of the worst case scenario used. This observation was also made for the other two climate scenarios whose results did not show significant differences.