Comportement hydrogéologique d'une couverture monocouche sur des résidus miniers sulfureux: Essais en colonne et simulations numériques

One of the main environmental problems facing the mining industry results from the surface storage of reactive tailings. Such tailings result from the milling process, where the extracted ore is crushed and treated to concentrate selected minerals and metals. When in contact with water and air, sulphide minerals contained in the tailings can oxidise and generate acid mine drainage (AMD). These acidic waters have a low pH (< 3) and may contain high concentrations in dissolved sulphates and metals. To meet environmental regulations, mining companies need to prevent or at least reduce AMD production. The oxidation reactions can be controlled by excluding one of the constitutive components of the reactions, i.e. water or oxygen. The main methods used under humid climates to prevent AMD formation are oxygen barrier such as water covers and covers with capillary barrier effects (CCBE). An alternative method is the use of the elevated water table technique applied in conjunction with a monolayer cover. To date however, relatively little work has been done on this technique for practical situations. This project aims at studying the behaviour of a monolayer cover implemented with an elevated water table on reactive tailings. The investigation includes the evaluation of the effect of various parameters on the capacity of the cover to act as an oxygen barrier or as an evaporation barrier, depending on the position of the water table. The study is loosely based on some of the characteristics of the Selbaie mine site situated in the north-west of Québec. This tailings impoundment has been partly reclaimed with a monolayer cover. The tailings and the cover material used in the laboratory column tests conducted here were collected from this site. The characterisation of the tailings and cover properties was done in the laboratory to determine their grain size distribution, relative density, optimum Proctor unit weight, saturated hydraulic conductivity, water retention curve, oxygen diffusion and reaction coefficients, and mineralogical composition. The results indicate that the values obtained for the saturated hydraulic conductivity (ksat), water retention curve (WRC), and diffusion coefficient (De) are comparable to estimated values calculated from predictive models. A field work campaign was also conducted to assess some aspects of the in situ hydrogeological behaviour in terms of water table position and water content, and to collect additional samples of the cover material (till) to evaluate its heterogeneity.