Few things pose more of a threat to a mine’s legacy than acid mine drainage (AMD). Over the course of a mine’s life, and especially once the bustle of activity dies away, the creation of acid in many tailings impoundments is prevented by ever-evolving cover technologies. For the last 10 years, the Industrial NSERC Polytechnique-UQAT Chair in Environment and Mine Wastes Management has focused on improving tailings impoundment covers, and thereby the future of today’s mine sites.
Chair holders Michel Aubertin in Montreal and Bruno Bussière in Rouyn-Noranda, along with industry partners, have been exploring how there is no one-size-fits-all cover solution. Depending on site characteristics, the approach to cover design must account for factors such as climate, mineralogy and topography, and be adjusted to meet the specific needs of the location. As a result, designs can consist of single or multiple layers of different materials intended to control the flow of water and prevent oxygen from reaching the tailings, which causes AMD.
When Barrick Gold reclaimed Les Terrains Aurifères (LTA), a tailing storage facility located near Malartic, Quebec, sulphides were identified in some of the residues – a key factor in the creation of AMD. In response, Barrick, a partner with the Chair, opted to construct a cover with capillary barrier effects (CCBE). This is a multi-layered cover; each layer is constructed with materials of different grain size, and each has its own specific role to play. The number of layers typically varies from two to five, depending on site, climate, topography and mineralogy.
“The cover at the LTA mine site uses natural materials – no geosynthetic materials were involved,” says Anne-Marie Dagenais of Golder Associates, another Chair partner that was involved in planning and construction of the LTA cover. “It’s better to use local materials when available,” she adds. “In the case of the LTA site, there was an old tailing storage area besides the one being remediated. The tailings from this neighbouring site have very low sulphide content and are not acid generating, so we used those as a water retention material for the LTA cover.” The use of low sulphide tailings in layered covers was originally proposed by researchers from the Chair.
In a CCBE, the capillary barrier effect happens in unsaturated conditions when a fine-grained material layer is placed over a coarser one. These two types of material have different hydrogeological properties which, when combined, can restrict the flow of water from the upper layer to the lower one. The coarser material drains quickly and limits downward movement of water from the fine-grained material layer located above. The upper, finer layer remains nearly saturated and prevents the flow of oxygen through to the reactive tailings beneath.