Performance of the oxygen limiting cover at the LTA site, Malartic, Quebec
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Controlling acid mine drainage (AMD) produced by reactive mine wastes is one of the most challenging environmental issues currently facing the mining industry. The LTA impoundment (property of Barrick Gold Corp.) located near Malartic, Quebec, contains sulphide tailings that have the potential to generate acidic leachate. The selected closure option was to construct a cover with capillary barrier effects (CCBE) shortly after the end of tailings deposition. The main role of the CCBE is to reduce the production of AMD by limiting the oxygen diffusion flux reaching the reactive minerals. The LTA cover consists of a moisture-retaining layer (0.8 m of non-acid generating tailings) sandwiched between two coarse-grained material layers (0.3 and 0.5 m thick for the top and the bottom layers, respectively). To be efficient, it has been determined that the cover layer that acts as the oxygen barrier should maintain a high degree of saturation (approximately >85%, which corresponds to a volumetric water content of about 37% for the moisture-retaining layer). The LTA site is one of the first mine tailings impoundments located in a humid climate that has been rehabilitated using a cover with capillary barrier effects aimed at limiting oxygen migration.
The performance of the LTA cover has been monitored since its construction in 1996. Instrumented stations have been installed in strategic locations on both the nearly flat area and on the sloping side of the tailings dykes. Previous studies by the authors showed that slopes can influence the hydraulic behaviour of CCBEs, which can affect their ability to limit AMD generation. Various parameters have been measured, including the volumetric water content and matric suction in the different layers of the cover. The evolution of volumetric water content measurements in the CCBE, for representative stations, is shown in the paper together with the corresponding suction values.
Two distinct types of hydraulic conditions have been observed on the nearly flat area of the LTA site: (1) active capillary barrier effects in the cover when the water table is deep (see figure), and (2) capillary-rise induced near saturation of the cover (but without the capillary barrier effect) when the phreatic surface is located close to the CCBE. In both situations, the measured volumetric water contents in the moisture-retaining layer are close to those at saturation (and are almost always above the design objective of 37%).
A small aerial portion of the cover constructed on the LTA site is located on the sloping side of the tailings dykes. Monitoring results have confirmed that it is more difficult to maintain the desired volumetric water content in the moisture-retaining layer along the upper parts of a slope (especially during prolonged dry periods). The enusing occasional desaturation may then temporarily influence the performance of the cover to limit oxygen migration. However, the volumetric water content is increased rapidly when precipitation occurs after a drought. This type of behaviour along the sloping areas was expected earlier in the design stage, as the numerical analysis had predicted such temporary up-slope desaturation. At the LTA site, the desaturation near the top of the slope is limited to a small portion of the cover (estimated at less than 5% of the total covered area) and it appears only for short periods of time.
Results shown in the paper indicate that the overall performance of the CCBE on the 60 ha site exceeds the design expectation. This case study also shows the importance of analyzing separately, at the design stage, the relatively flat and inclined portions of a CCBE (as was done for the LTA cover). The inclination influences the distribution of water in the CCBE, which can affect its ability to limit oxygen migration. Ignoring this effect could lead to misleading assumptions and inadequate cover configuration. Such analysis may help identify the changes required to the cover design to fully meet the performance criteria. In the case of the LTA site, it was deemed unnecessary to modify the cover as its overall response is meeting the original objectives. Finally, this study showed that non-acid-generating tailings from the nearby Malartic Goldfield property have the appropriate hydrogeotechnical properties for being used as the moisture-retaining layer in a CCBE.