Influence des changements climatiques sur la performance de couverture à effet de barrière capillaire : étude du cas Lorraine

Mine site reclamation, especially for acid mine drainage (AMD) generating sites, is one of the main environmental issues faced by the mining industry and the governments responsible for abandoned sites. In addition to these environmental risks specific to mining activities, climate change (CC) issues are now added. Significant variations in temperature and precipitation are expected across Canada in the coming decades. Mean annual precipitation, temperature, and the frequency and the duration of drought events are expected to increase in Québec south of the 50th parallel. At present, most mining reclamation works integrate superficially CC in their design increasing the environmental risks associated with the chemical and physical stability problems of mine wastes. The Lorraine mine site was abandoned at the end of the operation in 1968 without environmental consideration. Tailings were exposed to oxygen and water and sulfide oxidation occurred, leading to AMD generation for more than 30 years. The Ministry of Energy and Natural Resources (MERN) started reclamation of the Lorraine site in 1997 and a cover with capillary barrier effects (CCBE) was built as oxygen barrier in 1999 to limit the production of DMA. The performance of this technique depends on maintaining a high degree of saturation (Sr) (> 85 %) in one of the cover layers, called the moisture-retaining layer (MRL), in order to limit the migration of oxygen to the underlying reactive tailings. However, several uncertainties related to the influence of CCs on the CCBE performance are emerging because of their interaction with the environment, especially with climatic conditions. Increases in precipitation may have positive effects on maintaining the saturation of the MRL and reducing the oxygen flux to the reactive tailings. However, drought events could cause temporary desaturation of the MRL and, consequently, increase the potential for acid generation. The main objective of this study is to evaluate the influence of CC on the long-term performance of the Lorraine CCBE. More specifically, the objectives of the project are: (i) to develop a numerical model representing the current hydrogeological behavior of the Lorraine CCBE, (ii) to evaluate the influence of the mean annual climatic conditions by 2100 and (iii) to evaluate the influence of extreme drought conditions on the long-term performance of the CCBE. A 2D numerical model was developed with SEEP/W software (Geo-Slope International Ltd.). The model was first validated using three years of meteorological and hydrogeological data measured in the field between 2001 and 2003. The daily data of six climatic parameters were applied as a surface boundary condition thanks to the land-climate-interaction condition. (LCI): temperature, precipitation, relative humidity and wind speed obtained from the Earlton airport meteorological station, albedo defined as 0.2 and corresponding to a low vegetated soil as on the Lorraine site and solar radiation variations estimated by SEEP/W with the latitude of the site (47.7°). The validated model was then used with 3 CC scenarios for the period 2098-2100 (IPSL, MIROC and GFDL). The IPSL scenario is expected to be the worst case for the CCBE hydrogeological behavior. It projects a combination of a high increase of temperatures (+ 7 °C) and a low increase in precipitation (+ 0.7 %), which may lead to the MRL desaturation. GFDL was selected as median climate scenario, with a temperature increase of about 5 °C and an increase of annual precipitation of 26.1 %. The MIROC scenario presents the most advantageous climate scenario, with temperature and precipitation increasing by +9.9 °C and +51.1 %, respectively. Extreme drought conditions were also applied to the Lorraine CCBE numerical model. The methodology developed in this study consisted of focusing on two parameters: the duration of the drought (d) and the cumulative precipitation of the 30 days preceding the drought (CPBD). The maximum historical drought observed at the Earlton airport station was characterized with a duration of 42 days for a 3 mm CPBD. The same characterization work was performed on the projected climate data with the IPSL scenario and the relative change between the droughts of the historical period and the future period was calculated. Thus, an increase of 22 % in the duration of droughts and an increase of 50 % of the CPBD were calculated for the 2100 horizon. A future drought was thus defined and applied to the IPSL scenario considering a duration of 51 days and a CPBD of 6 mm. The results of the different simulations performed in this study indicate that the hydrogeological behavior of the Lorraine CCBE is impacted by the climate scenarios and the extreme climatic conditions tested. Piezometric levels drop by an average of 14 cm with the most pessimistic climate scenario (IPSL) and drop to a maximum of 42 cm when a future drought is applied. However, simulated volumetric water contents in the cover indicate that capillary barrier effects are well developed despite CC and drought events and the performance criteria (Sr> 85 % in the MRL) is reached with all scenarios and climatic conditions tested. This project highlights the influence of CC on Lorraine CCBE and confirms that the design of the cover is robust with respect to CC and extreme drought conditions tested. More generally, this study illustrates the utility of numerical modeling to quantify the influence of CC on the long-term performance of CEBCs to control AMD generation. Such works would then lead to the development of climatic indices adapted to the mining sector and modifiable for each site considering local climate scenarios. They could be part of a methodological approach to consider CC and future extreme climatic conditions into the design of mine reclamation infrastructures and facilitate the CC integration into regulatory frameworks. Keywords: Cover with capillary barrier effects (CCBE), Acid mine drainage (AMD), Mine site reclamation, Numerical modeling, Climate change