Mesures experimentales des concentrations en oxygene sur le terrain et modelisations numeriques pour evaluer le flux de diffusion dans la couverture du site minier LTA

Different types of covers acting as oxygen barrier can be installed over mine waste disposal sites potentially generating acid mine drainage (AMD). These include multilayered systems such as covers with capillary barrier effects or CCBE. This project complements the evaluation of the hydrogeological performance of the CCBE at the LTA site, but this time in terms of direct measurement of oxygen concentration, which had not been achieved so far. The objective of the project is to determine the vertical profiles of oxygen concentrations at different stations placed on the flat area and the slopes of the site.;The gas monitoring stations were placed close to existing stations equipped with TDR probes to measure the volumetric water content (thetaw) at different depths in the CCBE. The interstitial oxygen concentration was recorded at three depths in the moisture retention layer (MRN) of the CCBE, at one depth in the middle of the capillary break layer and at one depth in the tailings (close to the CCBE-Tailings interface). The dedicated tips in the MRN were placed to the same depth as the TDR probes. In-situ sampling of interstitial gas and oxygen concentration measurement campaigns were carried out from summer 2007 to summer 2009 at two stations in the cover on the surface of the tailings pond and at six stations in the sloped areas. During these campaigns, the soil gas was then pumped on site and analyzed using optical oxygen sensors. The volumetric water contents theta w were also measured. The oxygen concentration profiles obtained in all sampling stations are consistent. Oxygen concentrations decrease with depth. At the same depth, the oxygen concentrations are generally higher on the inclined area than on the flat area, especially at the base of the MRN. The values of volumetric water content thetaw show typical hydrogeological behavior of a CCBE; capillary barrier effects maintain a high degree of saturation in the MRN, which acts as a barrier to diffusion of oxygen to the underlying reactive tailings.;In parallel with in-situ determination of oxygen concentration profiles, several oxygen diffusion and /or consumption tests were performed in the laboratory to determine the impact of the degree of saturation on the reaction (consumption) rate coefficient (Kr) of the MRN tailings used in the MRN and the underlying LTA tailings. Results were interpreted with the POLLUTE code that solves the Fick laws defined according to the effective diffusion coefficients (De) and the reaction rate coefficient (K r). Good agreement between values calculated from the semi-empirical model proposed by Aachib et al. (2004) and values obtained with POLLUTE was obtained. The values of the reaction rate coefficient Kr obtained experimentally are generally lower than the values estimated with the Collin (1987) model. This model is based on the surface kinetics and Kr varies linearly with the sulphide minerals content regardless of the water content (or saturation degree). This difference is particularly marked for the materials almost dry as well as those close to saturation.;The results of the in-situ measurements of vertical profiles of oxygen through the CEBC and the underlying tailings at two stations on the covered surface of the tailings pond for the period from May 1 st to the end of December 2007 were compared with the vertical profiles of oxygen predicted using the numerical code VADOSE/W under natural climatic conditions. The water properties of the tailings and the CCBE layer materials used for the numerical simulations were taken from the literature, while the reactivity of materials was defined using representative parameters from the results obtained in the laboratory. 1-D simulations were used. Several scenarios were simulated to investigate the influence of the initial position of the water table and the reactivity of materials. Models with a good agreement between the profiles of oxygen measured and calculated were selected and used to calculate the cumulative flux of oxygen to the base of the CEBC for the diffusion active period from 1 May 2007 and December 2007. For this scenario, the results indicate that the annual oxygen flux does not exceed the design flux (110 g/m2/an) and the CEBC Site LTA was generally efficient (for simulated natural conditions). For these selected models, the behavior of the CEBC was also simulated by applying two (July and August, 2007) and three (June, July and August 2007) months without precipitations as boundary condition. The cumulative annual oxygen flux obtained in the case of three months without precipitations can locally and slightly exceed the design flux. (Abstract shortened by UMI.)...