Modelling the Environmental Implications of Bottom Seepage from Oil Sands Tailings Ponds

摘要

Bottom seepage of tailings water into an underlying permeable aquifer may result in long term environmental impact. Generally it is assumed that the tailings will consolidate at the bottom and seal against possible leakage. However, little research has been performed to evaluate this phenomenon. To simulate a tailings pond underlain by a permeable stratum, a 2 m high standpipe test with a bottom drainage layer was performed on typical oil sands fine tailings. The results from the standpipe test including the drainage volumes, pore water pressures, and void ratios were used to validate a finite strain consolidation model. The model used compressibility and hydraulic conductivity relationships obtained from large strain consolidation tests. It was found that the model is capable to qualitatively capture these behaviours in the standpipe. The model was then used to investigate the potential seepage through the bottom of a tailings deposit in a commercial scale. The results indicate that the downward seepage flux is exponentially attenuated due to consolidation immediately above the underlying permeable strata. On average, the downward seepage flux is less than 10% of the upward seepage flux and the downward seepage volume is less than 10% of the total seepage. A major thrust by industry to improve the rate of consolidation of the oil sands fine tailings is to increase the hydraulic conductivity of the tailings with the aid of flocculants or by reducing the amount of fine material in the tailings. As increased hydraulic conductivity may increase the rate of downward seepage, the results of a second standpipe test with higher hydraulic conductivity fine tailings was also assessed. The downward seepage fluxes and the downward seepage volumes for commercial scale tailings ponds containing these two types of tailings are compared.