Oil sands in northeastern Alberta, Canada, are strip mined and bitumen is separated from sand and clay by an alkaline hot water extraction process leaving millions of cubic metres of tailings for disposal. These include mature fine tailings (MFT) collected in settling ponds and consolidated or composite tailings (CT) that are produced by the addition of sand and gypsum to hasten dewatering and densification. This laboratory investigation monitored the potential for fugitive gas production in 0.9-m high columns that contained MFT or CT samples from three oil sands companies. Methane was found in columns that contained aged MFT, but not in those that contained fresh MFT. Dissolved or entrapped methane was found in columns containing each of the MFT samples and two of the CT samples. Ethylene was detected in columns containing MFT or CT. This may affect future plans to re-vegetate disturbed areas of the oil sands operations because ethylene strongly influences many phases of plant development. The densification rate of a methanogenic MFT was faster than that of a nonmethanogenic MFT that was placed in a similar column 6 years prior to the start of this investigation, suggesting that methane formation may increase the rate of densification. Key words: consolidated tailings, composite tailings, densification, ethylene, methane, methanogens, oil sands, sulfate-reducing bacteria.

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Approximately 20% of Canada's oil supply now comes from the extraction of bitumen from the oil sands deposits in northeastern Alberta. The oil sands are strip-mined, and the bitumen is typically separated from sand and clays by an alkaline hot water extraction process. The rapidly expanding oil sands industry has millions of cubic metres of tailings for disposal and large areas of land to reclaim. There are estimates that the consolidation of the mature fine tails (MFT) in the settling ponds will take about 150 years. Some of the settling ponds are now evolving microbially produced methane, a greenhouse gas. To hasten consolidation, gypsum (CaSO4 x 2H2O) is added to MFT, yielding materials called consolidated or composite tailings (CT). Sulfate from the gypsum has the potential to stimulate sulfate-reducing bacteria (SRB) to out-compete methanogens, thereby stopping methanogenesis. This investigation examined three MFT and four CT samples from three oil sands extractions companies. Each was found to contain methanogens and SRB. Serum bottle microcosm studies showed sulfate in the CT samples stopped methane production. However, if the microcosms were amended with readily utilizable electron donors, the sulfate was consumed, and when it reached approximately 20 mg/L, methane production began. Some unamended microcosms were incubated for 372 days, with no methane production detected. This work showed that each MFT and CT sample has the potential to become methanogenic, but in the absence of exogenous electron donors, the added sulfate can inhibit methanogenesis for a long time.

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