Sulfur Cycling in an Oil Sands Tailings Pond

Oil sands tailings ponds are used as the primary storage and settling basins for toxic tailings produced during oil sands processing in northern Alberta (Canada). As a result of microbial metabolism, methane production contributes to greenhouse gas emissions and was shown to affect tailings densification. In particular, sulfur cycling is supposed to play a key role both for the turnover of organic matter and the regulation of methane emissions. Adversely, the activity of sulfate-reducing bacteria (SRB) is likely to generate huge quantities of toxic H2S, which pose a strong concern for both gas-and water-phase environments in the vicinity of the ponds. In order to identify reactive zones of sulfur cycling and to assess the impact of microbial sulfate reduction on organic matter transformation and CH4 emissions, biogeochemical analyses of original tailings from two vicinal pond profiles were combined with a number of laboratory experiments performed under well-defined conditions. In conclusion, results give evidence that H2S outgassing from the pond is effectively prevented by the biochemical re-oxidation and primary incorporation of H2S into iron sulfide minerals. As demonstrated by the long-term incubation of original tailings in anoxic microcosms, considerable volumes of CH4 emissions may be prevented by the activity of SRB in sulfidic tailings between 3.57.5 m in situ. In addition, results show that microbial sulfate reduction is essential for the anaerobic mineralisation of labile organic matter with significance for tailings ponds carbon cycle and gas production.