S reactivity of an oil sands composite tailings deposit undergoing reclamation wetland construction

This study is the first to characterize the S stability of a composite tailings (CT) deposit undergoing pilot wetland reclamation in the Athabasca Oil Sands Region (AOSR, Alberta, Canada). As CT is sulfur, organic carbon and bacterially rich, the goal of this study was to characterize the in situ aqueous distribution of sulfur compounds across the wetland, sand cap and underlying CT zones of the deposit, in an effort to establish the potential for microbial sulfur cycling and generation of H2S, an explosive, corrosive and toxicity risk. Porewater samples from three depths spanning the different layers of the deposit, as well as wetland surface ponded water samples were collected for geochemical analyses (July and Sept 2013), and for microbial enrichments (both S reducing and S oxidizing bacteria) in June 2014. While porewater ΣH2S(aq) was detected at all depths across the three zones of the deposit, results identify that the sand cap layer required for construction, acts as a mixing zone generating the highest solution H2S concentrations (>500 uM or 18 mg/L) and H2S gas levels (over 100 and up to 180 ppm) observed. Porewater dissolved sulfate concentrations (0.14-6.97 mM) were orders of magnitude higher and did not correlate to the observed distribution of ΣH2S concentrations throughout the deposit. Unique to the sandcap, dissolved organic carbon positively correlated with the observed maxima of ΣH2S(aq) seen in this layer. The water management of the deposit is a critical factor in the observed S trends. Active dewatering of the CT resulted in migration of S rich water up into the sandcap, while downwelling labile organic carbon from the developing wetland acted in concert to stimulate microbial generation of the H2S in this structural layer to the highest levels observed. Functional enrichments identified that diverse S reducing and oxidizing microbial metabolisms are widespread throughout the deposit, indicating that these waste materials are biogeochemically reactive with implications for longterm stability. These results are of relevance to both the oil sands region, as well as other mine contexts where S rich wastes occur, identifying the need to consider the potential bacterially driven cycling of S and C in the generation of constituents of concern, as well as the water management of such waste deposits to minimize risk.