As oil production increases in Alberta’s Athabasca Oil Sands Region (AOSR), optimization of tailings management processes will be integral to the successful reclamation of tailings-based environments. Syncrude Canada Ltd. has established an innovative dry-storage method for their wastes known as composite tailings (CT) that supports mine closure objectives by providing a base for terrestrial reclamation landscapes. Syncrude’s Sandhill Reclamation Fen is the first instrumented research wetland of its kind to be developed in the AOSR and it overlays a sand-capped composite tailings deposit in a retired open-pit mine site. This stratified sulfur-rich environment is highly anthropogenically altered and consists of three distinct zones: a constructed wetland, a 10m layer of sand, and 40m of CT. As oil sands tailings systems are becoming globally significant sulfur reservoirs due to their size, sulfur content, and diverse microbial communities, understanding the mechanisms behind H2S generation in novel tailings structures will help inform our understanding of sulfur-rich environments. This study is the first to characterize the sulfur biogeochemistry in each zone of the Sandhill Reclamation Fen deposit in an effort to establish the potential for microbial sulfur cycling and explore the mechanisms controlling H2S generation. Porewater ΣH2S(aq) was detected at all depths, increasing with depth from the surface of the wetland (<1.1 μM) and peaking in the sand cap (549 μM). Across all sampling trips, ΣH2S(aq) concentrations were consistently highest in the sand cap, with sampling-associated H2S gas concentrations in the wells reaching 104-180 ppm. Abundance of dissolved sulfate (0.14-6.97 mM) did not correlate to the distribution of ΣH2S, and dissolved organic carbon (21.47-127.72 mg/L) only positively correlated with the observed maxima of ΣH2S in the sand-cap. Identical sodium and chloride distributions in the sand and CT supported the model of upward migration of CT-derived porewater and fines into the sand cap. Functional metabolic enrichments established the ability of endemic microbial communities from all depths of the deposit to oxidize and reduce sulfur. Experimental microcosms demonstrated 1) the dependence of ΣH2S generation on the presence of fine particles; 2) stimulation of endemic microbial sulfur reduction through amendment with labile carbon and 3) increased generation of ΣH2S in the presence of thiosulfate over sulfate. Field and experimental results indicated that the bioaccessibility of recalcitrant organic carbon in the deposit likely controls rates of ΣH2S generation at depth. While the mechanisms relating CT-derived fines to ΣH2S in the sand cap are still unconstrained, the sand layer is clearly a bioreactive mixing-zone supporting optimal conditions for ΣH2S accumulation. These findings inform our understanding of biogeochemical sulfur cycling in novel oil sands reclamation deposits and will advise on-going optimization of tailings-based landscape management practices. / Thesis / Master of Science (MSc)

<p>Surface mining of Alberta’s oil sands has led to significant land disturbance, making reclamation and sustainable development of this resource one of the largest challenges facing the industry today. Syncrude Canada Ltd. has developed an innovative technique to reclaim composite tailings (CT) through constructed wetland landscapes and is currently investigating the viability of a pilot-scale freshwater fen built over sandcapped CT. Unpredicted by abiotic geochemical modelling of CT behaviour, a minor episode of hydrogen sulfide (H₂S) gas release was encountered during the initial stages of fen construction indicating microbial activity was likely involved in H₂S generation within CT. This thesis investigates the S geochemistry of CT with depth and employed 454 pyrosequencing and functional enrichments to characterize the associated microbial communities in the first S biogeochemical study of oil sands CT. Porewater H₂S was detected extensively throughout the deposit with background levels ranging from 14 – 23 µM and a maximum of 301.5 µM detected at 22-24 m of depth. Reduced Fe (Fe²⁺) was also detected, but confined within surficial depths sampled, ranging from 1.2 – 38.5 µM. Mass balance calculations identify that the Fe²⁺ generated within the surficial zone of the CT deposit is sufficient to effectively sequester ambient concentrations H₂S generated in this deposit through FeS precipitates. Results identifying (1) distinct zones of porewater Fe²⁺ and H₂S, (2) co-occurrence of the highest [H₂S] and lowest dissolved organic C (DOC) at 22-24 m consistent with heterotrophic sulfate reducing bacteria (SRB) activity, and (3) the presence of mixed valence Fe biomineral, magnetite, throughout the deposit, are all consistent with microbially-mediated Fe and S cycling occurring within this CT deposit. The cultivation independent identification of several known iron reducing bacteria (IRB) and SRB within CT microbial communities, in conjunction with observed positive growth of IRB and SRB functional metabolic enrichments, demonstrates widespread capacity for microbial Fe and S activity throughout the CT deposit. Metagenomic characterization of CT microbial communities revealed high diversity (over 20 phyla) over the 5 depths examined. Multivariate statistical analyses (Unifrac) revealed that bacterial community composition and structure was driven by changed in DOC, ORP and salinity and that structuring corresponded with a surficial zone of Fe³⁺ reduction and an underlying zone of SO₄^2- reduction. Despite the high organic carbon (OC) content of oil sands tailings, much of that C is not considered to be labile and accessible to microbes. Based on the results of this thesis, CT SRB appear to have a greater ability than IRB to utilize recalcitrant OC (e.g. bitumen, naphthenic acids) given the widespread occurrence of porewater [H₂S] and surficially restricted [Fe²⁺] despite accessible pools of Fe³⁺ and OC with depth. This enhanced understanding of biogeochemical S cycling within CT newly establishes the importance of microbial activity in these processes, identifying the need to incorporate microbially based understanding into on-going development of reclamation strategies in order to manage these waste materials effectively.</p> / Master of Science (MSc)