Experimental Models of Demonstration Pit Lakes: Evaluating Long-Term Effects on Water Quality, Biogeochemical Processes, and Metagenomic Profiles in Oil Sands Tailings
Creators: Bello, Akeem O
Publisher: name: University of Alberta Library
Year: 2024

<p>Bitumen extraction from oil sands reserves in northern Alberta, Canada, has already disturbed large expanse of land and created huge volume of tailings as wastes. The pore water of a tailing contains high concentrations of dissolved organic and inorganic compounds that adversely affect living organisms because of their toxicity upon exposure. Tailings comprises sand, silt, clay, unrecovered bitumen, and oil sands process-affected water (OSPW) and are temporarily being stored in settling basins known as tailing ponds in compliance with the zero-effluent discharge policy in Alberta. To this end, Permanent Aquatic Storage Structure (PASS) technology has been deployed to enhance the dewatering of tailings in demonstration pit lakes (DPL) by adding a coagulant (alum) and a flocculant (polyacrylamide, PAM). It is yet to be understood, however, how these additions would impact the capping water quality and whether their dissociation could affect nutrient biogeochemical cycling leading to unwanted emissions of gases. Therefore, changes in water quality with regards to dissolved organics and polymer additives were investigated along with biogeochemical processes and the likely genes.<br>
DPL models (75.4L) containing 22.6L of tailings and capped with 45.2L of lake water were established to investigate the fate of dissolved organics and polymer additives during long term storage of tailings in DPL under different oxygen conditions (oxic vs anoxic). Tailings settling resulted in 13.8 ± 0.14 cm of consolidation under oxic condition compared to 16.4 ± 0.71 cm under anoxic condition. Considerable flux of dissolved organics from the underlying tailings into the capping water was observed, resulting in an increase in the concentration of dissolved organic carbon, DOC in oxic and anoxic conditions by 5 – 37% and 19 – 72% respectively. Advective flux of DOC decreased gradually, ranging from 0.52 g/m2/d to 0.11 g/m2/d under oxic condition and from 0.65 g/m2/d to 0.13 g/m2/d under anoxic condition. Residual polymer additives present in the PASS-treated tailings were initially released into the capping water, with concentrations ranging from 1.13 mg/L in oxic conditions and 0.845 mg/L in anoxic conditions, before being slowly biodegraded over time to 0.123 mg/L in oxic conditions and 0.085 mg/L in anoxic conditions.<br>
To study if the addition of alum and PAM as PASS tailings treatment strategy affects microbial community structures and natural biogeochemical cycles leading to unwanted changes in water quality over time, smaller systems (20.4L) containing 6.7L of tailings and capped with 13.4L of lake water were established. One set of columns, termed treatment groups, were subjected to different temperature regimes of 5 °C, 20 °C, 25 °C, and 8 °C each lasting 2 months. The other set of columns, termed control groups, were operated at constant temperature of 22 °C. Tailings settling resulted in the release of porewater followed by increased DOC, chemical oxygen demand, and naphthenic acids and a decrease in turbidity. Temperature variations significantly affected dissolved oxygen and conductivity but did not alter microbial communities significantly, which were dominated by organisms involved in sulfur, carbon, nitrogen, and iron cycling. Analysis of biogas in headspace revealed that methane was not produced, but production of hydrogen sulfide was evident. Likely, sulfate reduction inhibited methanogenesis in model pit lakes. <br>
Finally, metagenomic analyses were conducted to investigate the microbial biogeochemical cycling in the 20.4L DPL models. Results revealed that bacterial communities dominated the model pit lake columns, with significant variations in abundance and composition between tailings and capping water. Biogeochemical cycling of carbon, sulfur, and nitrogen was prominent in both tailings and capping water. Carbon cycling accounted for 52-60%, sulfur cycling for 16-19%, and nitrogen cycling for 24-28% across all columns. Functional gene analysis further predicted genes for essential processes of nutrient cycling. The most prevalent processes were Wood-Ljungdahl Pathway and methanotrophy for carbon cycling, sulfite reduction and thiosulfate oxidation for sulfur cycling and, denitrification and dissimilatory nitrate reduction to ammonium for nitrogen cycling. Temperature variations did not affect nutrient cycling in tailings and capping water and there were no significant differences observed in the overall processes between the tailings and capping water indicating microbial resilience.<br>
In summary, this dissertation demonstrates that PASS treatment technology has the potential to promote the development of DPL into a self-sustaining ecosystem with resilient microbial communities and functional biogeochemical cycling while minimizing unwanted gas production.</p>