Abstract
End pit lakes (EPLs) have been proposed as a method of reclaiming oil sands fluid fine tailings (FFT), which consist primarily of process-affected water and clay- and silt-sized particles. Base Mine Lake (BML) is the first full-scale demonstration EPL and contains thick deposits of FFT capped with water. Because of the fine-grained nature of FFT, turbidity generation and mitigation in BML are issues that may be detrimental to the development of an aquatic ecosystem in the water cap. Laboratory mixing experiments were conducted to investigate the effect of mudline biofilms made up of microbial communities indigenous to FFT on mitigating turbidity in EPLs. Four mixing speeds were tested (80, 120, 160, and 200 rpm), all of which are above the threshold velocity required to initiate erosion of FFT in BML. These mixing speeds were selected to evaluate (i) the effectiveness of biofilms in mitigating turbidity and (ii) the mixing speed required to ‘break’ the biofilms. The impact of biofilm age (10 weeks versus 20 weeks old) on turbidity mitigation was also evaluated. Diverse microbial communities in the biofilms included photoautotrophs, namely cyanobacteria and Chlorophyta (green algae), as well as a number of heterotrophs such as Gammaproteobacteria, Desulfobulbia, and Anaerolineae. Biofilms reduced surface water turbidity by up to 99%, depending on the biofilm age and mixing speed. Lifting and layering in the older biofilms resulted in weaker attachment to the FFT; as such, younger biofilms performed better than older biofilms. However, older biofilms still reduced turbidity by 69% to 95%, depending on the mixing speed. These results indicate that biostabilization is a promising mechanism for turbidity mitigation in EPLs.
Highlights
Over 1.3 billion m3 of oil sands tailings are currently being stored in large aboveground impoundments, referred to as tailings ponds, in northern Alberta [1]
The fines content and water content of the fluid fine tailings (FFT) used in this experiment are considered to be generally representative of the FFT that may contribute to turbidity in Base Mine Lake (BML)
While the mixing duration and mudline linear velocities used in this work do not reflect all scenarios under which turbidity may be generated in BML, the results clearly show that the presence of biofilms at the mudline reduced turbidity generation under all mixing conditions evaluated in this work
Summary
Over 1.3 billion m3 of oil sands tailings are currently being stored in large aboveground impoundments, referred to as tailings ponds, in northern Alberta [1]. All of these tailings must be reclaimed and integrated into the surrounding landscape after oil sands mines are closed. Fine-grained tailings, referred to as fluid fine tailings (FFT), have solids that consist primarily of clay (less than 2 μm) and silt (less than 44 μm) particles. One proposed method of oil sands tailings reclamation is end pit lakes (EPLs), which are engineered water bodies comprised of thick FFT deposits (10–80 m) stored beneath a water cap (3–10 m) in decommissioned open pits. Due to the fine-grained nature of FFT and the slow consolidation rate, there are unique challenges associated with reclaiming oil sands tailings in EPLs [4]
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