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Introduction

The oil sands in Alberta, Canada, covering over 100000 km2, produce over 1.3 million barrels of bitumen per day [1]. Extraction of bitumen by surface mining of oil sands requires large amount of water and hydrocarbon solvents. The resulting byproduct creates large volumes of water, sands, clays, residual hydrocarbons, heavy metals, naphtha diluents, and naphthenic acids, which are termed as tailings. Due to operational policies preventing zero discharge of any liquids into the environment, these fine tailings are collected and confined into settling basins to create tailings ponds [1]. Despite the toxicity of tailings, microbial communities exist in tailings pond, which aid to accelerate tailings sedimentation while being able to degrade certain compounds in the tailings.

Physical & Chemical Environment

Composition of tailings pond

Along with the 20-30 wt% of solids (sands and clays) and slightly alkaline water (pH>7.5), mature tailings consist of 1-3 wt% of residual bitumen and naphtha, comprising of a mixture paraffins (n-alkanes), iso-paraffins (branched alkanes), olefins (alkenes), naphthenes (cycloalkanes), naphthenic acids (cyclopentyl and cyclohexyl carboxylic acids), and monoaromatics (benzene, toluene, ethylbenzene, and xylenes, or BTEX) [2][3]. Other minor elements include trace metals (Cr, Mn, Co, Ni, Cu, Zn, As, Sr, Mo, Ba), and ions (HCO3-, PO43-, NO3-, SO42-, Na+, K+, Mg2+, Ca2+, Cl-) [4]. Toxicity of tailings ponds to aquatic organisms is often associated with naphthenic acids, which have surfactant properties that penetrate the cell membrane [5].

Sedimentation & Gas Emissions

Closer to the inflow, sand settles quickly. However, tailings consolidation of fine clays occurs by gravity at a slow rate over 5-10 years [6]. Stratification therefore results with older and denser fine tailings settled to the bottom and fresh tailings deposited on top. Temperature also increases with depth due to lack of surface cooling and retention of heat at 30-60˚C by deposited tailings [7]. Densification remains an operational challenge, which prevents water from being recycled while delaying the land reclamation process. Anaerobic methanogenic activities by syntrophic bacteria and methanogenic archaea also enhance the rate of tailings sedimentation. Microbe-formed gas channels assist the escape of methane gas bubbles which also aid to drain pore water from deeper tailings. Each day, about 43000m3of methane (CH4) is released. Sulfate and nitrate reduction by sulfate and nitrate-reducing bacteria, however, impeded the actions of methanogenesis, thus slowing the rate of tailings densification. Densification is reached when the volumetric fraction of solids (Fs) increase to 85%(w/w) [8].

Active vs. Inactive pond

In active ponds, wastes from bitumen extraction are continually collected in the basin. This results in continual input of electron donors and acceptors of metal ions into the pond. In active ponds, gypsum is added to aid densification while microbial activities are dominated by anaerobes at lower depths. In inactive ponds, wastes from bitumen extractions are no longer collected. In this situation, there are no inputs of electron donor and acceptors while microbial activities are confined to the upper depths with lower anaerobic activities. Therefore, the status of the pond influences the microbial community found [9].