Chemical characterization of non-polar organic compounds transported by methane ebullition in an oil sands pit lake

Base Mine Lake Demonstration (BML) is the first full-scale commercial demonstration of pit lake in the Alberta oil sands region. Active microbial methane generation within the fluid fine tailings (FFT) underlying the lake results in gas ebullition that transports residual bitumen-associated organic compounds to the lake surface. However, the chemical composition of bubble transported organic compounds, the mechanisms of association with the bubbles, and the sources from which they are derived have yet to be well constrained. Here we analyzed volatile organic compounds within the bubble gas phase and bubble-associated bitumen trapped in winter ice. Results showed the bubble gas phase was dominated by methane gas, with several C5-C7 volatile hydrocarbons present at concentrations from 10 to 200 μmol/mol CH₄, >C7 hydrocarbons were also detected but fell below the quantification limit. Thus, ebullition represents a mechanism of removing low molecular weight hydrocarbons from the FFT and releasing them to the atmosphere. Concentrations and distributions of low molecular weight hydrocarbons differed between samples, likely due to the heterogeneity of BML FFT, but potentially also affected by extent of ebullition. For the ice trapped bubbles, total lipids extract (TLE) content ranged from <0.01 to 2.90 g indicating variation in the amount of hydrocarbons transported by ebullition. The chemical fingerprints of the non-polar fraction of transported bitumen showed consistency in all ice cores, implying the bitumen is derived from a similar source within BML. Comparing with dispersed bitumen within the FFT and bitumen "mats" residing on the surface of FFT, the chemical fingerprints showed greater consistency between bitumen "mat" and bubble-associated bitumen. This implies that bubble associated bitumen is primarily derived from bitumen "mats" rather than the dispersed bitumen present in the FFT. These results inform the design and construction of pit lakes with organic rich tailings in order to minimize transport of organics from tailings to the lake surface. They have further implications to other organic rich environments, where differential transport of different compound classes may occur from varying sources.