Heat budget for an oil sands pit lake

• Main Author: Chang, Sarah
• Format: Text
• Language: English
• Published: University of British Columbia 2020
• Subjects: Mine Lake; Ice cap
• Online Access: https://dx.doi.org/10.14288/1.0392976; https://doi.library.ubc.ca/10.14288/1.0392976

At Syncrude’s Base Mine Lake, a hydrocarbon sheen arising from a tailings deposit below the water column modifies the heat exchange between the lake and its surroundings. A heat budget was implemented to determine how heat fluxes are affected, and to assess resultant impacts to lake dynamics. The period of record spans May 2015 to April 2016, separated into open-water and under-ice periods. In the open-water period, individual heat fluxes were estimated using an existing parameterized model of the physical processes, the Air-Sea Toolbox, and validated using eddy covariance measurements and other methods to increase confidence in the results. The hydrocarbon sheen introduces resistance to evaporation at the water surface. This was modelled as a reduction in the relative humidity at the base of the air-water boundary layer. Latent heat flux due to evaporation dropped as a result by 10 % overall, or 20 % in low winds; in windy conditions, surface waves form, temporarily dispersing the sheen. The hydrocarbon sheen fundamentally impacts many of the physical processes of the lake. Reduced latent heat flux means a directly proportional drop in evaporation, which must be reflected in the water budget. As evaporation is a key cooling mechanism, elevated surface temperatures also result. Increasing the surface temperature also impacts other fluxes, partially offsetting this effect. Nevertheless, the lake remains warmer than would otherwise be expected, with potential consequences to biological activity. Reduced cooling also means a decrease in the buoyancy flux and in buoyancy-induced mixing. In winter, individual fluxes are again estimated using physical models, and a rough heat budget is developed. The hydrocarbon sheen does not play a major role in winter. However, the brackish water and heat exchange with the FFT deposit below the water cap introduce interesting under-ice dynamics. Rapid warming and occasional buoyancy-driven mixing are observed at the lake bottom, while at the surface, the majority of the fluxes relate to the growth and loss of the ice cap. These observations contribute to the understanding of heat fluxes and their impacts on water dynamics in tailings-affected systems.