Quantification of Trapped Diluted Bitumen Droplets in Gravel: an Experimental Study
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Adverse environmental effects caused by diluted bitumen (dilbit) spills have led to serious public concern. Compared to marine systems, the cleanup of oil-contaminated sediment in rivers is more difficult. Hyporheic flow, where surface water flows through the streambed, is one of the contributing factors for sediment contamination, which has received little research attention. Therefore, a series of laboratory experiments were conducted in this study: to quantify the mass of dilbit droplets that get trapped in the sediment in gravel-dominated rivers by hyporheic flow; to investigate the effects of velocity, gravel size and clean water wash on the interaction between dilbit droplets and gravel porous media; and to determine the predominant droplet retention mechanism. Dilbit droplets were generated in a mixing reactor and transported through a one-dimensional gravel-packed aluminum column. Gravel sizes of 3 mm and 8 mm, and flow velocities of 0.09 cm/s and 0.35 cm/s were investigated to identify the impacts of these influential parameters on the trapping of dilbit droplets in gravel. Clean water wash after the completion of droplet transport was also applied to some experiments to examine whether droplet trapping is reversible. In addition, mathematical models that included attachment and straining filtration mechanisms used in colloid transport were employed to fit the experimental data. Results showed that trapped dilbit droplets in gravel follows a hyperexponential (not log-linear) profile. Smaller gravel size (3 mm) or higher flow velocity (0.35 cm/s) led to a greater mass of dilbit retained in the gravel-packed column, especially near the pack inlet. Clean water wash was shown to remove part of the trapped dilbit droplets, especially near the gravel pack inlet. A filtration model that considered straining instead of attachment was able to better match the observations, which indicates that straining is more likely to be the primary mechanism for the trapping of dilbit droplets in gravel.