Transport and settling of particles in the ocean and estuaries

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Of the municipal plastic waste which is released into rivers and the ocean and which is buoyant, at most 3% is observed to be floating on the ocean surface. To help predict where the missing waste is located, the proposed research aims to examine three mechanisms through which buoyant plastics are transported and ultimately settle to the river bottom or ocean floor. Recently we discovered that kaolin clay is electrically attracted to the buoyant plastic, pliolite, when in contact with salt water. Because clay is more dense than sea water, this suggests that, when plastics in a muddy river enter the ocean, the accumulation of clay on the plastics will cause them to settle predominantly in estuaries, deltas and near the coast. This proposed research stream aims to quantify the enhanced settling of plastics as it depends on the type and size of plastic, the type and relative concentration of clay, and the salinity of the water in which these particles are suspended. Most muddy rivers entering the ocean are less dense than sea water, and so they float over the surface as a so-called "hypopycnal current". Nonetheless the clay eventually descends through the water column. Although an individual particle of clay settles very slowly, when in contact with salt water, the clay together with microplastics gather into flocs that, can settle thousands of times faster than an individual particle. As these flocs accumulate at the base of the hypopycnal current, research suggests that the fluid-particle mixture becomes convectively unstable further enhancing the settling speed. This proposed research stream will use particle-resolving numerical simulations to examine clay-clay and clay-plastic flocculation with an aim to develop predictions for the enhanced settling speed. For floating plastics in the open ocean, they are dominantly transported by currents, gyre circulations and surface waves. The third proposed research stream will examine a different mechanism, whereby particles are transported by internal tides, which are similar to the surface tide except that they move due to the buoyancy between the relatively warm near-surface waters and the colder abyss. Recently we discovered that when the internal tide is generated near a coast and propagates out to sea, particles near the surface are driven back toward the coast. However, particularly in the tropics, the waves steepen and drive particles in the wave-ward direction - again toward a coast where they eventually break. This proposed research stream will develop predictions for the surface transport of plastics by steepening internal tides. Each proposed study concerningly suggests that microplastics settle predominantly in coastal waters.  This research will therefore be useful to a broad community including environmentalists, marine biologists and Fisheries and Oceans Canada, who may better assess where to find, monitor and clean up plastic pollution in such biologically productive regions.