Transport of microplastics driven by turbidity currents developing over bedforms

The industry of plastics has grown exponentially over the last 70 years (Williams and Rangel-Buitrago, 2022). Although plastics are appropriately disposed, they have entered the natural environments, becoming an emerging contaminant. Due to both sunlight and mechanical abrasion due to waves and currents, plastic material degrades, breaking down into small plastic particles known as microplastics (MPs) when they have sizes below 5 mm (Sun et al., 2022). MPs are transported in suspension from their sources by rivers reaching the ocean. In their way, they can interact with suspended sediments (Mancini et al., 2023). For example, turbidity currents are mechanisms that transport sediment from continental landscapes into coastal areas and therefore into oceans (Pohl et al., 2020). Turbidity currents can transport particles in suspension due to the turbulence produced at the head of the current (Serra et al., 2025). Therefore, they can also transport MPs in suspension into the ocean. However, the transport capacity of turbidity currents is expected to depend on the granulometry of the bed. In the current work, the transport of MP by turbidity currents developing over beds of different granulometry (from bare soil to pebbles) is under study in a laboratory lock gate set up. Two different types of MPs (fragments and fibers) and two polymers (PET and PVC) were considered. Fibers with diameters of 45 mm and 25 mm and lengths of 5 mm and 3 mm were used. All these conditions accounted for a total of 27 experiments. The horizontal distance up to where MPs were transported was found to increase with the velocity of the gravity current and decrease with the settling velocity of the MPs. The granulometry of the bed had a slight impact on the velocity of the gravity current. However, the shape of the MPs particles impacted on the transport of MPs in such a way that the more elongated the particles (small Corey Shape Factors) resulted in longer distances. This can be caused by the alignment of elongated particles like fibers with the streamlines of the flow. A non-dimensional model of the MP transport as a function of the main parameters such as the granulometry of the bed, the settling velocity of MPs, the height of the water column and the shape of the MP particles (through the Corey Shape Factor) is proposed.

References

Williams, A., Rangel-Buitrago, N. 2022. Marine Pollution Bulletin. 176, 113429.

Sun, J., Zhen, H., Xiang, H., Fan, J. and Jiang, H. 2022. Science of The Total Environment. 838, 156369.

Mancini, M., Serra, T., Colomer, J., Solari, L. 2023. Science of the Total Environment. 890, 164363.

Pohl, F., Eggenhu7isen, J.T., Kane, I.A., Clare, M.A. 2020. Environmental Science and Technology. 54, 4180-4189.

Serra, T., Soler, M., Colomer, J. 2025. Sedimentary Geology. 476, 106802.