Plastic drift : Mapping the course of microplastic transport in turbulent riverine flows.

Microplastics present significant challenges to aquatic ecosystems, yet there remains a limited comprehension of the mechanisms governing their transport. Insights from natural sediment dynamics present a promising approach to address this knowledge gap. The concept of the transport stage is central to sediment transport analysis, characterizing the relationship between fluid shear velocity and particle settling velocity. However, inherent variability in microplastics physical properties such as shape and density raise doubts regarding the applicability of conventional sediment transport models to microplastics. To confront this issue, we conducted laboratory experiments to investigate the trajectories of 24 negatively buoyant microplastic particles, encompassing a spectrum of sizes, shapes, and densities. Employing a 3D particle tracking system, we documented the particles' movement within turbulent open channel flow, generating a dataset of 720 trajectories for subsequent analysis. Our findings indicate a robust relationship between the transport stage and both the mean forward velocity and vertical positioning of microplastic particles within the water column. Notably, particle shape emerged as a pivotal determinant of transport dynamics. Fibers exhibited lower mean forward velocities with a tendency to remain near the water surface. On the contrary, spheres tended to remain closer to the bed while exhibiting higher mean forward velocity. Further analysis revealed distinct modes of transport akin to those observed in natural sediment dynamics, encompassing rolling/sliding, saltation, and suspension. Overall, our study revealed a correlation between the transport stage and the temporal distribution of microplastics across distinct modes of transport. Building upon these laboratory insights, we drew a novel phase diagram tailored specifically to microplastics, analogous to practice in sedimentology. This innovative framework offers a comprehensive depiction of microplastic transport phenomena, which could improve the performance of prediction models. By improving our understanding and prediction of microplastic transport, this work can facilitate targeted interventions to mitigate their environmental impact. Also see: https://micro2024.sciencesconf.org/558476/document