A novel Eulerian-Lagrangian numerical framework to investigate microplastic transport at surface water-sediment interfaces.

Microplastics (MPs) have emerged as widespread and persistent contaminants in fluvial environments. Their transport pathways and retention mechanisms within riverbeds have recently attracted increasing attention. Although stochastic models and experimental studies have shown that streambed sediments can act as important sinks of MPs particles, the hydrodynamic drivers and particle-sediment interactions governing particle exchange across water-sediment interfaces remain insufficiently understood, particularly under complex streambed geometries and variable flows.

In this study, a novel three-dimensional Eulerian-Lagrangian model (MultiFlow3D) is used to investigate MPs transport at surface water-sediment interfaces, resolving turbulence and particle motion in both the free-flow region and the permeable streambed sediments. MPs are simulated as Lagrangian particles, while the streambed sediment is represented through a smooth transition volume penalization numerical treatment that represents the porous bed as spheres. In addition, particle-particle and particle-porous media collision is incorporated to enhance the physical realism of particle interactions.

The model is validated through the reproduction of published laboratory experiments, in which the hydrodynamic flow field and particle transport processes are validated separately. The hydrodynamic component is validated by comparing simulated velocity fields and pressure distributions with experimental measurements, while the particles interactions are validated by reproducing observed particle trajectories, infiltration locations, and retention.

Based on the validation, the influence of different riverbed geometries on the migration of MPs is investigated by testing both sinusoidal and uniform beds. The results indicate that, in most cases, high-pressure regions only develop on the upstream face of bedforms, causing MPs particles to predominantly infiltrate the sediment from the stoss side. However, a secondary high-pressure region may also form under certain conditions on the downstream side, allowing particles to enter the sediment from the lee side. Once infiltrated, most MPs particles remain confined to shallow subsurface layers, with limited penetration depth into the sediment bed.

This study provides mechanistic insight into the combined effects of hydrodynamic pressure distributions and bedform geometry on MPs transport across the water-sediment interface. The proposed modelling approach offers an efficient and physically consistent tool for investigating the environmental fate of MPs in permeable riverbeds and supports improved interpretation of experimental observations.