Transport dynamics of microplastics within aquatic vegetation featuring realistic plant morphology

Abstract

Despite the significance of rivers and streams in transporting terrestrial microplastics (MP) to the oceans, limited research has focused on the role of aquatic vegetation and their complex geometry in shaping the underlying mechanisms governing MP mixing and dispersion processes in riverine environments. This study, for the first time, investigates the transport and fate of non-buoyant MPs, specifically those with diameters of 188 nm and $6\mathrm{\mu }\mathrm{m}$ and a density of 1.04 g/cm${}^3$, in floating Eichhornia crassipes canopies under flow conditions typical of natural rivers (0.0167-0.0667 m/s). Physical modelling tests reveal that aquatic vegetation significantly alters the hydrodynamic structure and enhances the dissipation of turbulence in the water column, leading to decreased velocities, diversified length scales, and increased turbulent kinetic energy (TKE) in regions with higher frontal vegetation areas. This turbulence, in turn, facilitated momentum exchange and vertical mixing, particularly in regions with the most pronounced frontal area changes. Wider canopy spacing promoted the evolution of wake turbulence and facilitated wake expansion throughout the water column, generating coherent structures that effectively doubled the integral length scales with increasing distance between canopies from 0.5 m to 1.5 m. This adjustment resulted in a more uniformly dispersed downstream movement of MPs. Notably, the presence of canopies amplified MP diffusivity by 10-40 times compared to equivalent unvegetated conditions, transitioning the primary mixing mechanism from shear-induced velocity gradients to turbulence enhanced by plant-flow interactions. This study offers a robust framework for quantifying MP mixing and predicting longitudinal dispersion coefficients within the floating vegetated flows, by developing models that depict the vertical profiles of TKE and turbulent diffusivity featured by canopy morphology and spacing. The insights from this study make a significant contribution toward improving our ability to predict the mixing and fate of MPs in riverine environments and underscore the necessity of incorporating the complex dynamics of aquatic vegetation into environmental management and MP risk assessments.