Riparian vegetation as a natural barrier: experimental analysis of plastic particle retention in a vegetated reach

Plastic pollution is a growing environmental concern, with rivers acting as critical pathways for plastic particles entering marine ecosystems. In river corridors, riparian vegetation has been identified as a potential natural barrier for plastic particles while transported downstream. Knowledge of the trapping dynamics appears fundamental for understanding the role of riparian barriers in mitigating plastic pollution. However, to date the effects of the foliage distributions of riparian flexible plants and of turbulence on the plastic particle retention dynamics have not yet been studied. This work aims to fill these knowledge gaps including also the influence of the particle characteristics (i.e., density, size, and shape) and of the vegetation structure (i.e., number of branches and leaves). Laboratory experiments were performed in steady flow conditions over a fixed dune bed, using just submerged flexible foliated plastic plants. Results show that particle retention is highly influenced by density and shape. Among the tested disk-shaped particles, those with higher density (1.4 g/cm³) exhibited retention rates up to 93 %, largely at the channel bed structures. At equal density (1.04 g/cm³), irregular-shaped particles were retained at nearly twice the rate of disk-shaped ones (20 % vs. 9–10 %), with vertical dispersion influenced by turbulence. Turbulent Kinetic Energy (TKE), expressed as dimensionless TKE/U² values, showed a non-linear relationship with retention: values below ∼0.0015 were associated with enhanced trapping, while higher turbulence reduced retention, likely by promoting resuspension and weakening particle adhesion. Vertical profiles of TKE/U² varied with leaf configuration, suggesting that vegetation structure modulates turbulence intensity along the canopy. These findings provide a quantitative framework to evaluate microplastic trapping efficiency in vegetated riparian zones, offering insights into natural and engineered nature-based solutions for mitigation of plastic pollution in aquatic systems.