Microplastic dispersion and retention in bare, vegetated, and baffled freshwater systems : insights from physical and LES/RANS numerical modeling

This thesis presents an integrated investigation into the dynamic transport and fate of aquatic microplastics (MPs) in freshwater systems, combining laboratory experiments and advanced numerical simulations to reveal the mechanisms governing MP dispersion and retention. Physical experiments in artificial channels with vegetated canopies focused on the longitudinal dispersion of dense MPs (188 nm and 6 μm; 1040 kg/m3). Building on these experimental insights, two Eulerian–Lagrangian frameworks were developed in OpenFOAM, utilizing Large Eddy Simulation (LES) and Reynolds- Averaged Navier–Stokes (RANS) turbulence models with Lagrangian particle tracking (LPT) to simulate MP transport (50–100 μm; 940–1450 kg/m3) under complex flow conditions. Innovative stochastic differential equations—the Generalized Langevin Model for LES and the Drift Correction Model for RANS—were implemented to reconstruct subgrid-scale turbulence effects. The RANS-LPT framework was further extended to assess MP retention in permeable-baffled retention ponds, accounting for the effects of baffle porosity, number, and placement (5–100 μm; 1450 kg/m3), with a modified Darcy equation representing baffle-induced momentum losses. Results reveal that aquatic vegetation significantly modifies the hydrodynamic structure by reducing flow velocity, increasing turbulent kinetic energy (TKE), and diversifying turbulence length scales. These changes enhance mixing and lead to up to 40-fold higher MP diffusivity in vegetated compared to unvegetated flows, shifting the primary mixing mechanism from shear-induced velocity gradients to turbulence enhanced by plant–flow interactions. Large-scale turbulence and secondary vortices are identified as dominant drivers of MP redistribution during longitudinal transport, with particle inertia inversely affecting turbulence sensitivity. Smaller MPs (50 μm) exhibit uniform distributions due to their high sensitivity to turbulence, while 98% of 100 μm MPs rapidly deposit under gravity within 3.2 m of release. Retention ponds with optimized baffle configurations are shown to be effective strategies for mitigating MP pollution from natural runoff, achieving up to 100% retention for 100 μm MPs and over 80% for dense MPs in the 50–100 μm range. However, retention efficiency for MPs smaller than 25 μm was reduced (23–40%), attributed to intensified particle mixing and elongated residence time, particularly in systems with fine-porous baffles. Overall, this thesis provides new insights into MP dynamics in turbulent freshwater environments by integrating high-resolution physical measurements with robust numerical simulations. The developed modeling tools offer a foundation for predicting MP environmental fate and optimizing engineered water treatment strategies for more effective mitigation of microplastic pollution.