Assessing Microplastic Dispersion and Persistence in a Tidally Influenced Urban Estuary: Implications of Particle Shape for Environmental Modeling

Microplastic (MP) transport in estuarine systems is governed by the interaction between hydrodynamic forcing and particle-specific properties, including density and shape. This study applies a Computational Fluid Dynamics (CFD) modeling framework to investigate how particle shape influences the transport, residence time, and vertical–horizontal distribution of MPs in the Salado Estuary (Ecuador), a tropical urban estuary characterized by semidiurnal tidal dynamics. The model couples a Volume of Fluid (VOF) free-surface formulation with a k–ω SST turbulence closure and a Discrete Phase Model (DPM), incorporating non-spherical particle behavior through a simplified shape-factor-based representation (shape factor = 0.75) and a tidal User-Defined Function (UDF). Six polymer types commonly reported in the estuary (PA, PC, PET, PP, PS, and PVC) are simulated and compared under spherical and non-spherical particle representations. The results indicate that particle density remains the dominant control on vertical fate, while particle shape systematically modulates horizontal transport, residence time within the water column, and spatial redistribution under identical hydrodynamic conditions. For dense polymers, non-spherical particles exhibit delayed settling, reduced cumulative deposition, and enhanced downstream transport relative to spherical particles. In contrast, low-density polymers (PP and PS) display persistent near-surface transport with negligible or marginal deposition, indicating that when buoyancy dominates, particle shape has a limited influence on vertical fate. These findings demonstrate that commonly adopted spherical-particle assumptions in environmental modeling may underestimate transport extent and persistence for polymers prone to sedimentation, potentially biasing the identification of accumulation zones and exposure pathways. The qualitative consistency between simulated transport patterns and reported observations for the Salado Estuary supports the environmental plausibility of the modeling framework, without constituting quantitative validation. Overall, the study provides a reproducible CFD-based approach for exploring shape-related uncertainties in MP transport and supports its application in environmental modeling and assessment of tidally driven estuarine systems.