Modeling microplastic deposition in sandy streams with moving bedforms

Microplastic (MP) delivery from the terrestrial to aquatic environments is a global concern to many ecosystems and potentially also to humans. Currently, a limited number of models can accurately predict how MPs move through streams and rivers toward the oceans. The limited predictive power of classical colloid filtration theory and the lack of models that take into account the interactive effect between streambed characteristics, flow conditions and particle characteristics limit our ability to model the deposition of MP in streambeds. This study combines improved mechanistic prediction of colloid attachment with a model that predicts flow and transport of particles in a moving streambed to quantify MP deposition in streams. A set of numerical simulations were conducted using sand with D₅₀ of 0.3 mm and hydraulic conductivity of 0.12 cm/s. Such sand is predicted to form ripples with a length of approximately 17 cm and a height of 1.9 cm. Coefficient of attachment (K_att) was predicted for simulated MP particles of four different densities (900, 1050, 1140, and 1350 (Kg/m³), which are typical densities of Polypropylene [PP], Polystyrene [PS], Polyamide [PA], Polyethylene terephthalate [PET], respectively. In addition, model scenarios included three colloidal sizes (0.5, 1, 10 μm) and various overlying stream velocities of 0.1-0.5 m/sec. Such stream velocities were predicted to yield bed celerities between 0-130 cm/hr. Hyporheic exchange flux between the stream and the bed increased non-linearly with celerity and was found to be ten times greater for the fast celerity (130 cm/hr at stream velocity of 0.5 m/sec) as compared to slow-moving bedform with the same geometry (10 cm/hr at stream velocity of 0.2 m/sec). Difference hyporheic exchange fluxes are also expected to influence the rate of MP delivery to the bed and their deposition. Initial simulations show that increased bedform celerity and K_att lead to a shallower depth of MP deposition and a more compact distribution in the bed. Increased celerity reduces deposition depth by flattening hyporheic exchange flow paths. Therefore, despite an increased flux of MP into the bed under high stream water velocity, deposition occurs at shallower depths, and the chance for resuspension due to erosion of the bed sediment increases. Quantifying the deposition rates and residence time in the bed is essential for understanding the transfer of MP through streams and rivers toward the oceans, developing sampling strategies, and finding long-term solutions for reducing their concentrations and the associated risks.