Integrated numerical modeling to quantify transport and fate of microplastics in the hyporheic zone

Despite the significance of rivers and streams as pathways for microplastics (MP) entering the marine environment, limited research has been conducted on the behavior of MP within fluvial systems. Specifically, there is a lack of understanding regarding the infiltration and transport dynamics of MP across the streambed interface and within the hyporheic sediments. In this study, transport and retention of MP are investigated using a new numerical modeling approach. The model is built as a digital twin of accompanying flume experiments, which are used to validate the simulation results. The model accurately represents particle transport in turbulent water flow and within the hyporheic zone (HZ). Simulations for transport and infiltration of 1 µm MP particles into a sandy streambed demonstrate that the advection-dispersion equation can be used to adequately represent particle transport for pore-scale sized MP within the HZ. To assess the applicability of the modeling framework for larger MP, the experiment was repeated using 10 µm particles. The larger particles exhibited delayed infiltration and transport behavior, and while the model successfully represented the spatial extent of particle transport through the HZ, it was unable to fully replicate hyporheic transit times. This study is the first to combine explicit validation against experimental data, encompassing qualitative observations of MP concentration patterns and quantification of fluxes. By that, it significantly contributes to our understanding of MP transport processes in fluvial systems. The study also highlights the advantages and limitations of employing a fully integrated modeling approach to investigate the transport and retention behavior of MP in rivers and streams.