Assessing microplastic abundance and distribution in rivers: A mechanistic modeling approach

Rivers are major transport pathways for microplastics to reach the oceans. Microplastics have numerous different shapes; however, theoretical models generally assume microplastics as spherical particles. This study aims to investigate (i) spatiotemporal distribution and the potential sources of microplastics in an industrially polluted river, (ii) the relationship of microplastic abundance with river's morphological and hydrodynamic characteristics, and (iii) vertical transport of microplastics and the effect of particle and flow characteristics on settling and resuspension. To achieve these aims, water and sediment samples were collected from six sites of the Ergene River in May 2019 and Sep 2020. A mechanistic model was developed using the data of microplastics, and river hydrodynamics and morphology. According to the results, the Ergene River had excessive levels of microplastics compared to many other rivers. Microplastic concentrations in water correlated positively with depth but negatively with channel width. Fibers were the most abundant shape of microplastics suggesting that effluents from textile industries were the foremost contributor of microplastics in the river. The model results revealed that the residence time of microplastics in water was directly related to the flow characteristics, while initial concentration of particles in water dominates the other parameters in settling and resuspension fluxes of microplastics. According to the scenario analysis, settling and resuspension fluxes increased with increasing sphericity and size of the particles, as well as higher bed shear stress. The model results were sensitive to changes in shape factor developed for this model, therefore this parameter should be improved in future studies.