Flume experiments on transport and deposition behavior of microplastics in sediment bed environments

The number of scientific studies on the presence, abundance and distribution of microplastics (MPs) across aquatic compartments, including rivers, sediment beds, and groundwater has increased significantly. Fundamental processes impacting the mobility of MPs within the upper part of the hyporheic zone in freshwater sediments however are poorly understood. Thus, established sediment transport models fail to incorporate transport interactions between MPs and fluvial sedimentary environments. Flume experiments provide reference for detailed modeling establishing a context between the dynamics of sediments and MPs. In this study we conducted 42 experiments with three model sediments at two laboratory flumes to investigate the effect of density, size, and initial placement of spherical MPs into the flume. Experimental data revealed that the deposition depth of MP particles is affected by the porosity of the sediment and the ratio of grain diameter and the diameter of the MP particles. Also, bedform dynamics that were observed during experimental runs might influence MP deposition in sediments. Transport of spherical MPs of different sizes and densities in mixed gravel with minimal bedform motion has been found to be mainly affected by density and placement of MPs into the experimental set-up. Our findings provide reference for developing more efficient MP mass balance models to predict particle abundances in sediment bed environments.