Dynamics of microplastics particles in rivers: experimental settling processes and numerical models of the transport and deposition

Microplastic pollution is an environmental problem facing rivers, oceans, and coastlines. Estimations suggest that around 1.15 to 12.7 million tonnes of plastic waste enter the ocean every year from the global riverine system. In the environment, organisms ingest plastic they cannot distinguish from food, causing them harmful and lethal effects. Observations from rivers show that suspensions of microplastic are composed of particles, formed from different plastic polymers, with densities buoyant, non-buoyant or neutrally-buoyant in water. The microplastic particles are found transported in the river flow and deposited in the sediments. The vertical distribution of microplastic particles trapped in sedimentary deposits shows that the amount of plastic deposits decreases in deeper layers. However, all the types of plastics are found in similar percentages all over the layers, lacking current physical explanation. To understand the physical parameters that control the trapping of microplastic pollution within sedimentary deposits, a series of sedimentation experiments were designed using different mixtures of microplastic particles and sediment. The results highlight the relative importance of microplastic-sediment concentrations controlling plastic material distribution within the deposits. The sediment-microplastic deposits have become an active component of river systems generating changes in the bedforms; the presence of the plastics is increasing the average diameter of the materials in the bed, and therefore there is a need for a higher critical shear stress for erosion. A numerical model of a braided river with plastics was created to predict the physical changes in the bedforms due to the interaction of the plastics and sediment in the river bed. The model describes how a higher amount of plastic can be deposited in the bars and river banks near the sources, decreasing the erosion capacity of the river flow in these areas and promoting the formation of more extensive sediment bars and deeper or wide channels. The model allowed studying the deposition, resuspension, and transport patterns of microplastic loads, identifying key processes such as high concentrations of plastic(hotspots)