Threshold migration conditions of (micro) plastics under the action of overland flow

Surface runoff is a major pathway for the transport of plastics. However, most previous studies focus on the transport of microplastics in aquatic environment, whereas the migration of plastics from terrestrial environment to aquatic ecosystems receives limited attention. In this work, we investigated the migration of plastic on different surfaces via surface runoff. Results indicate that the threshold migration condition increases with the size and density of plastics, while decreases as the surface inclination increases. Plastics show a higher degree of mobility on smooth surfaces, but plastic films exhibit lower mobility due to the frictional forces induced by the downward pressure exerted by the water flow. Conversely, rough surfaces such as concrete and macadam can trap small fragments and rigid film plastics, and plastics can be embedded within the soil matrix under the water flow. In summary, smaller size, steeper incline, and greater water flow rate facilitate the movement of plastics on surfaces. Results from this work improve the understanding of the process of plastic migration from land to water, and are of great significance for the prevention and control of plastic pollution.