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.

Characteristics of (micro)plastic transport in the upper reaches of the Yangtze River.

Many studies focus on the transport of plastic from rivers to oceans while little attention was paid to the plastic transport in the upper reaches of rivers. Transport process of plastic from upstream to downstream in the whole river basin scale is still poorly understood. In this work, five sections in the upper reaches of the Yangtze River were investigated to characterize the features of plastic transport. Plastic abundance and flux were 293 to 156,667 n/m3 and 1.2 to 34,711 g/s, respectively. Plastic flux peaked at or right after the first flood peak in most sections, but plastic abundance was the highest in the normal or low water period. The first flood peak caused a temporary rise of plastic flux that last a short duration. Transport of plastic was not limited to water surface, and the Three Gorges Dam showed a peak elimination effect on plastic transport. Annual discharge of plastic was 1392 to 9532 tons and 6.2 × 1014 to 175 × 1014 particles at different sections. An increasing trend was observed for both plastic mass and quantity going downstream. Results showed that river plastic flux is highly variable and influenced by the dam, which should be considered in future to develop better monitoring strategies and to further improve the model.

Horizontal transport of macro- and microplastics on soil surface by rainfall induced surface runoff as affected by vegetations.

Surface runoff is considered as an important pathway that transport of plastic waste from terrestrial environment into the aquatic environment but the process is still poorly understood. In this work, runoff plot experiment was carried out to study the horizontal transport of macro- and microplastics between 50 mm and 0.25 mm in size on the soil surface by rain induced runoff. The influences of vegetation cover, characteristics of plastics, and rainfall scenarios were investigated. Results showed that the presence of vegetation significantly enhance the retention of plastics by about 20% under the experimental conditions. Lower density and smaller (<1 mm) plastics were found to have higher mobility. The herb plant (Photinia×fraseri Dress) showed a better interception efficiency on plastics than the shrub plant (Ophiopogon japonicus (Linn. f.) Ker-Gawl.) at the same planting density, while increasing plant density contributed litter to the interception of microplastics. Increase rainfall amount from 20 mm to 60 mm enhanced the transport of plastics while repeating 20 mm rainfall every 3 days did not affect the transport of plastics significantly. The same processes may involve in the transport of plastics and soil particles by rainfall induced surface runoff. Strategies controlling soil erosion could also be used to prevent plastics in soil from entering the aquatic environment. However, effects and risks of plastics retained in the soil are still unclear, which need to be investigated in future.