Changes and transport of microplastics in the riverbed of the mainstream below the Three Gorges Dam of the Yangtze River

Abstract

The Yangtze River has always been considered an important river for transporting microplastics into the sea. However, there is still a lack of understanding of the degree of microplastic pollution in the sediment of the middle and lower reaches of the Yangtze River, which has affected the estimation of microplastic flux into the sea. In this study, microplastics in sediments from 55 stations in the middle and lower reaches of the Yangtze River were investigated, including 48 mainstream stations, 5 confluence stations, and 2 lake stations. Microplastics were detected in 34 stations, ranging from 0 to 71.04 n/kg of dry weight. The average concentrations of microplastics in the mainstream, confluence and lake were 6.16 ± 8.87, 23.91 ± 11.2, and 43.95 ± 27.09n/kg of dry weight, respectively. Of all the microplastics found in this investigation, fiber (59.52 %) forms the main type in terms of shape; PET is the most abundant microplastic species, accounting for 51.19 %. Sewage treatment plants are the most likely source of microplastics in sediment, and sediment from tributary estuaries, lake mouths, and lakes is an important sink of microplastics. Terrain, sediment transport load, biofilm, and flow velocity are important factors affecting microplastic settlement. Although there are multiple microplastic enrichment areas, the high flow velocity during the flood season makes it difficult for microplastics to remain in the sediment of the Yangtze River bed. This indicates that microplastics in riverbed sediments have little influence on the flux into the sea during flood season. This study is the first large-scale investigation of microplastics in riverbed sediments in the Yangtze river basin.