[Distribution Characteristics of Microplastics and Their Migration Patterns in Xiangxi River Basin]

Microplastics as a prevalent pollutant in water bodies have recently attracted widespread attention. To investigate the spatial and temporal distribution characteristics of microplastics in freshwater rivers and their migration patterns, the surface water, sediments, and subsidence zone of the Xiangxi River, a tributary of the Yangtze River, were sampled and analyzed in November 2020 and April 2021, respectively. The results showed that the average abundance of microplastics in the surface water of Xiangxi River was (6.64±1.32) n·L^-1 in flat water and (5.00±1.07) n·L^-1 in dry water, the average abundance of microplastics in sediments was (0.56±0.13) n·g^-1 in flat water and (0.41±0.09) n·g^-1 in dry water, and the average abundance of microplastics in the subsidence zone was (0.53±0.15) n·g^-1 in flat water and (0.68±0.18) n·g^-1in dry water. There were significant differences in the abundance distribution of microplastics in the surface water, sediments, and subsidence zone (P<0.05). In the surface water and sediments, the particle size of microplastics was mainly distributed in the range of 0.1-0.5 mm, and in the subsidence zone, it mainly ranged from 1-5 mm. The color of microplastics was mainly transparent in the surface water and subsidence zone and blue in sediments. The morphology of microplastics in the Xiangxi River basin was mainly fiber, and the materials were mainly polyethylene (PE) and polypropylene (PP). There were many factors affecting the distribution of microplastics. The analysis results showed that the abundance of microplastics in the surface water was negatively correlated with the flow rate of the water body. The abundance of microplastics in the sediment was related to the substrate type of the riverbed and negatively correlated with the substrate particle size. Combined with the microplastic abundance data of each sampling site, it was found that there was a significant migration process of microplastics in the spatial distribution of the Xiangxi River in the watershed. Along the river longitudinal direction, the longitudinal migration of microplastics in the surface water was along the river direction, and in the vertical direction, it showed the mutual migration between the water body and the subsidence zone and the water body and sediments.