[Pollution Characteristics and Risk Assessment of Microplastics in the Xiamen Houxi River Watershed]

Microplastics, defined as minuscule plastic particles measuring less than five millimeters （5 mm） in size, have become pervasive in various environments. Watershed areas, perpetually subjected to escalating human activities, face a growing and persistent threat from microplastic pollution. This study aimed to investigate the characteristics and ecological risk posed by microplastic pollution in the Houxi River watershed of Xiamen. A comprehensive analysis was conducted, employing field sampling, stereomicroscopy, Fourier transform infrared spectroscopy, as well as the risk index （H） and pollution load index （PLI） model. The findings revealed a 100% detection rate of microplastics at various points within the watershed. The average abundance of microplastics in water, sediment, and soil was found to be （3.65±0.51） n·L^-1, （354.56±18.22） n·kg^-1, and （1 509.55±69.90） n·kg^-1, respectively. Significantly lower microplastic abundance was observed in the upper reaches of the watershed, attributed to the enhanced ecological protection in this area. In contrast, the middle and lower reaches, characterized by dense populations, exhibited higher microplastic levels due to increased production and domestic pollution. The majority of microplastics had a particle size of < 0.5 mm, constituting an average proportion of over 70%. Larger particles exhibited a smaller proportion. Fragmented particles dominated in shape, comprising over 50%, followed by fibers and films, with foam having the lowest proportion. The predominant polymer type identified in microplastics was PE, accounting for over 50%, followed by PP, while PET and PA represented the least. The regional microplastic risk index （H） consistently fell within the low-risk level I, yet it approached the level II risk. The pollution load index （PLI） of microplastics indicated a low-risk level I. These findings contribute valuable insights for regional microplastic pollution prevention and risk assessment efforts.