[Vertical Distribution Characteristics of Microplastics in Miyun Reservoir]

There are sparse reports on the vertical distribution characteristics of microplastics in drinking water sources with large water depths. To elucidate the vertical distribution characteristics of microplastics in the water of Miyun Reservoir， an important surface drinking water source in Beijing， a sampling campaign was conducted for surface， intermediate， and bottom water at different sites in both the flood and non-flood seasons. The abundance， morphology， and compositional characteristics of microplastics were investigated， and the spatial and temporal heterogeneity of microplastics was analyzed. The results showed that the abundance of microplastics in the water of Miyun Reservoir ranged from 0.4 to 7.6 n·L^-1， with an average abundance of （2.7±1.6） n·L^-1. The average abundance of microplastics in the Chaohe River reservoir area was higher than that in the Baihe River reservoir area. At the Baihe River inlet， microplastic abundance in the intermediate and bottom water layers was significantly higher than that in the surface water layer， whereas the Chaohe River inlet exhibited the highest abundance in the surface layer. Microplastic abundance was significantly higher during the flood season. Microplastic particles in the non-flood season were larger， more colorful， and comprised higher proportions of fibers and debris than those in the flood season. Vertically， fibers dominated all water layers， aggregating mainly in the surface and bottom water layers during flood season but were homogeneously distributed during the non-flood season. Microplastics smaller than 0.5 mm accumulated in the intermediate water during the flood season and migrated down to the bottom of the water column during the non-flood season. Polyethylene terephthalate （PET）， polyethylene （PE）， and polypropylene （PP） were the dominant microplastic types in Miyun Reservoir. PET mainly presented in the surface and bottom water layers， PE accumulated in the intermediate and bottom water layers， and PP clustered in the surface and intermediate water layers. The results of the ecological risk and exposure level assessment indicated that both the abundance risk level and exposure level of microplastics in Miyun Reservoir were low.