Understanding the seasonal variation of the microplastics occurrence and source in the water source: upstream of the Huangpu River in Shanghai as an example

The potential seasonal variations in the features and sources of microplastics (MPs) in reservoirs and the connected upstream rivers remain poorly understood. A field study of MPs in Huangpu River upper reaches indicated that surface water MP abundance was 4.75 ± 0.63 (Taipu River), 5.8 ± 0.9 (Lanlugang River), and 4.3 ± 0.9 items/L (Jinze Reservoir) during wet season, increasing to 12.9 ± 1.4, 18 ± 1.7, and 8.3 ± 1.7 items/L, respectively in drought season, whereas in sediment was 6704 ± 421, 3932 ± 200, and 4737 ± 408 items/kg during wet season, which decreased to 4045 ± 523, 4017 ± 430, and 2229 ± 434 items/kg during drought period. The MPs mainly comprised PE, PP fragments/fibers, and PET fibers. The significant seasonal variations in MP abundance were detected only within the surface water and sediments of rivers. Notably, significant seasonal variations in the features of MPs were observed in the surface water of Jinze Reservoir and the surface water or the sediments of rivers. During the wet season, multiple potential sources of MPs were identified in the reservoir, whereas the Taipu River served as the primary source during drought periods. Moderate or lower MP abundance was observed within the surface water of upper reaches of the Huangpu River, while higher levels were found in the sediments. The findings suggest Jinze Reservoir's MP control requires seasonal adjustments. While upper Huangpu River surface water shows lower risk with good safety, MP migration to sediments elevates risks, necessitating prioritized sediment management.