Agricultural plastic Legacy in river Sediments: Abundance, oxidation, and occurrence characteristics of microplastics...

Microplastics (MPs), as emerging contaminants, remain relatively understudied in agricultural river systems, particularly regarding their occurrence, oxidation degree, and associated risks. This study conducted a comprehensive investigation into the occurrence status of microplastics across 25 agricultural rivers in Henan Province, China. Results showed that the average abundance of MPs in agricultural river sediments reached 1016.2 ± 1264.98 items/kg, which is between that in urban-dominant rivers (2836.6 ± 7313.8 items/kg) and lakes/reservoirs (379.5 ± 346.6 items/kg), while being slightly higher than in marine environments (818.2 ± 2166.0 items/kg). The polymer composition was dominated by polypropylene (PP, 39.6 %) and polyethylene (PE, 36.9 %). Most MPs were < 0.5 mm (80 %), with particles (45.0 %) and fragments (34.9 %) as the main shapes, and transparent MPs (56.3 %) the most common. Spatial analysis revealed higher MP abundance in areas with intensive agriculture. Carbonyl Index (CI) analysis showed a high degree of oxidation (CI > 0.3) in 73.9 % of samples, reflecting oxidative aging characteristics, likely due to riverine environmental conditions and the prolonged retention of MPs. Risk assessment indicated that the study area overall exhibited a moderate pollution level. This study provides a scientific basis for MP pollution control in agricultural watersheds, recommending enhanced source control of agricultural non-point source pollution (e.g., agricultural plastic mulch residues) to protect rural aquatic ecosystem health.