Uneven Distribution of Microplastic Pollution in Paddy Fields Driven by Irrigation Hydrodynamics

ABSTRACT Irrigation is essential for agricultural production, serving not only to transport water but also to facilitate the migration of pollutants such as microplastics (MPs). Although the presence of MPs in agricultural soils is well documented, the effects of irrigation hydrodynamics on their distribution have rarely been explored. This study examines the occurrence and sources of MPs in various sections of paddy soils from the Quyuan Irrigation District (QID) in China. MP analysis was conducted using density separation, microscopy, and laser direct infrared spectroscopy (LDIR) methods. Results showed that MP abundance ranged from 73.30 to 3880.00 items/kg. Statistical analyses revealed that agricultural plastic residues were the primary source of MPs. As a result of irrigation activities, the abundance of < 0.5 mm MPs in paddy soils near branch canals was 2.17 times higher than that in soils near the main canal, indicating more severe MP pollution near the branch canals. In addition, the MP abundance in soil at the drainage outlet (DO) was 2560 items/kg, significantly higher than the 1440 items/kg found at the irrigation inlet (IN) ( p < 0.05). This was due to an increase in water velocity and flux, which generated stronger water impact forces that flushed most MPs into the DO soil. The irrigation hydraulic conditions and MP characteristics influenced the horizontal migration of MPs in surface paddy soils. Overall, this study elucidated the relation between MP pollution and irrigation hydrodynamics, providing valuable insights for precise monitoring and control of MP pollution in farmland.