Effects of microplastic types and particle sizes on soil water infiltration

【Objective】Microplastic pollution has become a pervasive environmental concern. Increasing evidence has shown that microplastics can alter soil physical properties and hydrological processes. This study aims to elucidate the effects of different microplastic types and particle sizes on water infiltration in soil.【Method】Column experiments were conducted using soils amended with three common microplastic types: polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC). Microplastic particle sizes ranged from 0.05 to 4 mm; columns packed with unamended soil served as the control (CK). The effects of microplastics on the infiltration process were assessed by comparing wetting front migration, cumulative infiltration, infiltration rate and spatial variations in soil water content among different treatments.【Result】For all microplastic types, the time required for the wetting front to reach the depth of 60 cm initially increased and then decreased with increasing particle size. In PE-amended soils, infiltration was faster than that in CK when particle sizes were smaller than 2 mm, but became slower when particle sizes exceeded 2 mm. In contrast, water infiltration in PP-amended soils was consistently slower than that in CK, while infiltration in PVC-amended soils was generally faster than that in CK. Although microplastic addition slightly increased cumulative infiltration, differences among particle size treatments were not statistically significant. Microplastic amendments enhanced the heterogeneity of soil water distribution, increasing water content in the shallow layer while decreasing it in the deep layer relative to CK. During the early stage of infiltration, cumulative infiltration rates in microplastic-amended soils were higher than those in CK; however, these rates gradually converged toward the control values at the later stage.【Conclusion】Hydrophobicity of the microplastics followed the order PVC＜PE＜PP; water infiltration in soil decreased with increasing microplastic hydrophobicity. Microplastic incorporation alters soil pore structure and matrix potential, leading to more complex infiltration dynamics.