Microplastic Transport by Overland Flow: Effects of Soil Texture and Slope Gradient Under Simulated Semi-Arid Conditions

Microplastic pollution in soils and surface waters is a growing environmental concern, yet the mechanisms governing transport by overland flow remain unclear. This study investigated the influence of soil texture and slope gradient on the movement of microplastics with different shapes and polymer compositions under simulated rainfall and typical agricultural conditions in a semi-arid climate. Small soil flumes were subjected to controlled rainfall simulations replicating typical rain patterns, and microplastic transport was quantified using collection flasks. The results indicated that neither soil texture nor slope gradient significantly affected total microplastic transport. However, fibres exhibited greater retention in the soil compared to other shapes. Polymer composition did not play a major role in microplastic mobility, except for polystyrene pellets, which were transported more readily than polyethylene pellets. Field observations of agricultural soils with a history of sludge application confirmed a predominance of fibres in the topsoil, reinforcing the tendency of this shape to resist mobilisation. These findings suggest that microplastic transport by surface runoff is primarily governed by particle shape and buoyancy rather than soil properties or slope inclination. Future research should explore the roles of particle size, rainfall intensity, and organic matter content in microplastic mobility under natural field conditions.