Rainfall-Induced Transport of Microplastics in Soils Depends on Soil Pore Structure

Soils are recognized as major sinks of microplastics (MPs), yet their mobility under field-relevant conditions remain poorly understood. Most studies investigating water-driven vertical transport of MPs have employed simplified experimental setups with repacked soils or artificial homogeneous porous media. However, natural soils are structurally heterogeneous and contain macropore networks that can serve preferential transport pathways, even for larger MPs. Incorporating key soil physical controls on transport is therefore crucial for improving the applicability of experimental findings to natural soil systems.

This study examined the vertical transport of MPs in undisturbed soil with an intact macropore system. Intact soil cores (11 cm height, 9 cm diameter) were collected from a clay loam agricultural topsoil. Soil pore architecture, including pore connectivity, tortuosity, and pore size distribution, was characterised using X-ray computed tomography (CT). To further facilitate interpretation of the MP transport experiments, we carried out non-reactive tracer experiments at constant water flow rate. Metal-doped polyethylene terephthalate (PET; 63–125 µm) fragments were subsequently introduced to the soil surface, and the cores were subjected to intermittent rainfall simulations at 5 mm day-1 under near-saturated conditions. MP transport was quantified by measuring the metal tracer in leachates and soil cores at different depths using ICP-MS.

By linking MP transport to soil pore architecture, this work aims to unravel the role of the soil pore structure in determining MP mobility in soils. We expect transport depth and rate of MPs are likely governed by pore-network geometry, such as connectivity, continuity and pore-to-MP size ratios. Thereby, this work contributes to a more field-realistic assessment of MP transport process and a step towards improving long-term predictions of MP exposure in soils.