Minimal preferential transport of microplastics through macropores in irrigated disturbed and undisturbed soil columns

Microplastics are environmentally persistent pollutants. Transport of microplastics away from root zones by infiltrating water is potentially crucial in remediating polluted agricultural soils. However, existing knowledge suggests that microplastics are retained and dispersed within the root zone under typical agricultural water balances. To investigate the underlying mechanisms, we sampled two soil types and conducted experiments in both disturbed and undisturbed soil columns. Microplastics were introduced into the surface, then transported deeper under multiple irrigation-drying cycles. Microplastic concentrations in soil matrices and fracture zones were separately measured, to investigate transport behavior arising from preferential flow. For all disturbed soil columns, even after 120 cm water (before correction for porosity) was applied over seven irrigation-drying cycles, over 80 % of the microplastic mass remained within the shallowest 5 cm. Furthermore, microplastics were detected beneath 40 cm depth only after seven irrigation-drying cycles. The undisturbed columns exhibited slightly greater transport, especially through fractures, but similar overall behavior with disturbed columns. Microplastic concentrations in soil matrices and fractures were highly correlated, suggesting primarily diffusive transport, and that the strong retardation of microplastic transport was similarly dominant within the matrices and fractures. Hence, mechanically remediating microplastic polluted soils by leaching is challenging, and preventing pollution is crucial.