Imaging of microplastic distribution–related unsaturated water flow in sand

Abstract Soil is the largest sink of microplastic (MP) in terrestrial ecosystems, yet little is known about the transport of MP in soils and its effect on water flow. Since pristine MP can generally be considered water repellent, the transport of MP and water flow are subject to feedback processes. To investigate MP transport in porous media and the interactions with water dynamics, we applied neutron and X‐ray imaging methods to columns filled with mixtures of sand (700–1200 µm) and MP (polyethylene terephthalate, 20–75 µm). Simultaneous neutron and X‐ray CT were used to image MP distribution in 3D before and after each experiment, while time‐series neutron radiography was applied to image water distribution during repeated wetting and drying cycles. The imaging revealed that MP impacted water infiltration. High local MP content caused an initial delay in infiltration, plausibly due to water repellency, but was eventually bypassed by water flow. Rapid and preferential propagation of the wetting front as well as reduced local water saturation above the wetting front were the outcome. The discrepancy between the initial delay and rapid stage infiltration was increased with an increase in total MP content. No significant vertical transport of MP could be observed during two wetting and drying cycles, probably since water bypassed regions containing MP. In conclusion, the interactions between hydrophobic MP and water impact the infiltration of water, with implications for the retention and fate of MP in soils.