Rainfall-induced lateral and vertical microplastic transport of varying sizes in agricultural fields

Agricultural soils, particularly those utilizing plastic products for crop production, are increasingly recognized as sources of microplastics (MPs) to aquatic ecosystems. In this research, we investigate the transport of polyethylene MPs of three different size ranges (53-63 μm, 125-150 μm and 425-500 μm) in an agricultural soil during a plot-based rainfall simulation. Using a combination of fluorescent particles and high-frequency photography, we tracked the number of MPs on the soil surface throughout the rainfall simulation, measured the depth MPs migrated into the soil profile and the number of MPs which were transported in surface runoff. Our results show that MPs had dynamic movement on the soil surface throughout the rainfall simulation. Approximately 20% of MPs sized 125-150 μm and 425-500 μm were exported from the plot in surface runoff with the remaining 80% of MPs thought to be retained in the soil. No significant differences were found in the number of MPs transported in surface runoff between MPs sized 125-150 μm and 425-500 μm. Microplastics were found to be enriched in eroded sediments. Microplastics of all sizes 53-63 μm, 125-150 μm and 425-500 μm were found in soil as deep as 8 cm with the majority of MPs found in the 0-2 cm soil depth. Results from this research not only indicate that MPs are quite mobile both vertically and laterally during rainfall events but also show that soils effectively retain and accumulate a sizeable proportion of MPs during heavy rainfall events.