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Erosive rainfall drives coupled overland and subsurface transport of plastic film residues
Summary
Farm fields covered in plastic sheeting (a common trick to boost crop growth) leave behind tiny plastic fragments in the soil, and this study found that heavy rain washes a meaningful chunk of these microplastics away in runoff water rather than letting them soak deeper into the ground. This matters because it means microplastics can spread from farmland into streams, rivers, and eventually our water and food systems—especially in dry farming regions where this plastic mulch is heavily used—raising concerns about long-term environmental and potential health exposure.
: Intensive plastic mulch application has improved agricultural productivity. However, fragmentary microplastic residues accumulate in soils, potentially degrading surface soil hydraulic properties while being prone to overland transport. As soil surfaces act as dynamic pollutant sources for both overland and subsurface transport during erosive rainfall, both transport processes are mechanistically coupled and must be conjunctively understood to characterize microplastic fate. Accordingly, a flume experiment was conducted to simulate coupled overland and subsurface microplastic transport at two slopes (5°, 10°) under 1.5 mm min -1 rainfall intensity for 1 h. The uppermost 1 cm of soil was spiked with 0.025% by weight microplastics, comprising a mixture of debris and particles, to simulate agricultural plastic mulch residues. Results showed that 4.8% and 6.4% of microplastics were transported overland at 5° and 10° slope, respectively. The overland transported microplastics occurred mostly within runoff, while a small proportion settled with the eroded soil sediments. The transport behavior of debris and particles differed substantially, including the sensitivities of the overland and subsurface fluxes to slope, temporal dynamics of concentration profiles within the runoff and eroded sediment, and mass distributions across runoff, sediment, and various soil depths. Therefore, microplastics are susceptible to overland transport due to low subsurface mobility, which may be most severe in arid croplands, where soil erosion, plastic mulching, and limited rainwater infiltration are prevalent. This study advances understanding of how rainfall-induced soil erosion may drive expanding shallow soil microplastic pollution, which is susceptible to further erosive rainfall, and distinct for microplastics of different characteristics.