Microplastics undergo accelerated vertical migration in sand soil due to small size and wet-dry cycles

Microplastics (MPs) are an emerging concern and potential risk to marine and terrestrial environments. Surface soils are reported to act as a sink. However, MP vertical mobility in the subsurface remains uncertain due to a lack of scientific data. This study focused on MP penetration in sand soil column experiments. Here we report the mobility of five different MPs, which consisted of polyethylene (PE) and polypropylene (PP) particles of various sizes and densities. We observed that the smallest sized PE MPs (21 μm) had the greatest movement potential. Moreover, it was found that when these MPs were subjected to greater numbers of wet-dry cycles, the penetration depth significantly increased, with an apparent linear relationship between depth and wet-dry cycle number (r = 0.817). In comparison, increasing the volume of infiltration liquid or the surface MP concentration had only negligible or weak effects on migration depth (r = 0.169 and 0.312, respectively). Based on the observed wet-dry cycle trend, we forecast 100-year penetration depths using weather data for 347 cities across China. The average penetration depth was calculated as 5.24 m (95% CI = 2.78-7.70 m), with Beijing Municipality and Hebei, Henan and Hubei provinces being the most vulnerable to MP vertical dispersion. Our results suggest that soils may not only represent a sink for MPs, but also a feasible entryway to subsurface receptors, such as subterranean fauna or aquifers. Finally, research gaps are identified and suggested research directions are put forward to garner a better understanding MP vertical migration in soil.