Polymer Type, Oxidation, Size, and Abundance of Microplastics in Subsoils versus Topsoils with Varying Land Use in Beijing, China

The occurrence of microplastics (MPs) in topsoil is well documented; however, recent evidence has also shown that MPs can reach the subsoil, which may eventually enter groundwater aquifers posing a potential threat to drinking water. In this study, we examined polymer-type specific, small-sized MPs (20-500 μm) in both topsoil (5 cm) and subsoil (50 and 100 cm) in the megacity of Beijing, China, using Laser Direct Infrared (LDIR) Chemical Imaging Spectroscopy, focusing on variation in abundance, size, polymer type, and oxidation characteristics across 6 land use types. A total of 6085 MP particles with 11 polymer types were identified. MP abundance in subsoils was significantly lower, and they were surprisingly larger in size and less oxidized. MP distribution in subsoils was enhanced in fine-textured and iron (Fe)-depleted soils. Based on these findings, we conducted additional column migration experiments using different textured vadose zones, either with or without hematite, as a typical Fe oxide. Two scenarios were set, including continuous water infiltration and wet-dry cycling conditions. Field data strongly reflected our lab experiments under wet-dry cycling, suggesting that preferential penetration rather than filtration served as the likely primary mechanism of MP occurrence in subsoil.