Vertical distribution and migration of microplastics in soils from Fars Province, southwest Iran

Microplastics (MPs) in soils have recently emerged as a significant environmental concern because of their potential impacts on ecosystems and human health. In this study, MPs have been determined in genetic soil horizons to a maximum depth of 140 cm along four transects that encompass various land uses (managed and unmanaged) in Fars Province, southwest Iran. Soils have also been characterised in terms of texture and chemistry using established methods. With little contemporary or historical application through agricultural practices, MPs were dominated by fibres of various colours and sizes and polymeric construction (mainly polyamides, polyesters and polyolefins), with remaining particles largely consisting of sheet-like fragments. MP abundance (up to about 200 per kg of dry soil) and size were heterogeneously distributed throughout the region and with respect to soil depth, regardless of land use, with inverse correlations with soil particle size observed at two locations. We infer that atmospheric deposition is the principal source at the soil surface and that MPs that evade erosion are able to readily migrate downwards to depths extending to at least that of the lowest horizon sampled. Migration appears to be independent of particle size or density and is likely driven by percolating precipitation but facilitated through bioturbation and soil cracking during dry periods. The persistence and vertical migration of MPs in soils may have adverse impacts on subterranean ecosystems and ground water quality.