Transport and Fate of Microplastics in Terrestrial Environments: The Role of Surface Runoff, Root-Mediated Infiltration, and Fragmentation-Driven Mobility

Microplastics are widely distributed in terrestrial environments due to processes such as plastic fragmentation, tire wear, and the reuse of plastic-rich materials in agriculture. Their transport is influenced by hydrological, biological, and physicochemical factors. This dissertation applies principles from classical sediment transport to microplastic movement, focusing on three key processes: surface runoff, root-mediated infiltration, and fragmentation-driven mobility. The role of particle characteristics—density, size, and shape—is examined through field studies and laboratory experiments. Tire wear particles (TWP) and their chemical markers (6PPD-Q, DPG, and 2-OHBT) are investigated across different road and soil conditions. While 6PPD-Q tends to accumulate in soils, 2-OHBT demonstrates higher mobility, highlighting runoff as a key dispersal pathway. Rhizotron experiments with wheat plants demonstrate that roots influence the transport of microplastics, with fibres adhering to the roots and fragments infiltrating deeper soil layers. Fragmentation increases the mobility of microplastics, potentially accelerating groundwater contamination. Column experiments confirm that smaller, irregular fragments move more readily through porous media than spherical particles. The results contribute to understanding microplastic transport, informing environmental risk assessment and pollution mitigation. They also support the need for sustainable soil and water management, as well as stricter regulation of plastic additives and their degradation products.