Subsurface transport and environmental risks of microplastic pollution: influence of land use and seasonal variability.

Microplastic (MP) contamination is an emerging environmental concern due to its potential impacts on soil health, hydrological processes, and aquatic ecosystems, yet its transport mechanisms across different land use types under seasonal variability remain insufficiently understood. This study systematically investigates the vertical transport, seasonal variability, and environmental risks of MPs across five land use types: riverbanks, dumpsites, agricultural, residential, and woodlands. Field sampling was conducted during pre-monsoon and post-monsoon seasons, with soil and sediment collected from five depth intervals (0-20, 20-40, 40-60, 60-80, and 80-100 cm). MP concentrations varied significantly across land uses, with dumpsites exhibiting the highest in the surface layer (39,113 ± 4750 particles/kg pre-monsoon, 35,920 ± 6294 particles/kg post-monsoon), while woodlands retained the lowest (530 ± 101 particles/kg pre-monsoon, 703 ± 199 particles/kg post-monsoon). Smaller MPs (< 0.3 mm) showed greater vertical transport, especially post-monsoon. Fragments dominated all land uses (45% in deeper riverbank soils), with fibers enriched at the surface (up to 30%). Transparent and white MPs were the most common (up to 34%) with color heterogeneity (black, blue, red) peaking in dumpsites. Low-density polymers such as polyethylene (PE) and polypropylene (PP) dominated deeper layers (up to 60% in woodland), while high-density polymers remained surface bound. Risk indices identified dumpsites as critical hotspots: Pollution load index (PLI) rose from 30.7 to 33.7, and potential ecological risk index (PERI) exceeded 4400 in both seasons. These findings underscore the critical role of land use and seasonal hydrological variability in MP transport, highlighting potential threats to ecosystems.