Understanding microplastic transport and retention in soil: insights from laboratory and field studies

Microplastic pollution in terrestrial environments poses significant risks to soil health, groundwater quality, and ecosystem functionality. This study integrates findings from laboratory and field experiments to elucidate the dynamics of microplastic transport and retention in soils under various conditions. Laboratory experiments examined the fate of low-density polyethylene (LDPE), polybutylene adipate terephthalate (PBAT), and starch-based biodegradable microplastics in sandy loam and loamy sand soils under controlled rainfall intensities (22 mm/h and 35 mm/h). Effluent and soil analyses, coupled with microplastic balance assessments, revealed recovery rates between 64% and 104%, underscoring the reliability of the experimental approach. Transport varied with soil type, rainfall intensity, and polymer characteristics, with loamy sand exhibiting higher wash-off rates. LDPE consistently showed greater mobility than biodegradable polymers, particularly under higher rainfall intensities. Field studies complemented these findings, using loamy sand soil columns subjected to natural precipitation and fluctuating groundwater levels over 6- and 12-month periods. Retention profiles and particle size analyses highlighted distinct behaviors: LDPE persisted across soil depths, PBAT exhibited moderate redistribution due to partial biodegradation and starch-based microplastics underwent significant fragmentation and deeper transport. The field's natural precipitation and wet-dry cycles enhanced microplastic mobilization and degradation compared to laboratory conditions. HYDRUS-1D modeling was employed across both settings. Laboratory simulations showed depth-dependent deposition, particularly in upper soil layers, while field models reflected material-specific degradation and redistribution. Notably, LDPE exhibited stable retention parameters, whereas biodegradable polymers demonstrated declining attachment and detachment coefficients over time, indicating their biodegradability. These findings underscore the critical roles of soil type, rainfall intensity, polymer properties, and environmental conditions in shaping microplastic behavior in soils. Integrating controlled laboratory experiments and long-term field studies provides a comprehensive understanding of microplastic fate, offering essential insights for modeling and mitigating their impact on terrestrial ecosystems.
Keywords: microplastic transport, soil contamination, soil column experiment, HYDRUS-1D