Transport and retention of polypropylene microplastics through vadose zones: Experimental observation and numerical...

Accurate prediction of microplastics (MPs) transport in soil-groundwater systems is crucial for effective pollution control and environmental risk assessment. While substantial progress has been made in understanding MPs transport in saturated porous media, experimental observations and mechanistic insights into MPs migration under unsaturated conditions remain limited. This study conducts column experiments to investigate the transport and retention of polypropylene MPs under different flow conditions. A modified model is developed that incorporates a water content-dependent adsorption coefficient to capture the influence of hydraulic conductivity on available adsorption sites. Results show that the advection-dispersion equation remains valid for describing MPs transport through the vadose zone: The errors are 5.6% and 3.6% for saturated and unsaturated flow, respectively. Compared with the conventional model, the proposed correction reduces the error by 2.0% under unsaturated flow conditions. At site-scale, the modified model predicts faster breakthrough and lower retention in the vadose zone. This suggests that conventional models may overestimate MPs retention and associated environmental risks. It further shows that nearly 50% of MPs are retained in the solid phase of the saturated zone at late times, approximately 15% higher than the estimate from the conventional model. The proposed water content-dependent adsorption coefficient captures the coupling between hydraulic conductivity and MPs adsorption, enabling accurate predictions of MPs transport and retention in the vadose zone. The model provides a reliable tool for risk assessment and the development of effective mitigation strategies.