Colloid transport in soil: mechanisms and modeling

Soil contamination by emerging pollutants such as microplastics and pathogenic bacteria presents growing environmental and public health concerns, particularly regarding groundwater quality. Understanding the mechanisms governing their transport in soil is critical for assessing contamination risks and informing mitigation strategies. This thesis investigates the transport behavior of fecal bacteria and microplastics through an integrated approach combining laboratory experiments, field studies, and numerical modeling, providing a comprehensive assessment of their fate in controlled and natural environments. The introductory chapter outlines the dual role of soil as both a sink and a pathway for contaminants, with emphasis on the physical, chemical, and biological processes controlling particle movement in porous media. Pathogenic bacteria from wastewater, manure, and sewage leakage and microplastics from agricultural and urban sources interact with soil properties and hydrodynamics, influencing their retention and mobility. Chapter 2 examines the remobilization of fecal indicator bacteria (Escherichia coli and Enterococcus moraviensis) in unsaturated porous media under transient flow. Column experiments revealed that E. coli exhibits greater mobility than E. moraviensis, especially in sandy soils. Modeling results highlight the influence of attachment-detachment dynamics and decay processes in shaping bacterial transport under fluctuating water contents. Chapter 3 investigates microplastic transport in soil columns under controlled rainfall intensities, focusing on polymer type (LDPE, PBAT, starch-based), particle size, soil texture, and rainfall rate. Higher wash-off occurred in loamy sand compared to sandy loam, and smaller particles were more mobile. LDPE showed greater washout than biodegradable polymers, likely due to its lower density. High recovery rates (64–104%) confirmed the robustness of the experimental methodology. These results informed the calibration of the HYDRUS-1D model in Chapter 4. In Chapter 4, a non-equilibrium transport model was applied to simulate microplastic movement in unsaturated soils. Model calibration using laboratory data highlighted the influence of soil hydraulic properties and moisture fluctuations. While breakthrough curves were accurately reproduced, retention at depths beyond 10 cm was underestimated, indicating the need for further refinement. Chapter 5 extends the investigation to long-term field experiments, assessing microplastic retention under natural precipitation and fluctuating groundwater levels over 6–12 months. Biodegradable microplastics showed signs of fragmentation and deeper redistribution due to degradation processes. HYDRUS-1D simulations captured general retention patterns but revealed notable differences between laboratory and field outcomes, underscoring the importance of field calibration. The synthesis chapter integrates insights from all approaches, emphasizing the complex interplay of contaminant characteristics, soil properties, and hydrodynamics in governing transport. Laboratory studies enable controlled parameterization, modeling provides mechanistic interpretation, and field studies capture long-term variability. Together, they demonstrate that while predictive modeling is valuable, its reliability depends on robust, long-term field validation.