Microplastics transport in subsurface environments: Mechanisms and multi-scale modeling

Microplastics (MPs) are persistent contaminants in soils, the vadose zone, and groundwater, where they may act both as pollutants and vectors for co-contaminants. Although their sources and occurrence in subsurface environments have been increasing documented, the translation of this knowledge into mechanistic and predictive transport models remains limited. Unlike occurrence-oriented reviews, this review adopts a model-oriented perspective and examines how MP source characteristics, particle properties, and transport mechanisms can be represented across pore, column, vadose-zone, and field scales. We synthesize recent advances in three interconnected areas: (a) source and occurrence patterns relevant to model boundary conditions; (b) physical, geochemical, and biological processes controlling MP transport, retention, and remobilization; and (c) numerical modeling approaches, including continuum transport models, variably saturated vadose-zone models, and pore-scale simulations. We show that MPs cannot be treated simply as dissolved solutes or conventional colloids because their broad size distribution, irregular morphology, density contrast, deformability, and aging-dependent surface properties produce non-Fickian transport, size-dependent straining, interfacial trapping, and dynamic attachment-detachment behavior. Recent models increasingly extend advection-dispersion-reaction frameworks by incorporating particle-specific retention, dual-domain exchange, air-water interfacial processes, and multi-scale parameterization, while lattice-Boltzmann, computational-fluid-dynamics, and discrete-element simulations provide mechanistic constraints on particle-pore interactions. However, major gaps remain in representing unsaturated and preferential-flow conditions, bio-mediated transport, field-scale parameterization, and the upscaling of micro-CT and pore-scale observations. We therefore propose a focused multi-scale modeling framework that links source terms, process-based model closures, targeted experiments, and data-driven parameterization to improve prediction and risk assessment of MP contamination in subsurface environments.