Construction and mechanism interpretation of capillary model based on LNAPL migration kinetics

Quantifying the kinetic mechanisms of LNAPL migration in the unsaturated zone is critical for precision characterization and control of contamination. To elucidate its complex spatiotemporal distribution, a capillary model was developed by coupling macro-scale fluid migration patterns with micro-scale distribution morphologies. Based on observed distinct zonal and phased migration characteristics of LNAPL in the unsaturated zone, four distinct stratigraphic zones were delineated via the light reflection method. Key kinetic parameters, including relative permeability-saturation-capillary pressure (K-S-P) relationships, relative wetting contact angles (θ) and viscous frictional resistance (F_f), were quantified to characterize fluid-solid interfacial interactions. Results indicated that increasing water saturation (S_w) significantly enhanced θ and F_f, thereby inhibiting vertical LNAPL infiltration while promoting lateral expansion due to the capillary retention effect. Based on these insights, a capillary model was constructed by integrating media properties and fluid parameters into explicit mathematical expressions to interpret the kinetic mechanisms of migration patterns, pathways, morphologies, and forces of LNAPL-water displacement process. In the capillary model, LNAPL and water establish separate channels when simultaneously flowing through a medium, where saturation clearly indicates the migration patterns while θ and F_f serve as the boundary conditions determining LNAPL mobility. The derived expressions for gravity, capillary force, and viscous frictional resistance provide a theoretical foundation for the accurate prediction of LNAPL contamination plumes in complex subsurface environments.