Effect and mechanism of the moisture content on the kinetic retardation of LNAPL pollutant migration by the capillary zone

Light nonaqueous-phase liquids (LNAPLs) are the main source of organic pollution in soil and groundwater environments. The capillary zone, with varying moisture contents, is the last barrier against the infiltration of LNAPL pollutants into groundwater and plays an important role in their migration and transformation. However, the effect and mechanism of the moisture content in the capillary zone on LNAPL pollutant migration are still unclear. Herein, to explore the effect of the moisture content on LNAPL pollutant migration, a series of sandbox migration experiments were simulated using diesel oil as a typical LNAPL pollutant and the capillary zones of fine and silty sand as research objects. Several numerical models were constructed based on the recorded migration process of LNAPL pollution fronts in the capillary zones of different media during experiments. The mechanism by which the moisture content in the capillary zone retarded LNAPL pollutant infiltration was revealed using a combination of the construction method of microstructural pores and the theory of multiphase flow. These findings unequivocally indicate that an increase in the moisture content leads to a decrease in the relative permeability of the NAPL phase in unsaturated media, which is the fundamental reason for the retarded kinetic migration of LNAPL pollutants. The results of this study lay a solid foundation for designing comprehensive and effective remediation strategies for LNAPL contamination in soil and groundwater.