Insights into LNAPL saturation distribution in capillary zone based on light transmission and mechanical analysis

Light non-aqueous phase liquids (LNAPLs) are common industrial contaminants, posing significant environmental risks. Understanding the distribution of LNAPL in the vadose zone is crucial for developing effective remediation strategies. This study combined capillary modeling with sandbox experiments across dry sand to capillary zones to analyze the spatial and temporal distribution of LNAPL using modified light transmission techniques. The research examined the effects of particle size, viscosity, and the slope of phreatic surface on the spatiotemporal distribution behavior of LNAPL. Key findings reveal that capillary pressure transitions from gas-LNAPL driving to LNAPL-water resistance, which significantly influences the vertical infiltration of LNAPL in dry sand and horizontal migration in the capillary zone. This transition leads to the formation of a "levitational" lens above the groundwater table. Moreover, finer particles elevate LNAPL-water capillary resistance, forming a new "double shark-fin" vertical saturation profile. Higher viscosity narrows "shark-fin" profiles by impeding vertical migration. Enhanced hydraulic gradients expand distribution vertically/horizontally, elevating saturation peaks by 50%. Notably, optical transmission imaging detects sub-residual LNAPL in "shark-fin" saturation overlooked by conventional models. Underestimation of this critical zone directly compromises contamination severity assessment. Our study corrects residual saturation benchmarks for accurate risk management, informing more effective LNAPL remediation strategies.