Interfacial capillary barriers and direction-dependent DNAPL migration in vadose zone

Heterogeneous porous media, characterized by contrasting interfaces such as those between sand and clay, are ubiquitous in vadose zones. However, the transport mechanisms of Dense Non-Aqueous Phase Liquids (DNAPLs) across these interfaces remain poorly understood. This study uses an innovative stratified nuclear magnetic resonance (NMR) methodology which enables spatially-resolved T₂ spectral analysis via a spin-echo single-point imaging (SE-SPI) sequence to investigate the spontaneous infiltration behavior and underlying mechanisms of DNAPL across variably configured sand-clay interfaces. Through experiments simulating distinct interfacial scenarios, real-time, non-destructive monitoring of DNAPL spatial distribution was achieved. The results show that DNAPL transport depth and vertical distribution are critically governed by both the direction of interfacial crossing and the lens type within stratified media. Specifically, when migrating from clay to sand, DNAPL accumulates above the interface, while the reverse direction leads to predominant retention below. Notably, clay lenses exert a more pronounced capillary barrier effect than sand lenses, significantly reducing vertical migration. The study quantitatively establishes that interfacial capillary barrier effects, represented by newly proposed metrics such as retention rate and capillary retention efficiency, critically determine the redistribution patterns of DNAPL. This study clarifies the governing role of interfacial capillary barriers and provides quantitative metrics necessary to predict DNAPL fate and optimize mitigation efforts in heterogeneous subsurface environments.