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

Area of Science:

• Environmental Science
• Geosciences
• Hydrogeology

Background:

• Heterogeneous porous media with sand-clay interfaces are common in vadose zones.
• Understanding Dense Non-Aqueous Phase Liquid (DNAPL) transport across these interfaces is crucial but poorly understood.

Purpose of the Study:

• To investigate DNAPL spontaneous infiltration and transport mechanisms across sand-clay interfaces.
• To analyze the influence of interfacial direction and lens type on DNAPL distribution.
• To quantify the role of capillary barrier effects in DNAPL redistribution.

Main Methods:

• Utilized a stratified nuclear magnetic resonance (NMR) methodology with spin-echo single-point imaging (SE-SPI).
• Enabled spatially-resolved T2 spectral analysis for real-time, non-destructive DNAPL monitoring.
• Simulated distinct interfacial scenarios in variably configured sand-clay media.

Main Results:

• DNAPL transport depth and distribution depend on interfacial crossing direction and lens type.
• Migration from clay to sand causes accumulation above the interface; reverse migration leads to retention below.
• Clay lenses exhibit a stronger capillary barrier effect than sand lenses, impeding vertical migration.

Conclusions:

• Interfacial capillary barrier effects critically govern DNAPL redistribution patterns.
• Proposed new metrics (retention rate, capillary retention efficiency) quantify these effects.
• Provides insights for predicting DNAPL fate and optimizing remediation in heterogeneous environments.