Effect of entrapped water on NAPL migration in unsaturated fractures.

The migration of non-aqueous phase liquids (NAPL) in unsaturated fractures involves NAPL-water-air three-phase displacement. Understanding the impact of entrapped water on this migration process is crucial for predicting NAPL flow and solute transport in the fractured vadose zone. This study presents visualized flow experiments to systematically investigate NAPL (n-octane) invasion behaviors in initially dry and water-entrapped unsaturated single fractures by varying fracture inclination angle and injection flow rate. The experimental results reveal that entrapped water fundamentally shifts NAPL flow patterns from fingering flow in dry fractures to film flow propagating along entrapped water droplets in water-entrapped fractures. Compared to fingering flow in dry fractures, film flow in water-entrapped fractures exhibits enhanced lateral expansion, longer fracture penetration time, higher breakthrough saturation, larger perimeter-area ratio, and stronger ability to invade large-aperture channels. The morphology of the film flow is jointly regulated by injection flow rate and fracture inclination angle. Lower flow rates and larger inclination angles result in thinner films, while sufficiently high flow rates at small inclination angles can trigger a transition from film flow to fingering flow in the main flow channels. We also find that the distribution of NAPL-occupied apertures consistently follows a normal distribution, independent of flow rate and fracture inclination angle, and aligns with the statistical properties of the fracture aperture field. These findings elucidate the regulatory role of entrapped water on NAPL migration and advance the mechanistic understanding of NAPL invasion in unsaturated single fractures, thereby informing groundwater contamination risk assessment and remediation design.