Three-phase immiscible fluid movement in the vicinity of textural interfaces

The movement of light nonaqueous-phase liquids (LNAPLs) in heterogeneous vadose environments is not fully understood. We investigated LNAPL movement near sloping fine-over-coarse-grained textural interfaces, forming capillary barriers. Flow experiments were conducted in a glass chamber (50×60×1.0 cm) using two uniform silica sands (12/20 and 30/40 sieve sizes). LNAPL (Soltrol® 220) was released at the sand surface during constant water irrigation and visible light transmission was used to observe LNAPL and water flow paths. LNAPL movement strongly depended on the water saturation in the fine-grained sand layer above the textural interface. At low water saturation, LNAPL was fully diverted along the interface, whereas at moderate water saturation partial penetration of the LNAPL into the coarse-grained sand layer was observed. At high water saturation, LNAPL was fully diverted parallel to the interface, supported on the zone of high water saturation. Additional hysteretic effects on LNAPL movement were observed at high water saturation. The results demonstrate the critical role water saturation plays in determining LNAPL disposition in a heterogeneous vadose zone.