Assessment of p-Xylene Migration Behaviour in Unsaturated Soil Systems: A 3-D Sand Flow Chamber Study

The influence of soil water content and contaminant injection pressure on the migration behaviour of p-Xylene (one of the organic pollutants in Benzene Toluene Ethylbenzene & Xylene) is investigated in the unsaturated zone using the controlled laboratory sandbox experiments. The results show that, at equivalent migration distances, higher soil water content leads to longer migration durations and reduces transport velocities, whereas greater injection pressure promotes faster contaminant movement. These findings highlight the governing role of water saturation in the water–oil–gas three-phase system, in which increased water content decreases the relative permeability of the oil phase and retards capillary-driven BTEX transport. Unlike many previous studies in which contaminants commonly migrate downward toward the groundwater table, the BTEX in this study did not reach the water table and exhibited partial upward migration. Overall, the results provide experimental-scale insight into key mechanisms governing BTEX transport in unsaturated sandy media and serve as a reference for improving predictions of contaminant fate and migration pathways in field settings.