Experimental study on non-aqueous phase liquid multiphase flow characteristics and controlling factors in heterogeneous porous media

Enhanced understanding of non-aqueous phase liquid (NAPL) infiltration into porous media is important for the effective design of remediation strategies. Six model multi-flow experiments in two-dimensional polymethyl methacrylate tank were carried out to verify the influences of soil initial water content, local low permeability lens, lithologic sharp interface and leakage position on light non-aqueous phase liquid (LNAPL) and dense on non-aqueous phase liquid (DNAPL) transport velocities in soil, with diesel and perchlorethylene as LNAPL and DNAPL, respectively. The evolutions of the plume were recorded through the transparent side of the tank by CCD camera and the contaminant fronts were traced using gel pen on a plastic film coated on tank wall at appropriate intervals. The controlling factors of free NAPL migration paths and speeds were analyzed thoroughly, and capillary pressure equilibrium models of NAPL-water/NAPL-air interface and the partition movement theory were put forward. Results show that the effect of NAPL types: soil initial water content, local low permeability lens, leakage position and lithologic sharp interface on NAPL migration paths and velocities are diverse. The NAPL movement principles should be presented in accordance with partitions as related to the initial water content namely dry soil area, capillary zone and water-saturated zone. The vertical movement and lateral extension of NAPL are interdependent, and the vertical migration is dominant in initial leakage stage. The vertical migration is hindered by lateral growth when meeting with local low permeability lens, lithologic sharp interface or capillary fringe. Vertical migration rate reduction is mainly caused by the pore resistance, NAPL residual and lateral spreading and the causes of corresponding increase of horizontal expansion speed are the decrease of vertical velocity and the increase of buoyancy. The sufficiently short time allows to neglect chemical reactions, dissolution and sorption of NAPL and heterogeneous porous media.