Numerical study of variable-density flow and transport in unsaturated-saturated porous media

Dense contaminant or solute transport in the unsaturated-saturated aquifer commonly exists in the natural environment. However, the influence of hydraulic properties of the unsaturated soil on penetration process of variable density flow in an unsaturated-saturated aquifer system is rarely investigated. In this study, a variable density, variably saturated flow and transport model based on FEFLOW is used to systematically investigate the penetration process of the solute through the unsaturated-saturated aquifer system, with a focus on the influence of the hydraulic parameters of the unsaturated soil on the penetration process. Vertical center of mass, and its speed and acceleration, are used to identify different stages of the penetration process through the unsaturated-saturated aquifer. The hydraulic parameters in the van Genuchten-Mualem model (VGM) affect the water content distribution in the unsaturated zone, subsequently the penetration time through the unsaturated zone. Through influencing the thickness of the transition zone (0.1-0.9 maximum concentration of the plume) when the plume reaches the water table, the hydraulic parameters affect the accumulation time of the solute at the boundary layer in the saturated zone. Thus, the process in not only the unsaturated but also the saturated zones is influenced by the change of the hydraulic properties of the unsaturated zone. Furthermore, the results show that the instabilities start to occur at the zone with high water content (90% of maximum saturation) in the capillary fringe. Freshwater intrudes into the zone with high water content when fingers are completely developed at the entire system. The length of freshwater intrusion above the water table has a negative correlation with the fitting parameter alpha in the VGM model. The influence of the density difference of the solute on the penetration processes is insignificant in the unsaturated zone but significant in the saturated zone. Sensitive analysis for the hydraulic properties of the unsaturated soil on the penetration process shows that pore size distribution index is the main factor influencing the penetration time through the unsaturated zone, the penetration time through the entire aquifer, the accumulation time at the boundary layer, and the development time of the fingers. Two other parameters, fitting parameter alpha and residual saturation, have minor effects.