Numerical investigation on light non–aqueous phase liquid flow in the vadose zone considering porosity effects on soil hydraulic properties

In the management of light non–aqueous phase liquid (LNAPL)-contaminated ground, numerical simulation is widely used to analyze LNAPL flow in the unsaturated soil (vadose) zone. Porosity effects on the hydraulic properties of unsaturated soils are highly simplified in existing mathematical models. Some important features, such as the nonlinear relation between porosity and permeability/displacement pressure, cannot be well captured. To address this problem, a new mathematical model was developed in this study, considering porosity effects on hydraulic properties of soils, including the retention behavior and permeability function of LNAPL and water. The newly developed model was implemented in MATLAB using the finite difference method and then verified by the results of a centrifuge test. Then, parametric studies were conducted to investigate the flow of LNAPL upon an active leakage at the ground surface. Based on the computed results, the influence of several factors, such as porosity magnitude, porosity distribution, and soil layering, was revealed. In particular, an increase in the porosity leads to a significant increase in the volume of LNAPL leaked into the ground, the vertical front depth, and the area of contaminated ground. This is mainly because the porosity affects not only the intrinsic permeability but also the relative permeability because (a) the intrinsic permeability of soils is larger at a higher porosity (b) when the porosity is higher, the equilibrium water saturation at a given capillary pressure is smaller. Consequently, LNAPL can achieve a larger degree of saturation and higher relative permeability.