Experimental and numerical assessment of Light Non-Aqueous Phase Liquid (LNAPL) subsurface migration behavior in the vicinity of groundwater table

Groundwater contamination by constituents of hydrocarbon fluids may occur from the earth's surface or subsurface. Key sources of contamination include spills and leakages from oilwells, underground storage tanks (USTs), and oil transport-tankers. These light non-aqueous phase liquid (LNAPL) contaminants pollute the soil and its continuous migration, may even end up contaminating aquifer systems. This highlights the need to understand the fate and migration behavior of LNAPL contaminants in the subsurface, especially in the unsaturated zone – with a view to limiting the contaminants transport. This study presents a numerical approach for describing and predicting the fate of LNAPL contaminant-transport in the subsurface. Multiphase flow concept was adopted, where two LNAPL phases – oil, and gas – were mainly considered. Diesel and crude oil are the hydrocarbon contaminants used, while unconsolidated sand was used as the porous matrix. Two contaminant flow scenarios were simulated experimentally — surface and subsurface imbibition. Mass balance equations and constitutive functions from the several important phenomena that influence LNAPL contaminant flow, in the subsurface, are discussed. Extended Darcy's law in combination with van Genuchten model were applied in the 2D numerical model description for mass balance equation and constitutive relationship, respectively. The numerical simulation was executed using COMSOL Multiphysics® v. 5.5. The numerical simulation results showed a good correlation with the experimental results, and suggest that exposure time, fluid viscosity, density, contaminant supply, amount released, and the hydraulic properties of the porous matrix, are the most important parameters in LNAPL contaminant subsurface migration. This study thereby concludes that if the fluid thermodynamic properties and the hydraulic properties of the porous matrix are known, the numerical model can accurately predict the migration behavior of hydrocarbon contaminants. In that way, it becomes possible to effectively track oil-spills and leakages propagating into the vicinity of groundwater table and freshwater aquifer. The study outcome is useful in designing and implementing efficient hydrocarbon contamination remediation approaches and saving time by taking the right action.