Laboratory and model simulations of a LNAPL spill in a variably-saturated sand, 2. Comparison of laboratory and model results

A two-dimensional, multiphase flow experiment was modelled using a finite-difference, multiphase flow and transport code. The laboratory experiment consisted of a lighter-than-water, non-aqueous-phase liquid (LNAPL) spill in a variably-saturated sand. The numerical model allows a non-hysteretic, a partially hysteretic and a fully hysteretic solution. The fully hysteretic algorithm accounts for fluid entrapment, saturation hysteresis and hysteresis in the relative permeability terms. The partially hysteretic model allows air-phase entrapment within the LNAPL as the LNAPL migrates through the unsaturated zone. The fully hysteretic model results were compared to the laboratory pressures and saturations. The importance of hysteresis is clearly illustrated; however, the inclusion of hysteresis substantially increases the computation and storage requirements. The partially hysteretic model predicts the movement of the LNAPL front relatively well but does not account for the hysteretic conditions which persist during the redistribution of the LNAPL after the spill was complete. The laboratory experiment was modelled using an implicit in pressure, explicit in saturation (IMPES) solution and a fully implicit solution. The fully hysteretic IMPES solution with two-point upstream weighting of the relative permeability terms resulted in the best representation of the experimental data for the model scenarios evaluated.