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 fin ite-difference, multiphase flow and transport code. The laboratory exp eriment consisted of a lighter-than-water, non-aqueous-phase liquid (L NAPL) spill in a variably-saturated sand. The numerical model allows a non-hysteretic, a partially hysteretic and a fully hysteretic solutio n. The fully hysteretic algorithm accounts for fluid entrapment, satur ation hysteresis and hysteresis in the relative permeability terms. Th e partially hysteretic model allows air-phase entrapment within the LN APL as the LNAPL migrates through the unsaturated zone. The fully hyst eretic model results were compared to the laboratory pressures and sat urations. 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 t he hysteretic conditions which persist during the redistribution of th e LNAPL after the spill was complete. The laboratory experiment was mo delled using an implicit in pressure, explicit in saturation (IMPES) s olution and a fully implicit solution. The fully hysteretic IMPES solu tion with two-point upstream weighting of the relative permeability te rms resulted in the best representation of the experimental data for t he model scenarios evaluated.