Centrifuge modeling of LNAPL movement in the vadose zone

The aim of this research is to investigate the transport process of light non-aqueous phase liquids (LNAPLs) in the subsurface environment. The approach to this objective involves the combination of a scaled physical model and a mathematical model. The mathematical model was verified by physical scale modeling using a geotechnical centrifuge. The comparison of physical and mathematical models cross-validates different approaches, and consequently provides more understanding of uncertainty in the prediction for LNAPL transport in the subsurface environment.; A total of twelve centrifuge tests were performed using a 1-m radius balanced arm centrifuge at the University of California at Davis. The model tests simulated two-dimensional LNAPL migration in the vadose zone of a uniform sand deposit. The groundwater conditions, LNAPL spill rate, and total spill volume were taken as variables to be considered. A pore fluid pressure measurement system was developed and changes of the pore water pressure and LNAPL pressure with time were monitored during each test. Migration of LNAPL was also monitored through CCD camera, and captured images were used to estimate size and location of the LNAPL contaminated area. The dependency of the LNAPL migration on the boundary conditions was identified from the centrifuge test observations.; The centrifuge test results were compared with a finite difference model, STOMP, developed at Pacific Northwest National Laboratory. The lens-shaped plumes from the centrifuge tests showed more distinct boundaries than those for the numerical models. It is considered that the S-P scaling technique used in STOMP causes overestimation of the oil movement. Overall, good agreement was observed between the centrifuge test results and the numerical simulation results for general LNAPL migration patterns under various initial and boundary conditions. It confirmed that both the centrifuge model and STOMP characterized fundamental immiscible multiphase flow. The degree of agreement of between the experimental data and the simulations was reasonable, and the comparison served to illustrate the usefulness of the centrifuge modeling as a physical model to provide data for long-term LNAPL migration for the verification of numerical model.