"Ground-water contamination due to surface spills or subsurface leakage of hydrocarbon fuels, organic solvents and other immiscible organic liquids is a widespread problem which poses a serious threat to ground-water resources. In order to model the movement of such materials in the subsurface, it is necessary, in general, to consider flow of water, nonaqueous phase liquid (NAPL) and air, and transport of individual chemical components, which may move by convection and dispersion in each phase. A mathematical model was developed for multiphase flow and multicomponent transport in porous media with water, NAPL and air or any subset of these phases. Numerical procedures for solving the system of coupled flow equations, based on various formulations of the governing equations, were compared. Accurate representation of three-phase permeability-saturation-capillary pressure (k-S-P) relations is crucial to model multiphase fluid movement and accurate models for interphase mass partitioning are critical to describe species transport. A detailed physically-based model for hysteresis in three-phase k-S-P relations was described. Simplified models, which consider effects of nonwetting fluid entrapment, were shown to provide a reasonable compromise between accuracy, on the one hand, and efficiency and robustness, on the other. Laboratory studies of light and dense NAPLs in a 1 x 1.5 meter sand tank, involving measurements of water and NAPL pressures and saturations and component concentrations, are described. These studies were used to validate the mathematical model for multiphase flow and transport."