A mathematical model was developed and implemented for evaluating fate of a hazardous substance in the unsaturated zone of soil. The model simulates vadose processes of volatilization, degradation, sorption/desorption, advection, and dispersion. Four vadose zone physical phases are considered including water, oil, soil grains, and soil-pore air. The Vadose Zone Interactive Processes (VIP) model is appropriate for use at RCRA and CERCLA sites since site-specific processes affecting transport of hazardous chemicals through the vadose zone are incorporated in the model. A RCRA land treatment system was chosen as the basis of model development. Laboratory experiments were conducted to observe transport and apparent degradation of three polynuclear aromatic hydrocarbon (PAH) compounds (anthracene, fluoranthene, and naphthalene) in sandy loam soil in separate soil columns under two flow conditions. Vadose zone concentrations of PAH compounds were evaluated as function of time (90 days) and depth (1.5 m).