A mathematical model (VIP) was developed and implemented for evaluating the fate of a hazardous substance in the unsaturated zone of the soil. The model simulates vadose zone processes including volatilization, degradation, adsorption/desorption, advection, and dispersion. Four physical phases in the vadose zone are considered including water, oil, soil grains, and soil-pore air (unsaturated pore space). The Vadose Zone Interactive Processes (VIP) model is appropriate for sites under RCRA and CERCLA (Superfund) categorization since site-specific soil-waste 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 the model development. The model is implemented on IBM-PC, -XT, and -AT compatible equipment and has an interface to accept input files from LOTUS spreadsheets.

Laboratory experiments were conducted to observe the 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 the PAH compounds were evaluated as function of time (90 days) and depth (1.5 m). Results of the experiments were compared with the model predictions.

The VIP model closely simulated the fate and behavior of the three compounds through depth and through time in the vadose zone conditions in the soil columns. It was possible to use literature values for partition coefficients, apparent degradation rates, and interpore water velocity in the model for all three compounds.