A multiphase unsaturated zone contaminant transport (MUCT) model has been developed to simulate the simultaneous vertical flow of water and a second immiscible fluid in unsaturated porous media. The model couples multiphase flow equations with a multiphase solute transport equation. The multiphase flow equations are solved semi-analytically by applying finite difference computations. Multiphase solute transport is governed by a simplified three-phase flow advection-dispersion equation. The solute transport equation is solved analytically under the assumptions of a steady flow field and local partition equilibria between phases. The solute transport is assumed to be strictly one dimensional; only longitudinal advection and dispersion are included. The MUCT model is evaluated by applying generic examples, mass balance, and sensitivity analyses to demonstrate the potential application of the model. Results of the generic simulations could be applied in ground water modeling to characterize the hydrocarbon source loading in terms of the release rate and intensity of the pollutant. The OILENS model (Charbeneau and Shultz, 1988) is selected and combined with the MUCT model to study the impact of a hydrocarbon release event on ground water quality. The combination of MUCT and OILENS also defines the interfacial boundary conditions with the existence of a light immiscible hydrocarbon between the unsaturated zone and the saturated zone. The Horizontal Plane Source model (Galya, 1987) is selected and merged into the MUCT model to extend the capability of the MUCT model to study the impact of ground water contamination in the saturated zone. In order to better understand the ground water contamination problem of the existence of a less dense non-aqueous phase liquid (LNAPL), a field study (Chang et al., 1990) was conducted at an aviation gasoline spill site in Traverse City, Michigan. Results of this investigation verified that the infiltration water carried contaminants from the residual oil into the ground water beneath the capillary fringe zone. Soil gas analyses showed that concentrations of the organic compounds in the unsaturated zone increased with depth and the time after the commencement of infiltration. Strong evidence indicated that biodegradation was occurring beneath the residual oil zone
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