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VLEACH employs a number of major, often tenuous as but it can be useful in making preliminary assessments of the potential impacts of contaminants within the vadose zone. The most important assumptions are listed below: • Linear isotherms describe the partitioning of the pollutant between the liquid, vapor and-soil phases. Locator instantaneous equilibrium between these phases is assumed within each cell. • The vadose zone is in a steady state condition with respect to water movement. More specifically, the moisture content profile within the vadose zone is constant,which in reality rarely occurs. Although moisture gradients cannot be simulated,the user can estimate the impact of various moisture contents by comparing results from several simulations that cover the common or possible ranges in soil moisture conditions. • Liquid phase dispersion is neglected. Hence, the migration of the contaminant will be simulated as a plug(within each polygon). This assumption causes higher dissolved concentrations and lower travel time predictions than would occur in reality. • The contaminant is not subjected to in situ production or degradation. Since organic contaminants, especially hydrocarbons, generally undergo some degree of degradation in the vadose zone, this assumption results in conservative concentration values. • Homogeneous soil conditions are assumed to occur within a particular polygon. This assumption will rarely hold true in actual field applications. Although spatial gradients cannot be simulated, the user can estimate the impact of non-uniform soils by comparing results from several simulations covering the range of soil properties present at the site. However, initial contaminant concentrations in the soil phase can vary between cells. • Volatilization from the soil boundaries is either completely unimpeded or completely restricted. This assumption may be significant depending upon the depth of investigation and the soil type. In particular, after a depth of one meter volatilization to the atmosphere will decrease significantly. • The model does not account for non-aqueous phase liquids or any flow conditions derived from variable density. 3.2 Input Parameters Data used in the model are of two types: l) site-specific physical properties of the environment, and 2) simulation data that apply generally and are not specific to an individual site. The site-specific data are defined for each polygon, allowing for variation 3