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groundwater was'estimated to be 46 feet below grade,with an annual vertical fluctuation of 7 to 10 feet. The information that was obtained during the drilling investigation was used as input data-- to ata`to the fate and transportmodeling program known as VLEACH. The following sections describe the model and'its.use in,assessing the S T&ERailroad site. fill 3.0 DESCRIPTION OF TSE MODEL The computer modeling program known as Vadose Zone Leaching Modeling (VLEACH) was initially developed in 1990 by CHZM.Hill for the U.S. Environmental Protection' Agency, Region iX. VLEACH is a one-demensional finite difference vadose zone model for estimating the impact due to the mobilization and migration of a sorbed organic contaminant located in the vadose zone on the underlying groundwater resource. Since its' development, it has;been utilized at numerous sites to assess the'potential groundwater. impacts from existing soil contaminants anal in soil`vapor projects 11 Model Conceptualization,Assumptions, and Limitations When.developing amodel simulation, it is important to fully understand the implication of the VLEACH conceptualization. VLEACH describes the movement of an organic contaminant with and,between three different phases: (1) as;a solute dissolved in water, (2)-as a gas in the vapor phase, and (3) as an adsorbed compound in the solid phase. Equilibration between the phases occurs according to distribution coefficients defined by the user. In particular,.VLEACH simulates vertical transport by advection in the liquid phase acid by gaseous diffusion in the vapor phase. These processes are conceptualized as occurring in a number of distinct, user-defined polygons whose'size and shape can be set by the modeler. The polygons are also vertically divided into a'j,series of user-defined cells. Figure 3-A&B illustrates how the site conditions are conceptualized into one or more polygons.. initially, VLEACH calculates the equilibrium distribution of contaminant mass-between the liquid, gas, and sorbed phases. Transport processes are then simulated. Liquid advective transport is calculated based on-values defined b.y the user for infiltration and. soil water content. The contaminant in'the vapor phase migrates into or out of adjacent . cells based on'the calculated concentration gradients that exist between adjacent cells'. After the mass is.exchanged between the cells, the total mass in each cell is recalculated and re-equilibrated between the different phases.- These steps are conducted for each time step, and each polygon is simulated independently. At the end of the model simulation, the results from each polygon are compiled to determine an overall area-weighted groundwater impact for the entire modeled area. 2