VLEACH, A One-Dimensional Finite Difference Vadose Zone Leaching Model, is a computer code 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. The model was initially developed by CH2M Hill for the U.S. Environmental Protection Agency, Region IX in 1990. In particular, the model was designed specifically on the Phoenix-Goodyear Airport Superfund site where it was used successfully to evaluate groudwater impacts and volatilization of volatile organic contaminants (Rosenbloom et al., 1993). Since that time the code has been utilized at numerous sites to assess the potential groundwater impacts from existing soil contaminants and in soil vapor projects.
VLEACH describes the movement of organic contaminant within and between three different phases: (1) as a solute dissolved in water, (2) as a gas in the vapor phase, and (3) as an absorbed 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 and by gaseous diffusion in the vapor phase.
These processes are conceptualized as occurring in a number of distinct, user-defined polygons that are vertically divided into a series of user-defined cells. The polygons may differ in soil properties, recharge rate, and depth to water. However, within each polygon homogeneous conditions are assumed except for contaminant concentration, which can very between layered cells. During each time step the migration of the contaminant within and between cells is calculated. Hence, VLEACH can account for heterogeneities laterally but is limited when simulating vertical heterogeneity.
Initially, VLEACH calculates the equilibrium distribution of contaminat mass between the liquid, gas, and sorbed phases. Transport processes are then simulated. Liquid advective transport is calculated based on values defined by 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 areaweighted groundwater impact for the entire modeled area.
For computational purpose each polygon is divided vertically into a series of cells. When developing a model simulation, it is important to fully understand the implications of the VLEACH conceptualization. The following assumptions are made in the development of VLEACH.
1. Linear isotherms describe the partitioning of the pollutant between the liquid, vapor and soil phases. Local or instantaneous equilibrium between these phases is assumed within each cell.
2. The vados zone is in a stady state condition with respect to water movement. More specifically, the moisture content profile within the vados zone is constant. This assumption will rarely occur in the field. Although moisture gradients cannot be simulated, the user can estimate the impact various moisture contents by comparing results from several simulations that cover the common or possible ranges in soil moisture conditions.