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08/06/96 09:47 0510 372 5220 IT CORP MARTINEZ t 2. MODEL CONCEPTUALIZATION,ASSUMPTIONS, AND LIMITATIONS VLEACH describes the movement of an organic conta1 unant within and between three } different phases_ (1) as a solute dissolved in water, (2) as a gas in the�ya�r phase, and (3) as an ' adsorbed compound in the solid phase (see Figure 1). 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 inthc vapor phase. i These processes are conceptualized as occurring in a inurnbcr of distinct, user-defined polygons that am vertically divided into a series of user-defined cells. The polygons may differ in t soil properties, recharge rate, and depth to water(see Figure 2)'. However, within each polygon homogeneous conditions are assumed except for contaminant concentration, which can vary between layered cells(see Figure 3). During each time step the nugra"of time contaminant within t ' and between cells is calculated. Hence,VLFACH can account for heterogeneities laterally but is limited when simulating vertical heterogeneity. r Initially,VLEACH calculates the equilibrium distribution of contaminant mass between the 1 liquid,gas,and sorbed phases. Transport processes we then simulated.1�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 ceps 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. ' Theses s are conducted for each times ,and each polygon is simulated independently. At the � steps mP Po YS ,� �M end of the model simulation,the results from each polygon ate cotnpiledtodetermine an overall area- weighted groundwater impact for the entre modeled area. For computational purposes 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 trade in the development of VLEACH. { Ip ;; 1. Linear isotherms describe the partitioning of the pollutant between the liquid,vapor and soil _ phases.Local orinstantaneous equilibrium between these'phasesis assumed within each cell. f 2. 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. This assumption will rarely occur in the field. Although moisture gradients cannot be simulated,the user can estimate the impact of various moisture contents by,!icom sarin results from several comparing simulations that cover the common or possible ranges in,soil zrioisture conditions. 3 "