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VLEACH

Detailed Description

VLEACH

VLEACH





Introduction

VLEACH, a One-Dimensional Finite-Difference Vadose Zone Leaching Model, is a U.S. EPA program which describes the movement of an organic contaminant within and between three 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. The leaching is simulated in a number of distinct, user-defined polygons vertically divided into a series of user-defined cells. At the end of the simulation, the results from each polygon are used to determine an area-weighted ground-water impact for the modeled area. VLEACH includes the executable, source code and technical support.

VLEACH is a menu-driven computer program 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.

VLEACH describes the movement of the organic contaminant within and between three different phases. 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 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 vary 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 contaminant 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 area-weighted ground-water impact for the entire 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.

VLEACH includes a preprocessor for creating/editing data files.



VLEACH Assumptions

The following assumptions were made in the development of VLEACH:

  • 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.
  • 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 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. 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 occur in the field. Although spatial gradients cannot be simulated, the user can estimate the impact of nonuniform 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 nonaqueous phase liquids or any flow conditions derived from variable density.



VLEACH Input Parameters

Simulation Data for VLEACH
  • Title - Simulation Description
  • Number of Polygons - Each polygon has a unique set of parameter data
  • Timestep - Model timestep
  • Simulation Time - Total length of simulation
  • Output Time Interval - Time to print ground-water impact and mass balance
  • Profile Time Interval - Time to print vertical concentration profile results
  • Organic Carbon Distribution Coefficient (KOC)
  • Henry's Constant (KH)
  • Water Solubility
  • Free Air Diffusion Coefficient

Polygon Data - (for each polygon) for VLEACH
  • Title - Defines the simulation
  • Area - Area of polygon
  • Vertical Cell Dimension - Vertical height of the cells within the polygon
  • Recharge Rate - Groundwater recharge rate through the vadose zone
  • Dry Bulk Density
  • Effective Porosity
  • Volumetric Water Content
  • Soil Organic Carbon Content
  • Concentration of Recharge Water
  • Upper Boundary Condition for Vapor - Contaminant concentration in atmosphere above soil surface
  • Lower Boundary Vapor Condition for Vapor - Contaminant concentration in groundwater at base of vadose one lower
  • Cell Number - Number of cells within the polygon
  • Plot Variable - Indicates whether a plot file containing the soil contaminant profile will be created
  • Plot Time - Time at which soil contaminant profile data will be produced for the plot file
  • Initial Contaminant Concentration - Initial contaminant concentration in the soil within a single or set of cells

A graphical user interface for VLEACH is now available in the WHI UnSat Suite. The VLEACH model is also included.



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