VLX: A CONVENIENT SPREADSHEET FOR MODELING VADOSE ZONE CONTAMINANT LEACHING TO GROUNDWATER

VLX is a spreadsheet with a Visual Basic script that computes vadose zone leaching of organic compounds. VLX was designed to incorporate the functions of the old USEPA VLEACH program (Ravi and Johnson, 1997) on a convenient spreadsheet, and to increase its versatility. VLX supports the modeling of a soil column with heterogeneous physical properties (bulk density, porosity, water content, organic carbon). VLX models a one-dimensional soil column discretized into cells. The VLX user inputs the physical properties of soil at each cell, the chemical properties of the subject compound, and the initial concentration of the subject compound at each cell in the soil. VLX computes the partitioning of the initial concentrations of the compound between dissolved, vapor, and adsorbed phases. VLX then computes, by the explicit finite difference method, the fluxes of the subject compound resulting from vapor phase diffusion, aqueous phase diffusion, and aqueous phase advection into and out from each cell for a limited time step. Then VLX re-equilibrates the distribution of the subject compound between dissolved, vapor, and adsorbed phases. Following the re-equilibration, fluxes are computed for the next time step.

Results are displayed in tabular and graphical forms. Results include the changes in the concentration profile at user-specified times during the run, and mass balances through the time of the run, and the mass fluxes between the surface and the water table. Examples are shown of vapor migration in a uniform soil type, a uniform soil type with a variable moisture column, and a column with soils of greatly varying properties. Finally, an application of VLX to compute the effective recharge concentration for input to an MT3D groundwater contaminant transport model is illustrated.

Reference: Varadhan Ravi and Jeffrey A. Johnson (1997) VLEACH: A One-Dimensional Finite Difference Vadose Zone Leaching Model Robert S. Kerr Environmental Research Laboratory Center for Subsurface Modeling Support.