# Gas Phase Transport Principal Sources: VLEACH, A One-Dimensional Finite Difference Vadose Zone Leaching Model, Version 2.2 – 1997. United States Environmental.

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Gas Phase Transport Principal Sources: VLEACH, A One-Dimensional Finite Difference Vadose Zone Leaching Model, Version 2.2 – 1997. United States Environmental Protection Agency, Office of Research and Development, National Risk Management Research Laboratory, Subsurface Protection and Remediation Division, Ada, Oklahoma. Šimůnek, J., M. Šejna, and M.T. van Genuchten. 1998. The HYDRUS-1D software package for simulating the one-dimensional movement of water, heat, and multiple solutes in variably- saturated media. Version 2.0, IGWMC - TPS - 70, International Ground Water Modeling Center, Colorado School of Mines, Golden, Colorado, 202pp., 1998.

Effective Diffusion Tortuosity (T = L path /L) and percolation (2D)

Total Mass At Equilibrium:

Henry’s Law Dimensionless: Common: atm m 3 mol -1

VLEACH VLEACH simulates vertical transport by advection in the liquid phase and by gaseous diffusion in the vapor phase

VLEACH VLEACH describes the movement of solutes within and between three different phases: –solute dissolved in water –gas in the vapor phase –adsorbed compound in the solid phase Equilibration between phases based on distribution coefficients

Processes are conceptualized as occurring in a number of distinct, user- defined polygons that are vertically divided into a series of user- defined cells

Voronoi Polygons/ Diagram Voronoi_polygons –close('all') –clear('all') –axis equal –x = rand(1,100); y = rand(1,100); –voronoi(x,y)

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 Hence, VLEACH can account for heterogeneities laterally but does not simulate vertical heterogeneity During each time step the migration of the contaminant within and between vertically adjacent cells is calculated

DEPTH TCE CONCENTRATION (ft)(µg/kg of soil) 1 – 20100 20 – 3050 30 - 4010 40 - 500

Chemical Parameters Organic Carbon Partition Coefficient (Koc) = 100 ml/g Henry’s Law Constant (K H ) = 0.4 (Dimensionless) Free Air Diffusion Coefficient (Dair) = 0.7 m 2 /day Aqueous Solubility Limit (Csol) = 1100 mg/l

Soil Parameters Bulk Density (rb) = 1.6 g/ml Porosity (f) = 0.4 Volumetric Water Content (q) = 0.3 Fraction Organic Carbon Content (foc) = 0.005

Environmental Parameters Recharge Rate (q) = 1 ft/yr Concentration of TCE in Recharge Water = 0 mg/l Concentration of TCE in Atmospheric Air = 0 mg/l Concentration of TCE at the Water Table = 0 mg/l

Computational Parameters Length of Simulation Period (STIME) = 500 years Time Step (DELT) = 10 years Time Interval for Writing to.OUT file (PTIME) = 100 yrs Time Interval for Writing to.PRF file (PRTIME) = 250 yrs Size of a Cell (DELZ) = 1.0 ft Number of Cells (NCELL) = 50 Number of Polygons (NPOLY) = 1

Output

Something missing?

Dispersion! Dispersivity is implicit in the cell size (  l) and equal to  l/2 (Bear 1972) Numerical dispersion but can be used appropriately

Dispersion M.C. Sukop. 2001. Dispersion in VLEACH and similar models. Ground Water 39, No. 6, 953-954.

Hydrus

Solves –Richards’ Equation –Fickian solute transport –Sequential first order decay reactions

Governing Equation Provide linkage with preceding members of the chain

Hydrus Input Files

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