# EPA On-line Tools for Site Assessment Calculation

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## Computing the Effective Diffusion Coefficient

The effective diffusion coefficient for layer

*i*of a porous media can be estimated using the Millington relationship:

**Contaminant Terms:**

- D
_{air}is the free-air diffusion coefficient for the contaminant of concern [cm^{2}/sec]. - D
_{water}is the diffusivity in water for the contaminant of concern [cm^{2}/sec]. - H is the dimensionless Henry's Law Constant for the contaminant of concern [unitless].

**Soil Terms:**

- n
_{i}is the total porosity of layer*i*of the porous media [unitless]. - θ
_{w,i}is the water-filled porosity of layer*i*of the porous media [unitless]. - θ
_{a,i}is the air-filled porosity of layer*i*of the porous media (θ_{a}= n - θ_{w}) [unitless].

The overall effective diffusion coefficient for systems composed of

*m*layers (e.g., capillary fringe and the unsaturated zone) is:

where L

_{i}is the thickness of layer i, L

_{T}is the distance between the source of contamination and the bottom of the slab (sum of all layer thicknesses) and D

_{i}

^{eff}is the effective diffusion coefficient for layer i.

The overall effective diffusion coefficient used in determining target soil-gas concentrations only considers diffusion through the single-layer unsaturated zone.

The overall effective diffusion coefficient used in determining target groundwater concentrations incorporates both diffusion through the capillary fringe and diffusion through the unsaturated zone.

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