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Vol. 9  No. 8

DIFFUSION

&
RBCA TOOLKIT FOR CHEMICAL RELEASES

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Diffusion

You're sitting in a room and someone enters at the other side of the room and sits down. Moments later, you smell their perfume from across the room. How is this possible?  Diffusion. This same process occurs in water.  Imagine a bucket of water sitting completely still.  A drop of dye is placed in the middle of the bucket, what happens to that dye?  Of course the dye begins to spread in the water, despite there being no movement of the water.  Again, this is diffusion.  This newsletter will describe what diffusion is, why it occurs, and at what scale.

What is diffusion?

Diffusion is defined as the mixing caused by the movement of the molecular constituents under the influence of their kinetic energy.  This kinetic energy of the constituents is the energy it possesses due to it's motion.

Where does diffusion occur?

As previously mentioned, diffusion occurs in a variety of environments.  Technically,  diffusion occurs in all materials at all times, however net diffusion ceases when the concentration gradient is non-existent (when the concentration is homogeneous).

What are the scales of diffusion?

The values of diffusion vary depending on the medium and the molecular constituent.  For example, diffusion in gases is larger than in liquids, and diffusion in liquids is larger than in solids.  This is due to the molecular spacing, the further the molecules are spaced, the more room there is for them to diffuse. In a porous medium (such as groundwater), the diffusion within the liquid phase is less than in a pure liquid (such as in a river) because of collision with the solids of the medium hinders diffusion.

What are some example values of diffusion?

The following information is approximate and is based upon the values for diffusion taken directly from Domenico and Schwartz (1998).

 

RBCA Toolkit for Chemical Releases includes diffusion in many of it's calculations in determining the fate and transport of contaminants of concern.

In next month's newsletter we will discuss the effective diffusion coefficient; the coefficient typically used in groundwater transport calculations.  This will include several calculators giving a variety of methods for determining this parameter.

References

Domenico P.A. and Schwartz, F.W. (1998). Physical and Chemical Hydrogeology. Published by John Wiley & Sons, Inc. New York, NY.

Fetter, C.W. (1994). Applied Hydrogeology; Third Edition.  Published by Prentice-Hall, Inc., Englewood Cliffs, NJ.

Freeze, R.A. and Cherry, J.A. (1979). Groundwater. Published by Prentice-Hall, Inc., Englewood Cliffs, NJ.

 

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RBCA Toolkit for Chemical Releases

The RBCA Toolkit for Chemical Releases includes a series of fate and transport models for predicting the ocncentrations of chemicals of concern (COC) at points of exposure (POE) located downwind or downgradient of source areas for air or groundwater exposure pathways, respectively. Under Tier 1 and Tier 2 relatively simple analytical models are to be employed for the calculations.

There are two different types of calculations incorporating diffusion terms in RBCA Toolkit for Chemical Releases; cross media transfer and lateral transfer.

Cross Media Transfer

The cross media transfer is the transport from one media to another. Exposure pathways involving transport of COCs from one medium to another (e.g., soil-to-air, soil-to-groundwater) require estimation of the corresponding cross-media transfer factor.Various analytical expressions are available for estimating soil-to-air volatilization factors as a function of site soil characteristics and the physical/chemical properties of volatile organic COCs.

Diffusion plays an important role in these calculations and the effective diffusivity (to be discussed in next month's newsletter) can be calculated for a variety of scenarios.  These include:

• Effective diffusivity  in vadose zone soils
• Effective diffusivity above the water table
• Effective diffusivity through foundation cracks
• Effective diffusivity in the capillary fringe

Lateral Transfer

The lateral transfer is considered to be within the same media. During lateral transfer transport within air or groundwater COC concentrations in the flow system will be diminished due to mixing and attenuation effects. There are two different models available in the RBCA Toolkit for Chemical Releases:

• DAF - Lateral groundwater dilution attenuation factor. To account for attenuation of affected groundwater concentrations between the source and the POE, the Domenico analytical solution transport model is used.  This model includes advection, dispersion (typically including the diffusion component), sorption and biodegradation.
• ADF - Lateral Air Dispersion factor. This 3D Gaussian dispersion model of airborne contaminants from the source area to a downwind POE.

For more info or to purchase click here:  RBCA Toolkit for Chemical Releases

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