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
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
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
Effective diffusivity above the water
Effective diffusivity through foundation
Effective diffusivity in the capillary
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
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.