Contaminant Risk Factors - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 367

explains why the inverse of the specific gravity is used. The partitioning coefficient is a
measure of a contaminant's tendency to sorb to particulate matter. This factor is important
because the higher the partitioning coefficient, the higher the likelihood a contaminant
will attach to particulate matter. For example, a contaminant with a low specific gravity
and high sorptive potential has the potential to be very mobile in the atmosphere com-
pared to one with a high specific gravity and low sorptive potential.
Values for the partitioning coefficient can be obtained from numerous sources,
including USEPA (1996a), Wiedemeier (1999), USEPA (2002a), and Suthersan and Payne
(2005).
Persistence of a contaminant in the air is generally expressed in the gas phase and par-
ticulate phase (McKone and Enoch 2002; USEPA 2008). Persistence times for most organic
compounds are much less in the gas phase than when they are attached to particulate
matter.
Finally, the CRF AIR is calculated by multiplying the inverse of the chemical compound's
toxicity (T), by mobility (M) and persistence (P) in the gas and particulate phases (Equation
10.10). Multiplying by the inverse of the mobility value is not necessary.

1

CRF = × [( M × P ) + ( M × P )] (10.10)

AIR gas gas particulate particulate

( T )

where
CRF AIR is the soil contaminant risk factor
T is the toxicity
M gas is the mobility of a contaminant in the gas phase
P gas is the persistence of a contaminant in the gas phase
M particulate is the mobility of a contaminant sorbed to particulate matter
P particulate is the persistence of a contaminant sorbed to particulate matter

The inverse of the toxicity value must be used because the integers assigned for toxicity
values decrease with increasing carcinogenicity (USEPA 2009). The presence of mobility
and persistence factors for gas and particulates accounts for the significant differences in
the half-lives depending on whether the contaminant is present as a gas, or if it is sorbed
onto particulate matter (USEPA 2009).
Using Equation 10.10, an example calculation of the CRF AIR for a Chemical XYZ is
written as

1

CRF for chemical XYZ = × [( M × P ) + ( M × P

iculate )] AIR gas gas particulate part

( T )

Step 1 : Obtain toxicity value:
The toxicity of XYZ chemical was obtained from the literature and has a value of 0.04.
Step 2 : Determine the mobility and persistence values for the gas phase:
Mobility in the gas phase (M gas ) = Henry's law constant (H) = 0.228
Persistence in the gas phase (P gas ) = literature value of 4 days

M ( . 0 228 ) × P ( ) 4 = 0 228 4 . × = 0 912 .

gas gas