Contaminant Risk Factors - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 358

All the necessary information has been obtained, and the retardation value of 1.11 is
calculated using Equation 10.3:

( ρ b Kd )( )

XYZchemical retardation R = = + 1

η

( . )( . 1 7 0 017

0 25

)

XYZchemical retardation R = = + 1 = 1 11 .

.

Plug in Henry's law constant (H) obtained from the literature into Equation 10.2; the
Henry's law constant for XYZ chemical is 0.228.

M H R = ( )( )

M = 0 228 1 11 . × . = 0 253 .

Step 3 : Determine the persistence value. The persistence value was obtained from the lit-
erature and is 0.2 years.
Step 4 : With toxicity, mobility, and persistence values obtained, the CRF SOIL is calculated
using Equation 10.6:

1

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

SOIL

( T )

1
0 04

CRF for chemical XYZ = × ( . 0 253 ) × ( . ) 0 2 = 1 265 .

SOIL

( . )

CRF for chemical XYZ = 1 265 .

SOIL

Contaminant Risk Factors in soil for common LNAPL VOCs in the Rouge River watershed
are listed in Table 10.8 (Rogers et al. 2007b; Kaufman et al. 2009). As with the CRF GW, the
values span the four most common types of soils found in urban areas of the United States,
so their relative magnitudes can be used as a starting point for risk assessments in other
urbanized watersheds with similar geological units.
As shown in Table 10.8, the values for LNAPL compounds are greatest for soil composed
of clay, and reflect the tendency of LNAPLs to sorb more strongly to finer grained soils.
Table 10.9 lists the CRF SOIL for common DNAPL VOCs (Kaufman et al. 2009).
The CRF SOIL for DNAPLs are much greater than those of LNAPL compounds listed in
Table 10.8. Not only are the DNAPL compounds more toxic, but they are also much more
persistent in the environment than the LNAPL compounds (Kaufman et al. 2009). For
instance, the LNAPL compound benzene—a very toxic chemical—has a half-life of 0.2
years, whereas the DNAPL compound tetrachloroethene—also very toxic—has a half-life
of 18 years because of its sequential degradation to vinyl chloride. If released into the
environment at the same time, the DNAPL tetrachloroethene would last approximately 90
times longer than the LNAPL benzene.
Table 10.10 lists the CRF SOIL for common PAHs (Kaufman et al. 2005; Rogers et al. 2007b).