Environmental Engineering Reference
In-Depth Information
Kd
=
(
Foc Koc
)(
)
=
( .
0 0003 58 9
)(
. )
=
0 017
.
The remaining values necessary to calculate the retardation value are the bulk density of
the aquifer material and the effective porosity. The bulk density of the aquifer material
was obtained through the collection and analysis of soil samples and was 1.7 g/cm 3 . The
effective porosity was estimated from literature values to be 25% or 0.25.
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
.
.
Now we can complete the mobility calculation by plugging in the Henry's law constant (H)
obtained from the literature into Equation 10.2. The Henry's law constant for XYZ chemi-
cal 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 GW is calculated
using Equation 10.5.
1
1
CRF
for chemical XYZ
=
×
×
( )
P
GW
(
T M
)
(
)
1
0 04
1
0 253
CRF
for chemical XYZ
=
×
×
( . )
0 2
=
19 76
.
GW
( .
)
( .
)
CRF
for chemical XYZ
=19 76
.
GW
Tables 10.2 through 10.4 display the groundwater contaminant risk factors (CRF GW ) for
common VOC LNAPL, VOC DNAPL, and PAH compounds, respectively, in the Rouge
River watershed (Kaufman et al. 2005; Rogers et al. 2007a). Table 10.5 summarizes those
values and includes other selected compounds. The values span the four most common
types of soils found in urban areas of the United States, including clay, silty clay, sandy
silty clay, and sand. As such, their relative magnitudes can be used as a starting point for
risk assessments in other urbanized watersheds with similar geological units.
In Table 10.2, the CRF GW values for benzene are greater than other volatile organic com-
pounds (VOCs) within the LNAPL group. This is because benzene exhibits the highest
combined values of toxicity, mobility, and persistence in groundwater compared to the
other LNAPL compounds examined.
 
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