Environmental Engineering Reference
In-Depth Information
TABLE 9.8
Retardation Coeffi cients and Relative Velocities as a Function of Fraction of
Organic Carbon
Fraction of
Organic Carbon,
f oc (%)
Retardation Coeffi cient, a R
Ratio of 1,1-Dichloroethylene
Velocity to Methyl Chloroform
Velocity
Methyl Chloroform
1,1-Dichloroethylene
0.01
1.07
1.03
1.04
0.02
1.14
1.06
1.08
0.05
1.35
1.15
1.17
0.1
1.69
1.30
1.31
0.2
2.39
1.59
1.50
0.5
4.47
2.49
1.80
1.0
7.95
3.97
2.00
Source: After Gauthier, T.D. and Murphy, B.L., 2003, Environmental Forensics 2003(4): 205-213.
a
These calculations of retardation coefi cient R = 1 + (ρ b K d e ) assume bulk soil density ρ b = 1.6 g/cm 3 , water-i lled
porosity Θ = 0.35%, and partition coefi cient K d = K oc × f oc . Because f oc can vary within an aquifer and may be higher in
i ne-grained deposits where hydraulic conductivity is lower, its heterogeneous distribution may impart uncertainty to
the analysis of ratios to estimate the timeframe of a release. Overall retardation can be estimated by comparing the
distance that methyl chloroform and its breakdown products have migrated compared to that of 1,4-dioxane.
One case study provides a remarkable example of the application of ratio analysis to interpret the
timeframe of a release. The estimate predicts the date of release to within one week of the actual
date (Wing, 1997). On the morning of August 14, 1984, a pipe supplying a vapor degreaser at a
precision medical instrument plant in Santa Clara, California, failed and released as much as 20
gallons of methyl chloroform. Three hundred cubic yards of contaminated soil were removed within
TCA source
<500 ft
500-1000 ft
>1000 ft
100
90
Median ratio at
source = 0.25
80
70
60
Predicted ratio at 1000 ft,
assuming mean groundwater
velocity of 0.6 ft/day, reaction
half-life of two years, and 0.2
mole of 1,1-dichloroethylene
produced from each mole of
methyl chloroform
50
40
30
20
10
0
0.01
0.1
1
10
100
Ratio of 1,1-DCE (ppb) to 1,1,1-TCA (ppb)
FIGURE 9.1 Field and modeled volume concentration ratios of methyl chloroform (1,1,1-TCA) and 1,1-
dichloroethylene (1,1-DCE). Predicted volume concentration ratio is given at 1000-ft distance from plume
source. The modeling assumes a mean velocity of 0.6 ft/day, reactant half-life of 2 years, and 0.2 mol of 1,1-
dichloroethylene produced from each mole of methyl chloroform. (From McNab, W.W., et al., 1999, Historical
case analysis of chlorinated volatile organic compound plumes. Lawrence Livermore National Laboratory
report UCRL-AR-133361. With permission.)
 
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