Table 3. Soil vapor mass established by the Gore-
found to agreed within a factor of 30%. Once in-
Sorber module and concentrations established for
vial methods or in-field solvent extraction meth-
grab samples taken for in-vial sample handling
ods become a common practice, colocated soil
and HS/GC analysis.
samples will offer a powerful tool for QA pur-
poses.
In-vial HS/GC
Gore-Sorber
In terms of contaminant distribution both the
(g TCE/g)
Map
module
soil vapor survey and grab sample analysis meth-
(g TCE)
location
76 cm
91 cm
Mean
ods established very similar trends for TCE in the
1
5.7
6.8
6.2
892.28
near surface vadose zone. Moreover, a strong re-
2
15
16
16
1012.03
lationship was established between the mean of
3
4.3
4.6
4.4
827.25
the colocated grab samples analyzed by HS/GC
4
3.1
5.5
4.3
923.29
and the soil vapor Gore-Sorber results (r2= 0.944,
5
0.11
0.11
0.11
88.34
6
0.017
0.023
0.020
82.92
Fig. 5). The correlation coefficient was only slightly
7
0.016
0.014
0.015
33.82
less significant than that of the two colocated grab
8
0.010
0.014
0.012
109.29
samples (Fig. 4). The semilog relationship found
9
0.013
0.0046
0.0088
43.15
between the two methods could be a function of
10
0.0066
0.0077
0.0072
58.70
the TD/GC/MS detector response or a change in
11
0.095
0.10
0.098
273.50
12
0.11
0.088
0.099
239.27
the Tenax-TA sorbent affinity with increasing
13
1.5
1.6
1.6
648.18
loading of analyte. Even with this degree of
14
2.1
5.2
3.6
724.12
nonlinearity, the TCE concentration (mass/mass)
15
0.39
0.32
0.36
390.55
in the soil at discrete locations on this site could
16
<0.001
<0.001
0.10
be adequately predicted over a range of 0.010 to
17
<0.001
<0.001
0.00
16 g/g, based on passive soil gas measurements
18
<0.001
<0.001
0.00
and the relationship shown in Figure 5. The worst
case value (passive soil vapor amount established
uted at the majority of sampling locations, on at
for location no. 5) for this comparison shows only
a discrepancy of 5.5 between the predicted and
least a scale of 15 cm. This degree of homogeneity
measured TCE soil concentration. This discrep-
for TCE in colocated samples is consistent with
ancy is not nearly as misrepresentative as the
several other studies performed at this site (Hewitt
random 10 to 1000 range of disagreement re-
et al. 1992, Hewitt et al. 1995, Hewitt and Lukash
ported for comparisons between colocated soil
1996). With the exception of samples collected
samples collected and analyzed using in-vial pro-
within the first 30 cm of the surface, or when two
tocols vs. those collected and analyzed using com-
distinctly different types of soil substrate are en-
mon regulatory agency approved procedures (fill-
countered, colocated samples have often been
18
16
Slope = 1.09
y-intercept = 0.388
r 2 = 0.957
14
12
Best
10
Fit
8
6
4
Figure 4. Comparison of TCE
concentrations (g/g) estab-
2
lished for colocated grab
samples taken for HS/GC
0
4
8
12
16
analysis.
Concentrations at 76 cm (g TCE/g)
7
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