1200
y = 606.38 + 304.65 log(x)
r 2 = 0.94436
800
400
300
200
400
100
Figure 5. Comparison of TCE measure-
ments established by mean of colocated
0
0.1
0.2
0.3
0.4
grab samples analyzed by HS/GC and
passive soil vapor established by the
0
10
20
Gore-Sorber modules.
Grab Sample (g TCE/g)
ing a collection bottle to capacity then shipping to
acidification, mercuric chloride, etc.) are not taken
an off-site laboratory for subsampling and analy-
to prevent biological degradation (Hewitt 1995a,b).
sis [Urban et al. 1989, Illias and Jaeger 1993, Hewitt
At present, comparisons between soil vapor
1994b, Hewitt et al. 1995]).
survey methods and grab samples are perhaps
best suited for near-surface locations. Soil samples
retrieved from depths beyond 6 m often take 15
DISCUSSION
minutes or longer before they can be subsampled.
Intact soil cores held in open tubes may or may
The most significant finding of this study is
not retain representative VOC concentrations over
that strong relationships are very likely between
this time period (Hewitt and Lukash 1996). In
passive soil vapor VOC measurements and VOC
addition to the time concern, many down-hole
concentration established by grab samples. Sec-
sampling systems pull a vacuum on the soil core
ondly, TCE contamination in the vadose zone is
during retrieval, or require the sample be extruded
largely caused by vapor transfer, because a ho-
from a sample chamber prior to subsampling.
mogeneous distribution of analytes is unlikely if
Therefore, many present sampling systems facili-
they were principally to exist as separate liquid
tate or allow volatization losses even before dis-
or solid phases. This characteristic of TCE con-
crete grab samples can be collected. However,
taminant transport was recently demonstrated ex-
once relationships have been established for near
perimentally and modeled for the unsaturated
surface sampling locations, soil vapor technolo-
zone (Conant et al. 1996). Their study showed
gies would provide means of estimating VOC
that in an unconsolidated subsurface zone, a va-
concentrations in unsaturated subsurface regions
por plume will radiate out from a point where a
that cannot be easily or quickly accessed for grab
separate phase or residual source exists, forming
sample collection.
gradient contours of contamination.
However, to accurately establish the relation-
ship between soil vapor survey methods, passive
LITERATURE CITED
or active, and grab samples requires that extreme
precautions are taken to minimizes VOC losses.
Chiou, C.T. and T.D. Shoup (1985) Soil sorption
With regard to volatilization, grab sample col-
of organic vapors and effects of humidity on sorp-
lection must be performed using a rapid non
tive mechanism and capacity. Environmental Sci-
disruptive transfer method, coupled with either
ence and Technology, 19: 11961200.
in-vial handling and analysis or solvent-immer-
Conant, B.H., R.W. Gillham and C.A. Mendoza
sion. An additional concern not covered in this
(1996) Vapor transport of trichloroethylene in the
report is that if aromatic compounds are involved,
unsaturated zone: Field and numerical modeling
grab sample analysis should occur within 2 days of
investigations. Water Resources Research, 32: 922.
collection if measures (i.e., solvent-immersion,
Griffith, T.J., G.A. Robbins and T.M. Spittler
8
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