Establishing a Relationship Between

Passive Soil Vapor and Grab Sample

Techniques for Determining Volatile

Organic Compounds

Alan D. Hewitt September 1996

SPECIAL REPORT 96-14
Abstract: A passive soil vapor and an in-vial sample

handling and analysis method were compared for estimating volatile organic compound (VOC) contamination

in the near-surface vadose zone. These two methods of

VOC site characterization, although very different operationally, established very similar trends for trichloroethylene (TCE) contamination. The correlation (r

2 =

0.944) of the results from these two methods shows a

much better agreement than what has been reported

between comparisons of in-vial methods (or solvent

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immersion) and conventional soil sample collection

and handling methods often used for site characterization activities. The strong correlation between these two

methods and from grab samples taken 15 cm apart

indicates that this analyte is homogeneously distributed as compared to metals and semivolatile organic

compounds. For contaminants such as TCE, soil vapor

measurement technologies offer a promising means of

estimating subsurface concentrations in locations were

grab samples cannot be easily obtained.
Special Report 96-14

Establishing a Relationship Between

Passive Soil Vapor and Grab Sample

Techniques for Determining Volatile

Organic Compounds

Alan D. Hewitt September 1996

Prepared for

U.S. ARMY ENGINEER WATERWAYS EXPERIMENT STATION

Approved for public release; distribution is unlimited.

US Army Corps


of Engineers

Cold Regions Research &


Engineering Laboratory
PREFACE

This report was prepared by Alan D. Hewitt, Research Physical Scientist, Geological Sciences Division, Research and Engineering Directorate, U.S. Army Cold Regions

Research and Engineering Laboratory (CRREL), Hanover, New Hampshire. Funding

for this work was provided by the U.S. Army Engineer Waterways Experiment

Station, Vicksburg, Mississippi, through Environmental Quality Technology Project

AF25, Ann B. Strong, Project Monitor.

The author thanks Dr. Thomas F. Jenkins and Thomas A. Ranney of CRREL for

technical review of the text. He also thanks Mark J. Wrigley, Wayne M. Wells, Paul

Andrews of W.L. Gore & Associates for initiating this collaborative study, and

providing and analyzing the Gore-Sorber modules; and Karen Myers and Bobby

Jones of the Waterways Experiment Station for assisting in the development and

analysis (PT/GC/MS) of the vapor-fortified secondary soil standard.

This publication reflects the view of the author and does not suggest or reflect

policy, practices, programs, or doctrine of the U.S. Army or of the Government of the

United States.

The contents of this report are not to be used for advertising or promotional

purposes. Citation of brand names does not constitute an official endorsement or

approval of the use of such commercial products.

ii
INTRODUCTION

Traditionally, passive soil vapor techniques

have been used to establish the presence and relative distribution of volatile organic compounds

(VOCs) in the near-surface vadose zone. These

methods of screening are often used during the

initial phase of a site investigation to provide

insight for the placement of monitoring wells and

to delineate areas for intensive sampling. In contrast, discrete soil sample (“grab samples”) techniques are used for risk assessment, to establish

both the horizontal and vertical profiles of contamination in areas of concern and to assess the

progress of remediation.

To date, we are unaware of a study showing a

strong relationship between the results from a

passive soil vapor survey and soil VOC concentrations resulting from grab sample collection and

analysis. The task of establishing a relationship

between these two VOC assessment technologies

is confounded by both environmental and operational variables. Among environmental variables,

temperature, soil moisture, organic carbon content, grain size, mineralogy, texture, and barometric pressure all influence VOC concentrations

in both the soil gas and sorbed phases (Conant et

al. 1996). Among operational variables, these two

approaches to VOC characterization have very

little in common. Passive soil gas survey methods

require a long-term exposure (several days to a

couple of weeks) to establish an in-situ, timeintegrated amount for a unknown volume of soil.

Furthermore, because this application is usually

designated for screening purposes, concentration

determinations are not rigorous (i.e., single point

calibration). Grab samples on the other hand represent a single point in time and are obtained in

an intrusive fashion, by collecting a discrete quantity of soil. Analysis of grab samples is performed

using a specified laboratory procedure, which is

usually validated by accompanying quality assurance (QA) and quality control (QC) data.

VOCs in an unsaturated porous medium partly

exist in a gaseous state due to their high vapor

pressures. Indeed, it is this physical characteristic

of VOCs that has led to the success of both passive and active soil vapor techniques. On the other

hand, because a portion of the contaminant VOC

is present in the gaseous phase, intrusive sampling protocols have to be rapid and minimize

disruption of the soil structure to prevent losses.

Hewitt and Lukash (1996) have shown that in less

than 40 minutes, more than 90% of the low boiling point VOCs (i.e., trichloroethylene [TCE]) can

be lost from an intact soil stored in an uncovered

core liner. Moreover, procedures that require multiple handling operations often result in underestimating soil VOC concentrations by one to three

orders of magnitude (Urban et al. 1989, Hewitt

1994a, Hewitt et al. 1995).

In recognition of the problems with past intrusive soil sample collection, handling and analysis

techniques, in-vial methodologies are now being

promulgated. The U.S. Environmental Protection

Agency is currently proposing two in-vial methods, 5021 and 5035, for the third update of the

Test Methods for Evaluating Solid Waste, SW-846

(U.S. EPA 1986). Using an in-vial approach requires that a discrete soil sample be collected and

transferred directly to a vial with hermetic seals,

that then has to be punctured by a needle in order

to perform a VOC analysis. Furthermore, Hewitt

and Lukash (1996) recommend that these soil

samples be collected within a few minutes of exposing a fresh surface and transferred to the collection/analysis vial with minimal disruption to

the native soil structure.

Establishing a Relationship Between

Passive Soil Vapor and Grab Sample Techniques for

Determining Volatile Organic Compounds

ALAN D. HEWITT
If in-vial sample handling and analysis methods work as intended, then VOCs present as vapor, sorbed, or liquid (pure product or mixture)

should be included in the measurement. Therefore, using in-vial sample handling and analysis

procedures should increase the chances of establishing a relationship with a vapor survey method.

To test this hypothesis, soil mass concentrations

established using an in-vial handling and analysis method were compared to soil vapor analyte

amounts obtained with a passive collector, GoreSorber modules. Use of Gore-Sorber modules is a

relatively new method of performing a passive

soil gas survey for VOCs and some semivolatile

organic compounds (Wrigley 1995). The sorbent

present in the modules used in this study was

Tenax-TA, a material that has strong affinity for a

broad range of organic compounds, independent

of sample moisture. Experimentally, comparisons

between the Gore-Sorber modules and well-established grab sample VOC measurement methods were performed with both laboratory-fortified and field-contaminated soils.

MATERIAL

Each Gore-Sorber module consists of long

(≈120-cm) hollow Gore-Tex membrane cord that

allows for installation and retrieval, and contains

three separate granular Tenax-TA sorbent packs

(40 mg) positioned at one end. The Gore-Tex membrane, which also serves as a wrapping for each

pack, is an expanded polytetrafluoroethylene

(ePTFE), transparent to gases while preventing

direct contact of the sorbent with solid or aqueous matrices. Thus, during deployment in the

field, the sorbent module is both wrapped and

sheathed in an inert, hydrophobic, microporous

membrane, allowing only for the penetration of

vapors.

EXPERIMENTAL

Laboratory-fortified soil

The capabilities of the Gore-Sorber module,

headspace gas chromatography (HS/GC), and

Table 1. VOC concentrations (μg/g) established for vapor-fortified QA

soil samples.

Aqueous Methanol

extraction extraction Tenax-TA TD/GC/MS***

HS/GC PT/GC/MS (μg/g)

Analyte* (μg/g) (μg/g) 1 d† 9 d 15 d

TDCE 5.5±0.6** 5.8±0.55 0.73 1.81 1.68

CDCE 51±0.9 51.7±1.21 8.1 15.6 14.5

CF 89±5.0 85.3±1.58 17.4 30.2 30.0

C-tet ND†† 44.3±1.01 11.7 18.4 18.2

Ben 63±0.4 61.7±1.49 14.2 26.4 24.2

TCE 8.0±0.3 8.3±0.44 3.24 4.56 3.99

1,1,2 TCA ND 6.3±0.31 2.74 3.54 3.44

Tol 56±0.6 49.7±1.76 22.4 30.8 27.5

PCE 91±3.0 74.2±0.96 57.6 70.1 47.6

E-Ben 13±0.1 11.7±0.31 7.10 9.00 8.64

p-Xyl 26±0.4 22.7±0.32 13.5 16.9 15.6

o-Xyl 41±0.7 36.9±0.85 22.5 27.2 22.8

1,4 DCB 10±0.1 12.4±0.31 9.45 11.5 9.85

Napht 12±1.0 9.3±0.76 4.24 7.69 7.01

 * TDCE, trans-1,2-dichloroethylene; CDCE, cis-1,2-dichloroethylene; CF, chloroform;

C-tet, carbon tetrachloride; Ben, benzene; TCE, trichloroethylene; 1,1,2 TCA,1,1,2

trichloroethane; Tol, toluene; PCE, tetrachloroethylene; E-Ben, ethylbenzene; pXyl, para Xylene; o-Xyl, ortho Xylene; 1,4 DCB, 1,4 dichlorobenzene; Napht, naphthalene

** Mean and standard deviation of sample triplicates

† Equilibration period (days)

†† Not detectable by instrumental configuration.

*** Thermal desorption gas chromatography mass spectrometry.

2
purge-and-trap gas chromatography mass spectrometry (PT/GC/MS) Method 8260 (U.S. EPA

1986) were initially compared using vapor fortified soil samples. These secondary quality assurance (QA) standards were fortified with 14

VOCs, ranging in concentration from 5 to 100

μg/g (Table 1). Briefly, vapor fortification involves exposing a desiccated soil held in small

(1-mL) glass ampoule to vapors emanating from

an organic solution, inside of a sealed chamber

(Hewitt 1994b). This method of producing performance evaluation samples is precise, mimics

an environmental pollution pathway, and uses a

1-week or longer exposure period. Furthermore,

once fortification has been completed and the

ampoules heat sealed, analyte concentration remain stable for up to 6 months.

To analyze the VOCs present in one of these

vapor-fortified samples, the entire ampoule is

placed inside of a container (usually a volatile

organic analysis [VOA] vial or equivalent vessel)

with the appropriate solvent or sorbent for the

chosen method of analysis. For this study Type 1

water was used for HS/GC, methanol for Method

8260 (U.S. EPA 1986), and a single Tenax-TA sorbent pack for the Gore-Sorber module. Once the

VOA vial or vessel has been capped, the soil is

dispersed for extraction/equilibration, by hand

shaking, causing only the glass ampoule to break.

For this study three separate secondary standards

were supplied to W.L. Gore & Associates. To test

the VOC affinity of the Tenax-TA sorbent pack

with these QA samples, one was allowed to equilibrate for 1 day, while the other two were allowed

to equilibrate for 9 and 15 days.

Field samples

Discrete grab and passive soil vapor samples

were collected at 18 separate locations, at the

CRREL (Fig. 1). This site has previously been characterized as having TCE contamination in both

the vadose zone and groundwater, ranging in

concentration from <0.001 to >100 ppm (μg/g or

mg/L, Hewitt 1994b, Hewitt and Shoop 1994).

Consequently, the locations chosen for this comparison study took full advantage of this feature.

In general, the soils typical of this site are siltysands with a moisture content of around 10% and

an organic carbon content of <1.0%. However,

the substrate was dominated by either silty-clays

or sand at a couple of the sampling locations.

The near-surface holes, from which grab

samples were collected and then the Gore-Sorber

modules were placed, were made using a Veihmeyer tube. A Veihmeyer tube uses a sliding bar

hammer to push its hollow stem into the soil

subsurface, and, once removed, a

3.2-cm-i.d. open channel remains

from the ground surface to the

depth of interest. Grab samples

were collected and the Gore-Sorber

modules were initially exposed

(installed) on 4 December 1995. For

the purpose of this report, all of the

samples taken from a single location (Fig. 1) will be referred to as

colocated. These sampling locations were created, 42 grab samples

collected, and 18 Gore-Sorber modules installed in approximately 4

hours by two people.

For in-vial sample handling and

HS/GC analysis, two grab samples

(≈3 g each) were taken from each

location (hole), one at 76 cm and a

second at 91 cm. In addition, at six

locations grab samples from a

depth of 84 cm were collected and

placed directly into jars with a

Gore-Sorber module. The vessels,

used for shipping and storage of

the sorber modules, were 125-mL

Laboratory Building

10

9

8

6

5

7

1 3 4

2

11

12

13

14

15

16

17

Figure 1. Map of sampling locations. 18

3
glass jars with Teflon-faced cushioned lids, which

formed a hermetic seal upon capping. Prior to

adding the grab samples to these jars, the ePTFE

cord was shortened to eliminate unnecessary

weight.

Grab samples were collected from the bottom

of the open channel (hole) using a 116-cm-long,

1.9-cm-i.d. Plexiglas subcorer equipped with a

plunger. This sampling device works in the same

manner as a plastic syringe with its tip removed

(Griffith et al. 1988). Discrete plugs of undisturbed

soil were obtained immediately after removing

the Veihmeyer tube, by inserting the open barrel

of the Plexiglas subcorer approximately 2.5 cm

beyond the hole’s present depth. The plug of soil

was then quickly transferred directly to either a

prepared VOA vial or jar containing a Gore-Sorber

module. These plugs of soil, transferred to jars

containing sorbent modules, remained intact

throughout the study (maintained the cylindrical

shape created by the subcorer).

Soil samples taken for the in-vial collection and

HS/GC analysis protocol were placed into preweighed 22-mL VOA vials containing 10 mL of

Type 1 water. Before inserting a soil plug, a sheet

of Parafilm “M,” which had covered the vial to

prevent spilling or evaporation, was removed.

Immediately after the sample had been extruded

directly into the VOA vial, it was sealed with a

Teflon-faced septum and aluminum crimp top.

Using this protocol, grab samples from the depth

of interest were collected, transferred, and enclosed in a vessel with hermetic seals, within a

minute of exposing a fresh surface.

The moist weight of each soil sample was obtained by weighing the collection/analysis VOA

vial before and after sampling. Likewise the jars

with Gore-Sorber modules, into which grab

sample were placed, were also weighed before

and after the sampling event to obtain the samples’

moist weight. These jars were allowed to sit at

room temperature (22°C) for 16 days after sample

collection.

The Gore-Sorber modules were quickly positioned at the bottom of the 3.2-cm-i.d., 91-cmdeep hole by using an insertion tool to push the

sorbent end of the cord into the exposed substrate. These modules that were left in the field

were retrieved after 16 days, and placed into their

individual shipping and storage jars. During the

16-day exposure period (4 to 20 December 1995)

the holes into which the sorber had been placed

were sealed with a cork stopper that was covered

with a Styrofoam dish. Wooden stakes were also

used to locate each sampling hole. During the

sampling period there was a continuous snow

cover (≈30 cm at installation, ≈76 cm at retrieval)

and a frost zone that penetrated 5 to 10 cm below

the ground surface. The same day the sorbent

modules were retrieved from the field, these

sample jars and the ones into which grab samples

had been place were packed in a cooler with blue

ice and shipped to W.L. Gore & Associates for

analysis.

ANALYSIS

Grab samples placed in VOA vials containing

water were analyzed the day after collection using a HS autosampler (Tekmar 7000) coupled to a

GC (SRI, model 8610-0058) equipped with a 15-

m, 0.53-mm diam. MXT-1 capillary column (Restex

Corp.), and photo-ionization/flame ionization sequential detectors. This same system was also

used to determine the VOCs in the vapor-fortified samples. Before placing the VOA vials that

either contained field grab samples or vapor-fortified samples onto the HS auto sampler, they

were shaken for 2 minutes to assist in attaining an

equilibrium partition of VOCs among the phases

present. In accordance with in-vial analysis protocols, a portion of the headspace VOC vapors

were removed from these VOA vials by puncturing the septum with a needle.

Analyte concentrations were established relative to aqueous headspace standards prepared by

adding small (<10 μL) quantities of a methanol

(MeOH) stock solution to autosampler vials containing 10 mL of Type 1 water. This HS/GC method of analysis has a detection limit of approximately 0.001 μg TCE/g. Instrumental settings for

the autosampler and gas chromatograph, and information concerning the performance of this

method for establishing VOC concentrations in

soil, have been previously documented (Hewitt

et al. 1992, Hewitt 1995).

The vapor-fortified soil samples were also analyzed by PT/GC/MS following Method 8260 (U.S.

EPA 1986). Due the high concentrations of VOC

present (5–100 μg/g) this sample was first extracted with MeOH. Once immersed in MeOH,

VOCs associated with a soil matrix become dissolved in this solvent. Completeness and the rate

of extraction is a function of both organic carbon

content and macro/micropore structure inherent

to the matrix (Pignatello and Xing 1996). After

extraction into MeOH, VOC concentrations re4
main stable for extended periods (several months),

and the vessel can be intermittently opened for

aliquot removal without significant losses (Hewitt

1995b). Therefore, the immersion of a soil sample

into an organic solvent also accomplishes the intent of in-vial handling and analysis protocols,

relative to controlling volatilization losses.

Individual Tenax-TA packs from the GoreSorber modules were analyzed by thermal desorption gas chromatography mass spectrometry

(TD/GC/MS), using a Level 1 screening procedure (W.L. Gore & Associates). This semiquantitative procedure establishes a compound mass,

based on a single point calibration 5-μg standard

mass, and has a reported detection limit of 0.02

μg for TCE.

RESULTS

Laboratory-fortified soil

Table 1 shows the VOC concentrations determined for the vapor fortified soil samples by HS/

GC, PT/GC/MS (Method 8260) and by TD/GC/

MS analysis of the Tenax-TA packs after three

different exposure periods. All concentrations are

expressed on a mass per mass basis since the

amount of soil present was known. Quantitatively,

both HS/GC and PT/GC/MS (Method 8260) established similar concentrations for 12 of the 14

analytes (two compounds were not determined

by HS/GC analysis due to co-elution). Good agreement between these two methods of sample preparation and analysis has been established previously for soils with a low (<1%) organic carbon

content (Hewitt et al. 1992).

In comparison, the sorbent pack TD/GC/MS

analysis attained a maximum recovery after 9 days

of exposure, showing analyte concentrations 30%

to 93% of that established by MeOH extraction

PT/GC/MS analysis. The analyte recoveries established by the Tenax-TA sorbent is a function

of its affinity for the individual VOC vapors, and

their release from the desiccated soil. By plotting

the percentage recovery after 9 days of exposure

for the Tenax-TA sorbent relative to the MeOH

extraction results vs. the boiling points of the

analytes, an direct relationship is established (Fig.

2). Clearly, this sorbent’s ability to passively acquire VOCs from this matrix is dependent on the

boiling point (i.e., vapor pressure).

Qualitatively, the TD/GC/MS analysis of the

sorbent pack was able to rapidly (1-day exposure) identify all of the analytes present in the

vapor fortified QA soil sample (Table 1). Thus,

even under a desiccated condition where soils

show a much greater capacity for the sorption of

VOCs (Chiou and Shoup 1985) and analytes may

be held to crystalline surfaces by physical forces,

such as van de Waals (Sawheny and Gent 1990),

this technology is effective on a relative basis.

Field samples

The results shown in Table 2 are for locations

where a grab sample was collected at a depth of

85 cm and placed in a jar with the Gore-Sorber

module. This table also includes results for

colocated grab samples collected some 7 cm above

Figure 2. Percentage analyte

recovery by the Tenax-TA

sorbent relative to methanol

extraction vs. boiling point.

120

80

40

0 50 100 150 200 250

Boiling Point (°C)

Percent Recovery Tenax-TA/Methanol

Best 

Fit

Slope = 0.40

y-intercept = 16.3

r = 0.7464 2

5
and below, and analyzed by HS/GC. All values

are reported on a mass per mass basis by accounting for the number of sorbent packs present (only

one of the three replicate packets present during

exposure was analyzed). A comparison between

these two methods was evaluated by plotting the

mean TCE concentration from the two grab

samples analyzed by HS/GC vs. the concentration estimated from analysis of the Gore-Sorber

module. This limited comparison (five out of the

six locations) has a correlation coefficient (r2) of

0.964, and slope of 1.62 (Fig. 3). Failure to detect

TCE on the sorber module positioned at location

no. 7 is consistent with the detection limit reported for this technology (0.02 μg TCE) and the

concentrations established by the colocated grab

samples taken for HS/GC analysis (Table 2).

Unlike the 57% recovery of TCE from the laboratory fortified soil as determined by TD/GC/

MS analysis of the Tenax-TA sorbent (Table 1)

relative to HS/GC analysis, the correlation shown

in Figure 3 suggests the opposite trend for recovery of this analyte from colocated field grab

samples. That is, TD/GC/MS analysis of the GoreSorber module typically resulted in greater concentrations of TCE than established by HS/GC

analysis. In reality too few comparison were made

to offer an explanation for this observation. However, in the presence of moisture (i.e., ample water vapor), VOCs may not be as strongly retained

as by a desiccated soil; thus under field conditions Tenax-TA may have a greater apparent affinity.

Table 3 shows the TCE results obtained from

18 locations (Fig. 1), for both the two grab samples

taken for HS/GC analysis and the mass of TCE

sorbed by the passive soil vapor GORE SORBER

modules over the 16-day exposure period. In this

table (Table 3) the GORE SORBER results are reported only as the total amount of TCE determined, since the mass of soil from which the vapors came is not known. Not included in Table 3

is a single detection of cis-1,2-dichloroethylene,

and several for carbon tetrachloride and tetrachloroethylene, established by the analysis of the

Gore-Sorber module. In every case where these

other analytes were detected, the mass established

for TCE was at least 20 times greater (more often

>100×).

Looking first at the comparison between the

colocated grab sample results, a strong linear relationship is shown, with a correlation coefficient

of 0.957 (Fig. 4). This correlation and slope of

1.088 show that TCE was homogeneously distribTable 2. Soil sample concentrations established

for discrete grab samples by the Gore-Sorber module and by in-vial sample collection and HS/GC

analysis.

Gore-Sorber

In-vial HS/GC (μg TCE/g) (μg TCE/g)

Location 76 cm 91 cm Mean 85 cm

1 5.7 6.8 6.2 11.0

2 15 16 16 23.5

4 3.1 5.5 4.3 5.12

7 0.016 0.014 0.015 <0.02

13 1.5 1.6 1.6 2.19

14 2.1 5.2 3.6 2.31

Figure 3. Comparison of the

mean TCE concentrations

(μg/g) in grab samples established by HS/GC and by

Gore-Sorber modules.

Best 

Fit

15

10

5

0 4 8 12 16

Grab Sample (μg TCE/g)

Gore Sorber Module (μg TCE/g)

Slope = 1.62

y-intercept = 0.9853

r = 0.9645

25

20 2

6
uted at the majority of sampling locations, on at

least a scale of 15 cm. This degree of homogeneity

for TCE in colocated samples is consistent with

several other studies performed at this site (Hewitt

et al. 1992, Hewitt et al. 1995, Hewitt and Lukash

1996). With the exception of samples collected

within the first 30 cm of the surface, or when two

distinctly different types of soil substrate are encountered, colocated samples have often been

Table 3. Soil vapor mass established by the GoreSorber module and concentrations established for

grab samples taken for in-vial sample handling

and HS/GC analysis.

In-vial HS/GC Gore-Sorber

Map (μg TCE/g) module

location 76 cm 91 cm Mean (μg TCE)

1 5.7 6.8 6.2 892.28

2 15 16 16 1012.03

3 4.3 4.6 4.4 827.25

4 3.1 5.5 4.3 923.29

5 0.11 0.11 0.11 88.34

6 0.017 0.023 0.020 82.92

7 0.016 0.014 0.015 33.82

8 0.010 0.014 0.012 109.29

9 0.013 0.0046 0.0088 43.15

10 0.0066 0.0077 0.0072 58.70

11 0.095 0.10 0.098 273.50

12 0.11 0.088 0.099 239.27

13 1.5 1.6 1.6 648.18

14 2.1 5.2 3.6 724.12

15 0.39 0.32 0.36 390.55

16 <0.001 <0.001 0.10

17 <0.001 <0.001 0.00

18 <0.001 <0.001 0.00

found to agreed within a factor of 30%. Once invial methods or in-field solvent extraction methods become a common practice, colocated soil

samples will offer a powerful tool for QA purposes.

In terms of contaminant distribution both the

soil vapor survey and grab sample analysis methods established very similar trends for TCE in the

near surface vadose zone. Moreover, a strong relationship was established between the mean of

the colocated grab samples analyzed by HS/GC

and the soil vapor Gore-Sorber results (r2= 0.944,

Fig. 5). The correlation coefficient was only slightly

less significant than that of the two colocated grab

samples (Fig. 4). The semilog relationship found

between the two methods could be a function of

the TD/GC/MS detector response or a change in

the Tenax-TA sorbent affinity with increasing

loading of analyte. Even with this degree of

nonlinearity, the TCE concentration (mass/mass)

in the soil at discrete locations on this site could

be adequately predicted over a range of 0.010 to

16 μg/g, based on passive soil gas measurements

and the relationship shown in Figure 5. The worst

case value (passive soil vapor amount established

for location no. 5) for this comparison shows only

a discrepancy of 5.5× between the predicted and

measured TCE soil concentration. This discrepancy is not nearly as misrepresentative as the

random 10× to 1000× range of disagreement reported for comparisons between colocated soil

samples collected and analyzed using in-vial protocols vs. those collected and analyzed using common regulatory agency approved procedures (fillFigure 4. Comparison of TCE

concentrations (μg/g) established for colocated grab

samples taken for HS/GC

analysis.

Best 

Fit

18

16

14

12

10

8

6

4

2

0 4 8 12 16

Concentrations at 76 cm (μg TCE/g) 

Concentrations at 91 cm (μg TCE/g)

Slope = 1.09

y-intercept = 0.388

r = 0.957 2

7
ing a collection bottle to capacity then shipping to

an off-site laboratory for subsampling and analysis [Urban et al. 1989, Illias and Jaeger 1993, Hewitt

1994b, Hewitt et al. 1995]).

DISCUSSION

The most significant finding of this study is

that strong relationships are very likely between

passive soil vapor VOC measurements and VOC

concentration established by grab samples. Secondly, TCE contamination in the vadose zone is

largely caused by vapor transfer, because a homogeneous distribution of analytes is unlikely if

they were principally to exist as separate liquid

or solid phases. This characteristic of TCE contaminant transport was recently demonstrated experimentally and modeled for the unsaturated

zone (Conant et al. 1996). Their study showed

that in an unconsolidated subsurface zone, a vapor plume will radiate out from a point where a

separate phase or residual source exists, forming

gradient contours of contamination.

However, to accurately establish the relationship between soil vapor survey methods, passive

or active, and grab samples requires that extreme

precautions are taken to minimizes VOC losses.

With regard to volatilization, grab sample collection must be performed using a rapid non–

disruptive transfer method, coupled with either

in-vial handling and analysis or solvent-immersion. An additional concern not covered in this

report is that if aromatic compounds are involved,

grab sample analysis should occur within 2 days of

collection if measures (i.e., solvent-immersion,

acidification, mercuric chloride, etc.) are not taken

to prevent biological degradation (Hewitt 1995a,b).

At present, comparisons between soil vapor

survey methods and grab samples are perhaps

best suited for near-surface locations. Soil samples

retrieved from depths beyond 6 m often take 15

minutes or longer before they can be subsampled.

Intact soil cores held in open tubes may or may

not retain representative VOC concentrations over

this time period (Hewitt and Lukash 1996). In

addition to the time concern, many down-hole

sampling systems pull a vacuum on the soil core

during retrieval, or require the sample be extruded

from a sample chamber prior to subsampling.

Therefore, many present sampling systems facilitate or allow volatization losses even before discrete grab samples can be collected. However,

once relationships have been established for near

surface sampling locations, soil vapor technologies would provide means of estimating VOC

concentrations in unsaturated subsurface regions

that cannot be easily or quickly accessed for grab

sample collection.

LITERATURE CITED

Chiou, C.T. and T.D. Shoup (1985) Soil sorption

of organic vapors and effects of humidity on sorptive mechanism and capacity. Environmental Science and Technology, 19: 1196–1200.

Conant, B.H., R.W. Gillham and C.A. Mendoza

(1996) Vapor transport of trichloroethylene in the

unsaturated zone: Field and numerical modeling

investigations. Water Resources Research, 32: 9–22.

Griffith, T.J., G.A. Robbins and T.M. Spittler

1200

800

400

0 10 20

Grab Sample (μg TCE/g)

Gore Sorber Module (μg TCE)

400

300

200

100

0 0.1 0.2 0.3 0.4

y = 606.38 + 304.65 log(x)

r = 0.94436 2

Figure 5. Comparison of TCE measurements established by mean of colocated

grab samples analyzed by HS/GC and

passive soil vapor established by the

Gore-Sorber modules.

8
(1988) A new method for field analysis of soils

contaminated with aromatic hydrocarbon compounds. In FOCUS Conference on Eastern Regional

Water Issues, National Water Well Association,

September, Stamford, Connecticut, p. 223–248.

Hewitt A.D. (1994a) Comparison of methods for

sampling vadose zone soils for the determination

of trichloroethylene. Journal of the Association of

Official Analytical Chemists, 77(2): 458–463.

Hewitt A.D. (1994b) Preparation of spiked soils

by vapor fortification for volatile organic compound analysis. Journal of the Association of Official

Analytical Chemists, 77(3): 735–737.

Hewitt, A.D. (1995a) Enhanced preservation of

volatile organic compounds in soil with sodium

bisulfate. USA Cold Regions Research and Engineering Laboratory, Special Report 95-26.

Hewitt A.D. (1995b) Preservation of volatile organic compounds in soil subsamples. American

Environmental Laboratory, August.

Hewitt, A.D. and N.J.E. Lukash (1996) Obtaining

and transferring soils for in-vial analysis of volatile organic compounds. USA Cold Regions Research and Engineering Laboratory, Special Report 96-5.

Hewitt A.D. and S.A. Shoop (1994) Rapid assessment of trichloroethylene in ground water. Ground

Water Monitoring and Remediation, 3: 116–122

Hewitt, A.D., T.F. Jenkins and C.L. Grant (1995)

Collection, handling, and storage: Keys to improved data quality for volatile organic compounds in soil. American Environmental Laboratory, January–February.

Hewitt, A.D., P.H. Miyares, D.C. Leggett and

T.F. Jenkins (1992) Comparison of analytical

methods for determination of volatile organic

compounds. Environmental Science and Technology,

26: 1932–1938.

Illias, A.M. and C. Jaeger (1993) Evaluation of

sampling techniques for the analysis of volatile

and total petroleum hydrocarbons (TRPH) by IR,

GC, and GC/MS methods. Hydrocarbon Contaminated Soils, 3: 147–165. Chelsea, Michigan: Lewis

Publishers.

Pignatello J.J. and B. Xing (1996) Mechanisms of

slow sorption of organic chemicals to natural particles. Environmental Science and Technology, 30(1):

1–11.

Sawhney, B.L. and M.P.N. Gent (1990) Hydrophobicity of clay surfaces: Sorption of 1,2-

dibromoethane and trichloroethylene. Clays and

Minerals, 38: 14–20.

Urban, M.J., J.S. Smith, E.K. Schultz and R.K.

Dickinson (1989) Volatile organic analysis for a

soil, sediment or waste sample. In 5th Annual Waste

Testing and Quality Assurance Symposium, U.S. Environmental Protection Agency, Washington,

D.C., p. II-87–II-101.

U.S. Environmental Protection Agency (1986) Test

Methods for Evaluating Solid Waste. Vol. 1B, SW846.

Wrigley M.J. (1995) Application of a passive soil

vapor survey at a former manufactured gas plant.

Presented at The 27th Mid-Atlantic Industrial and

Hazardous Waste Conference, 9–12 July, Lehigh University, Bethlehem, Pennsylvania.

9
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September 1996

Establishing a Relationship Between Passive Soil Vapor and Grab

Sample Techniques for Determining Volatile Organic Compounds

Alan D. Hewitt

U.S. Army Cold Regions Research and Engineering Laboratory

72 Lyme Road Special Report 96-14

Hanover, New Hampshire 03755-1290

U.S. Army Engineer Waterways Experiment Station

Vicksburg, Mississippi

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16

Contamination In-vial methods Trichloroethylene

Grab samples Soil vapor techniques Volatile organic compounds

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL

A passive soil vapor and an in-vial sample handling and analysis method were compared for estimating

volatile organic compound (VOC) contamination in the near-surface vadose zone. These two methods of

VOC site characterization, although very different operationally, established very similar trends for trichloroethylene (TCE) contamination. The correlation (r2 = 0.944) of the results from these two methods shows a

much better agreement than what has been reported between comparisons of in-vial methods (or solvent

immersion) and conventional soil sample collection and handling methods often used for site characterization activities. The strong correlation between these two methods and from grab samples taken 15 cm apart

indicates that this analyte is homogeneously distributed as compared to metals and semivolatile organic

compounds. For contaminants such as TCE, soil vapor measurement technologies offer a promising means of

estimating subsurface concentrations in locations were grab samples cannot be easily obtained.

For conversion of SI units to non-SI units of measurement consult ASTM Standard E380-93, Standard Practice for Use of the

International System of Units, published by the American Society for Testing and Materials, 1916 Race St., Philadelphia, Pa.

19103.