ASTM D7663-12(2018)e1

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
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Designation: D7663 − 12 (Reapproved 2018)
Standard Practice for
Active Soil Gas Sampling in the Vadose Zone for Vapor
Intrusion Evaluations
This standard is issued under the fixed designation D7663; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
ε NOTE—Reapproved with editorial change in February 2018.
1. Scope 1.8 This practice offers a set of instructions for performing
one or more specific operations. This document cannot replace
1.1 Purpose—This practice covers standardized techniques
educationorexperienceandshouldbeusedinconjunctionwith
for actively collecting soil gas samples from the vadose zone
professional judgment. Not all aspects of this practice may be
beneath or near dwellings and other buildings.
applicable in all circumstances. This ASTM practice is not
1.2 Objectives—Objectives guiding the development of this
intended to represent or replace the standard of care by which
practice are: (1) to synthesize and put in writing good com-
the adequacy of a given professional service must be judged,
mercialandcustomarypracticeforactivesoilgassampling,(2)
nor should this document be applied without consideration of
toprovideanindustrystandardforsoilgassamplingperformed
a project’s many unique aspects. The word “Standard” in the
in support of vapor intrusion evaluations that is practical and
title means only that the document has been approved through
reasonable.
the ASTM consensus process.
1.3 This practice allows a variety of techniques to be used
1.9 This international standard was developed in accor-
for collecting soil gas samples because different techniques
dance with internationally recognized principles on standard-
may offer certain advantages for specific applications. Three
ization established in the Decision on Principles for the
techniquesarepresented:samplingatdiscretedepths,sampling
Development of International Standards, Guides and Recom-
over a small screened interval, and sampling using permanent
mendations issued by the World Trade Organization Technical
vapor monitoring wells.
Barriers to Trade (TBT) Committee.
1.4 Some of the recommendations require knowledge of
2. Referenced Documents
pressure differential and tracer gas concentration measure-
ments.
2.1 ASTM Standards:
D653 Terminology Relating to Soil, Rock, and Contained
1.5 The values stated in SI units shall be regarded as
Fluids
standard. Other units are shown for information only.
D854 Test Methods for Specific Gravity of Soil Solids by
1.6 Thispracticedoesnotaddressrequirementsofanylocal,
Water Pycnometer
regional, state, provincial, or national regulations or guidance,
D1356 Terminology Relating to Sampling and Analysis of
or both, with respect to soil gas sampling. Users are cautioned
Atmospheres
that local, regional, state, provincial, or national guidance may
D1946 Practice for Analysis of Reformed Gas by Gas
impose specific requirements that differ from those of this
Chromatography
practice.
D2216 Test Methods for Laboratory Determination ofWater
1.7 This standard does not purport to address all of the
(Moisture) Content of Soil and Rock by Mass
safety concerns, if any, associated with its use. It is the
D2487 Practice for Classification of Soils for Engineering
responsibility of the user of this standard to establish appro-
Purposes (Unified Soil Classification System)
priate safety, health, and environmental practices and deter-
D3404 Guide for Measuring Matric Potential in Vadose
mine the applicability of regulatory limitations prior to use.
Zone Using Tensiometers
D4696 Guide for Pore-Liquid Sampling from the Vadose
This practice is under the jurisdiction of ASTM Committee D18 on Soil and
Rock and is the direct responsibility of Subcommittee D18.21 on Groundwater and
Vadose Zone Investigations. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Feb. 15, 2018. Published July 2018. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 2011. Last previous edition approved in 2011 as D7663–11. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/D7663-12R18E01. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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D7663 − 12 (2018)
Zone (Withdrawn 2017) 3.3.6 blank sample, n—a sample that is intended to contain
D4700 Guide for Soil Sampling from the Vadose Zone none of the analytes of interest and which is subjected to the
D5088 Practice for Decontamination of Field Equipment usual analytical or measurement process to establish a zero
Used at Waste Sites baseline or background value; blank samples are named
D5092 Practice for Design and Installation of Groundwater according to their type and use (for example, field blank, trip
Monitoring Wells blank, equipment blank, reagent blank).
D5314 Guide for Soil Gas Monitoring in the Vadose Zone
3.3.7 contaminant, n—substances not normally found in an
(Withdrawn 2015)
environment at the observed concentration.
D5466 Test Method for Determination of Volatile Organic
3.3.8 dead volume, n—the total air-filled internal volume of
Compounds in Atmospheres (Canister Sampling Method-
the sampling system.
ology)
3.3.9 duplicate samples, n—two samples taken from and
D5504 TestMethodforDeterminationofSulfurCompounds
representative of the same population and carried through all
in Natural Gas and Gaseous Fuels by Gas Chromatogra-
steps of the sampling and analytical procedures in an identical
phy and Chemiluminescence
manner.
D6196 Practice for Choosing Sorbents, Sampling Param-
eters and Thermal Desorption Analytical Conditions for
3.3.10 effective porosity, n—the amount of interconnected
Monitoring Volatile Organic Chemicals in Air
voidspace(withinintergranularpores,fractures,openings,and
D6725 Practice for Direct Push Installation of Prepacked
the like) available for fluid movement: generally less than total
Screen Monitoring Wells in Unconsolidated Aquifers
porosity.
E741 Test Method for Determining Air Change in a Single
3.3.11 equipment blank, n—a sample of the gas which is
Zone by Means of a Tracer Gas Dilution
used to purge the sampling equipment between uses; sampling
E2024 Test Methods for Atmospheric Leaks Using a Ther-
equipmentblanksareusedtocheckthecleanlinessofsampling
mal Conductivity Leak Detector
devices and the thoroughness of the cleaning procedure.
F1815 Test Methods for Saturated Hydraulic Conductivity,
3.3.12 field blank, n—unused media carried to the sampling
Water Retention, Porosity, and Bulk Density of Athletic
site, exposed to sampling conditions (for example, connected
Field Rootzones
tothesamplinglines)andreturnedtothelaboratoryandtreated
3. Terminology
as an environmental sample; field blanks are used to check for
analytical artifacts or background contaminants or both intro-
3.1 This section provides definitions and descriptions of
duced by sampling and analytical procedures.
terms used in or related to this practice.Alist of acronyms and
a list of symbols also are included. The terms are an integral
3.3.13 fracture, n—a break in the mechanical continuity of
part of this practice and are critical to an understanding of the
a body of rock or soil caused by stress exceeding the strength
practice and its use.
of the rock or soil. Includes joints and faults.
3.2 Definitions:
3.3.14 free product, n—organic contaminants in the liquid
3.2.1 For definitions of common technical terms in this (“free” or non-aqueous) phase.
standard, refer to Terminology D653.
3.3.15 ground water, n—thepartofthesubsurfacewaterthat
3.3 Definitions of Terms Specific to This Standard: is in the saturated zone.
3.3.1 active sampling, n—a means of collecting a gas-phase
3.3.16 liquid phase, n—contaminant residing as a liquid in
substance that employs a mechanical device such as a pump or
vadose zone pore space, often referred to as “free product.”
vacuum assisted critical orifice to draw air into or through a
3.3.17 moisture content, n—the amount of water lost from a
sampling device.
soil upon drying to a constant weight, expressed as the weight
3.3.2 adsorption, n—a physical process in which molecules
per unit weight of dry soil or as the volume of water per unit
or gas, of dissolved substances, or of liquids adhere in an
bulk volume of the soil.
extremely thin layer to the surfaces of solid bodies with which
3.3.18 passive sampling, n—a means of collecting an air-
they are in contact.
borne substance that depends on gaseous diffusion, gravity, or
3.3.3 ambient air, n—any unconfined portion of the atmo-
other unassisted means to bring the sample to the collection
sphere; open air.
surface of sorbent.
3.3.4 attenuation factor (α), n—ratio of indoor air concen-
3.3.19 partitioning, n—the act or process of distributing a
tration to soil-gas concentration for a given compound.
chemical among different phases or compartments.
3.3.5 background level, n—the concentration of a substance
3.3.20 perched aquifer, n—a lens of saturated soil above the
that is typically found in ambient air (for example, due to
mainwatertablethatformsontopofanisolatedgeologiclayer
industrial or automobile emissions), indoor air (for example,
of low permeability.
from building materials or indoor activities) or the natural
3.3.21 permeability, n—the ease with which a porous me-
geology of an area.
dium can transmit a fluid under a potential gradient.
3.3.22 preferential pathway, n—a migration route for
The last approved version of this historical standard is referenced on
www.astm.org. chemicals of concern that has less constraint on gas transport
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D7663 − 12 (2018)
thanthesurroundingsoil;preferentialpathwaysmaybenatural 3.3.38 water table, n—the top of the saturated zone in an
(for example, vertically fractured bedrock where the fractures unconfined aquifer.
areinterconnected)orman-made(forexample,utilityconduits,
3.4 Acronyms and Abbreviations:
sewers, dry wells).
3.4.1 BLS—Below Land Surface (also known as below
3.3.23 porosity, n—the volume fraction of a rock or uncon- ground surface [bgs])
solidated sediment not occupied by solid material but usually
3.4.2 HDPE—High density polyethylene tubing
occupied by liquids, vapor, or air, or combinations thereof.
3.4.3 OD—Outer Diameter
Porosity is the void volume of soil divided by the total volume
3.4.4 PEEK—Polyetheretherketone
of soil.
3.4.5 PTFE—Polytetrafluoroethylene
3.3.24 purge volume, n—the amount of air removed from
thesamplingsystempriortothestartofsamplecollection.This
3.4.6 ppbv—part-per-billion on a volume basis
is usually referred to in number of dead volumes.
3.4.7 PRT—post-run tubing
3.3.25 reagent blank, n—sample of one or more reagents
3.4.8 QC—Quality Control
used in a given analysis.
3.4.9 SVOC—Semi-Volatile Organic Compound
3.3.26 saturated zone, n—the zone in which all of the voids
3.4.10 TO—Toxic Organic
in the rock or soil are filled with water at a pressure that is
3.4.11 USEPA—United States Environmental Protection
greater than atmospheric; the water table is the top of the
Agency
saturated zone in an unconfined aquifer.
3.4.12 VOC—Volatile Organic Compound
3.3.27 semi-volatile organic compound (SVOC), n—organic
compounds with boiling points typically in the range 240-260 3.5 Symbols
to 380-400 °C with polar compounds in the higher range. 3.5.1 Variables (typical units)
3.5.1.1 C = concentration (ppbv, µg/m,%)
3.3.28 soil gas, n—vadose zone atmosphere. Soil gas is the
3.5.1.2 C = detection limit concentration (µg/m )
DL
air existing in void spaces in the soil between the groundwater
3.5.1.3 d = diameter (cm)
table and the ground surface.
3.5.1.4 L = length (cm)
3.3.29 soil moisture, n—the water contained in the pore
3.5.1.5 M = mass (µg)
spaces in the vadose zone.
3.5.1.6 n = number of data points
3.3.30 sorbent sampling, n—the collection of an air sample
3.5.1.7 Q = flow rate (cm /min)
viaremovalofchemicalsfromagasbypassingthegasthrough
3.5.1.8 t = time (min)
or allowing it to come in contact with a sorptive medium; the
3.5.1.9 V = volume (cm )
chemicals are subsequently desorbed for analysis.
3.5.1.10 X = molecular weight of compound X (g/mol)
MW
3.5.1.11 α=attenuationcoefficientorfactor(dimensionless)
3.3.31 sub-slab vapor sampling, n—the collection of vapor
3.5.1.12 ∆P = change in pressure (Pa)
from the zone just beneath the lowest floor slab of a building.
3.5.1.13 τ = residence time (min)
3.3.32 tracer, n—a material that can be easily identified and
3.6 Superscripts
determined even at very low concentrations and that may be
3.6.1 — = mean value
added to other substances to enable their movements to be
followed or their presence to be detected.
3.7 Subscripts
3.7.1 i = pertaining to compound, time, or location i
3.3.33 tracer gas, n—a gas used with a detection device to
determinetherateofairinterchangewithinaspace,orbetween
4. Summary of Practice
spaces.
3.3.34 trip blank, n—clean, unused sampling media that is 4.1 This practice describes the active collection of soil gas
samples from soil pore spaces in the vadose zone or in fill
carried to the sampling site and transported to the laboratory
for analysis without having been exposed to sampling proce- material directly under building slabs to determine the concen-
tration of volatile organic compounds (VOCs). Three tech-
dures.
niques are presented: (1) sampling at discrete depths, (2)
3.3.35 vadose zone, n—hydrogeological region extending
sampling over a small screened interval, or (3) sampling using
from the soil surface to the top of the principal water table.
permanent vapor monitoring wells with one or more screened
Perched ground water may exist within this zone.
intervals. For sampling at a given depth, options include (i) a
3.3.36 vapor intrusion, n—the migration of a volatile
short stainless steel probe installed in a small diameter hole
chemical(s) from subsurface soil or water into an overlying or
drilled through building slab, (ii) disposable drive tips and
nearby building.
post-run tubing (PRT), or (iii) installation of sampling points
3.3.37 volatile organic compound (VOC), n—organic com- using tubing placed into a borehole and sealed in place with
poundswithboilingpointstypicallyrangingfromalowerlimit clay or other packing material. Several different combinations
between 50 °C and 100 °C, and an upper limit between 240 °C of equipment and materials can be used to actively collect soil
and 260 °C, where the upper limits represent mostly polar gas samples, and this practice is intended to allow all methods
compounds. thattypicallyresultinrepresentativeandreproduciblesamples.
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D7663 − 12 (2018)
Other techniques for assessing soil vapor concentrations exist been sampled, it may be possible to collect a representative
(for example, passive sampling), but are outside the scope of sample after a smaller volume of gas is purged, but the volume
this practice. The design of soil gas sampling programs (for of gas in the probe tubing or pipe must be purged at a
example, the number and location of samples necessary to minimum.Itisrecommendedthataminimumofthree(3)dead
characterize a site) also is outside the scope of this practice. volumes be purged from the sampling system immediately
Table 1 summarizes the key design aspects for the most prior to sample collection. Larger purge volumes typically are
common techniques. Examples of various installation ap- not necessary to achieve stable readings and should be avoided
proaches are shown in Fig. 1. forshallowerprobesorifthepotentialexiststhattheadditional
purging will affect the partitioning of the VOCs in the
4.2 Choice of Technique—In choosing a technique for
subsurface. Larger purge (and sample collection) volumes can
collecting and measuring soil gas concentrations, the user
result in migration of soil gas from locations some distance
shouldconsiderthestudyobjectives,sitegeology,chemicalsof
from the sampling probe. Preferential pathways within the soil
interest, target concentrations, type of building and its
may exist and so the uncertainty associated with the origin of
construction, potential for preferential pathways to be present,
the soil gas will tend to increase with increasing purge (and
potential for long-term or repeat sampling, the comparative
sample) volumes. The data, however, should still be represen-
capabilities of the techniques, and the complexity of the
tative of how VOCs will migrate in these subsurface condi-
equipment and procedures.
tions.
5.5 Effect of Sampling Rate—The faster the rate of
5. Significance and Use
sampling, the larger the pressure differential (that is, vacuum)
5.1 Soil-gassamplingresultscanbedependentonnumerous
that is induced at the point(s) where soil gas enters the
factors both within and outside the control of the sampling
sampling system. The relationship between the flow rate and
personnel. Key variables are identified and briefly discussed
the vacuum is primarily dependent on the gas-permeability of
below. Please see the documents listed in the Bibliography for
the subsurface materials. This pressure differential has the
more detailed information on the effect of various variables.
potential to affect the partitioning of the VOCs in the subsur-
5.2 Application—The techniques described in this standard
faceiftheVOCsexistintwoormorephases(forexample,free
practice are suitable for collecting samples for subsequent
phase,dissolvedphase,gasphase,sorbedontosoilparticles)at
analysis for VOCs by US EPA Method TO-15, US EPA
or near the sampling depth (for example, within1mofthe
Method TO-17, Test Method D5466, Practice D6196, ISO
sample probe ). Sampling at relatively high rates (for example,
16017-1, or other VOC methods (for example, US EPA
>200 mL/min) has the potential to introduce a positive bias to
Methods TO-3 and TO-12). In general, off-site analysis is
the results (that is, make the results more conservative). The
employed when data are needed for input to a human health
magnitude of any such bias is believed to be at most a factor of
risk assessment and low- or sub-ppbv analytical sensitivity is
two. If the sampling depth is not near the source of the vapors,
required. On-site analysis typically has lesser analytical sensi-
faster sampling rates (or larger sampling volumes) are not
tivityandtendstobeemployedforscreeninglevelstudies.The
expected to have a significant effect on data quality.
techniques also may prove useful for analytical categories
5.6 Effect of Induced Vacuum—If desired, the induced
other than VOCs, such as methane, ammonia, mercury, or
vacuum can be limited by some upper bound value (for
hydrogen sulfide (See Test Method D5504).
example, 2500 Pa [10 in. of water column]). The induced
5.3 Limitations:
vacuum, however, is dependent on variables such as soil
5.3.1 This method only addresses collection of gas-phase
moisture as well as length and internal diameter of sampling
species. Less volatile compounds, such as SVOCs, may be
line that may not be under the control of the user. Most
present in the environment both in the gas phase and sorbed
significantly,theuseofanupperlimitforinducedvacuummay
onto particulate matter, as well as in liquid phase. In soil gas,
preclude the use of preset flow control devices that allow
thegas-phasefractionistheprimaryconcern.Inotherpotential
unattended sample collection into evacuated canisters.
sampling locations (for example, ambient or indoor air),
5.7 Effect of System Volume and Length of Tubing—The
however, sampling for the particulate phase fraction may also
system volume should be relatively small to minimize the
be of interest.
volume of dead space that must be removed prior to sampling.
5.3.2 The data produced using this method should be
In practice, this typically means that 3-mm or 6-mm ( ⁄8 or
representative of the soil gas concentrations in the geological
⁄4-in.)ODtubingisusedforshallowprobes.Fordeeperprobes
materials in the immediate vicinity of the sample probe or well
(for example, ≥10 m), larger diameter installations may be
at the time of sample collection (that is, they represent a
preferabletominimizepotentialforpluggingovertime.Larger
point-in-time and point-in-space measurement). The degree to
diameter probes and tubing also may be needed for large
which these data are representative of any larger areas or
different times depends on numerous site-specific factors.
5.4 Effect of Purging of Dead Space—If a soil gas probe is
Hartman, B., B.A. Schumacher, J. Zimmerman, D.S. Springer, R.J. Elliott, and
M.C. Rigby. Results from EPA Funded Research Programs on the Importance of
to be sampled soon after installation, the gas within the probe
PurgeVolume, SampleVolume, Sample Flow Rate andTemporalVariations on Soil
and any sand pack will consist mostly of atmospheric air. This
GasConcentrations, Proceedings of Vapor Intrusion: Learning from the Challenges,
air must be purged before soil gas that is representative of the
Sponsored by theAir & Waste ManagementAssociation (AWMA). Providence, RI.
geologic materials can be obtained. If the probe has previously September 26-28, 2007
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D7663 − 12 (2018)
TABLE 1 Comparison of Installation Options for Soil-Gas Sampling
Direct-Push Probe or
Options Well with
Screened Annular
A
Topic Sub-Slab Probe Drive Point Interval Seal
Installation Hammer drill and5–15cm Direct-push Direct-push Hollow-stem
method stainless-steel tube rig with sacrificial rig with sacrificial auger, direct push
drive point drive point with coring
Typical Bottom 1.5 1.5 1.5
minimum of slab
sampling
depth BLS (m)
B
Typical None 2.5–5 15 –30 Can be customized
length of to any length. Typically at
sampling least 15 cm
depth
interval (cm)
C
Type of seal Clay, cement, wax, Gasket at bottom Clay layer Clay throughout
PTFE tape of rods. directly above the borehole annulus
Clay cap at screened interval
ground surface
Potential Low Low Very low Very low
for dilution
of sample by
D
ambient air
Typical 3 void volumes 3 void volumes 3 void volumes 3 void volumes
purge volume
Potential for Low Moderate Very low Very low
dilution of
sample by
soil gas from
depths other than
the sampling
depth interval
Potential Very low Low Moderate Low
E
for smearing
Potential Very low Moderate Low Low
for plugging
between uses
Suitability Seals may lose integrity Not typically used Not typically used Suitable for
for multiple over time more than once more than once multiple uses
uses
A
The type of wells described here have a screened interval that is isolated from the remainder of the monitoring well and connected to the ground surface via small-bore tubing. The use of traditional groundwater monitoring
wells for soil-gas monitoring is possible if the screened interval extends up into the vadose zone, but such wells will have a relatively large dead space volume and therefore require purging of relatively large gas volumes.
B
Sample is drawn from the preferentially permeable materials beneath the slab. The thickness of permeable materials varies and may not be known for a given site.
C
The term “clay” here refers to use of applied material such as hydrated bentonite and not native in-situ material.
D
All sampling options have an equal probability of ambient air dilution due to leaking fittings or tubing in the portion of the sampling train above the ground surface.
E
Screened interval or pipe opening may become plugged during installation due to smearing of soil.

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D7663 − 12 (2018)
FIG. 1 Illustration of Various Installation Options: A—Sub-Slab Probe, B—Direct-Push Drive Point, C—Direct-Push Screened Interval,
and D—Well with Annular Seal
volumesub-slabsampling.Thelengthofanytubingusedinthe less. When the ambient temperature is less than the soil gas
above-ground sample collection train also should be kept to a temperature, collecting samples at or near the maximum
minimum. If the ambient air temperature is less than the bulk
obtainable flow rate for a given location will minimize the
soil temperature, condensation may form in the above-ground
potential for condensation.
sampling lines and remove polar compounds from the sample
5.8 Effect of Connections and Fittings—The number of
stream. The potential is greater if excess tubing is present, so
connections and fittings also should be kept to a minimum, as
the length of tubing extending from the probe or well to
these represent potential points for leaks to occur. If possible,
connect to the sampling device should be kept to a meter or
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D7663 − 12 (2018)
and objective testing/sampling/inspection/etc. Users of this standard are
all connections should be made above ground and visually
cautioned that compliance with Practice D7663 does not in itself assure
inspected. For direct push approaches, this requires that slotted
reliable results. Reliable results depend on many factors; Practice D7663
drive caps and pull caps be used, to allow the tubing connec-
provides a means of evaluating some of those factors.
tion to the PRT adapter or implant to be made above ground
prior to probe installation. All fittings shall be leak checked
6. Materials and Manufacture
prior to use (See 7.3.1).
6.1 The procedures given in this practice are applicable to a
5.9 Effect of Annular Seal—Soil gas probes installed in an
widerangeofcommerciallyavailableequipmentandapparatus
augeredorcoredholewithathickslurryofbentoniteandwater
thataremarketedforuseinsoilgassamplingorcanbeadapted
in the borehole annulus above the sand pack have the least risk
for such use. General recommendations related to materials
of atmospheric air leakage down the borehole annulus or
and manufacture are given below.
cross-communication of soil gas between different intervals
6.2 All surfaces in contact with the soil gas sample should
during purging and sampling. This relative advantage com-
be clean, dry, and inert.All materials of construction should be
pared with other techniques is most apparent for geologic
stainless-steel, glass, PEEK, or PTFE. Other materials may be
materials with relatively low gas permeability.
substituted, but performance testing should be performed via
equipment blank samples and adsorption studies to verify that
5.10 Effect of Porosity—The effective porosity of a soil may
be different than the total porosity. Large spaces (“macro the material does not introduce a positive or negative bias to
pores”)suchasfracturesinfine-grainedsoilscanimpartahigh measured concentrations.
permeability to materials that would otherwise have a low
6.3 No metal parts contaminated with cutting oils should be
permeability. The emplacement of sampling probes in soil can
used if they will come in contact with the soil gas sample.
cause compression or closure of macropores, resulting in a
6.4 Flexibleorlow-densitytubingshouldnotbeusedinline
lower yield of soil gas than would otherwise occur through the
with canister or sorbent sampling devices. In particular, prob-
uncompressed soil or formation.
lems with polyethylene tubing have been reported. Short
5.11 Effect of Soil Moisture—The diffusion of vapors from
sections(thatis,≤10cm)ofTygonorothertubingmaybeused
subsurface sources to the vicinity of the sampling probe is
to temporarily connect portable field analyzers to the sampling
dependent on the presence of interconnected and air-filled
system (but the pressure drop caused by the sampling system
poreswithinthesoilcolumn.Therefore,soilmoisturecanhave
may affect the accuracy of the field analyzer).
a significant effect on the measurements. Increasing soil
6.5 Only compression fittings demonstrated to be leak-free
moisture levels will reduce the flux of contaminants through
at vacuums up to 101 000 Pa (1 atm) should be used. Vacuum
the soil column and increase partitioning to the dissolved
leak checks (see 7.3.1) are recommended for all fittings,
phase. As a result, the measured soil gas concentration within
regardless of brand. Never use PTFE tape with compression
the vadose zone will differ in areas of high soil moisture than
fittings.
for areas with low soil moisture. Knowledge of the soil
6.6 The use of granulated bentonite is preferred over pow-
moisture conditions is necessary in properly interpreting soil
der or large chips to seal off probes at the ground surface or to
gas results and may be useful for comparing results from
provideanin-groundsealabovethesamplingdepth.Hydration
multiple rounds of sampling performed at a site.
of the bentonite i
...