ASTM E2993-23

Standard Guide for Evaluating Potential Hazard in Buildings as a Result of Methane in the Vadose Zone

SIGNIFICANCE AND USE
5.1 Several different factors should be taken into consideration when evaluating methane hazard, rather than, for example, use of a single concentration-based screening level as a de-facto hazard assessment level. Key variables are identified and briefly discussed in this section. Legal background information is provided in Appendix X3. The Bibliography includes references where more detailed information can be found on the effect of various parameters on gas concentrations.  
5.2 Application—This guide is intended for use by those undertaking an assessment of hazards to people and property as a result of subsurface methane suspected to be present based on due diligence or other site evaluations (see 6.1.1).  
5.2.1 This guide addresses shallow methane, including its presence in the vadose zone; at residential, commercial, and industrial sites with existing construction; or where development is proposed.  
5.3 This guide provides a consistent, streamlined process for deciding on action and the urgency of action for the identified hazard. Advantages include:  
5.3.1 Decisions are based on reducing the actual risk of adverse impacts to people and property.  
5.3.2 Assessment is based on collecting only the information that is necessary to evaluate hazard.  
5.3.3 Available resources are focused on those sites and conditions that pose the greatest risk to people and property at any time.  
5.3.4 Response actions are chosen based on the existence of a hazard and are designed to mitigate the hazard and reduce risk to an acceptable level.  
5.3.5 The urgency of initial response to an identified hazard is commensurate with its potential adverse impact to people and property.  
5.4 Limitations—This guide does not address potential hazards from other gases and vapors that may also be present in the subsurface such as hydrogen sulfide, carbon dioxide, and/or volatile organic compounds (VOCs) that may co-occur with methane. If the presence of hydrogen sulfide or other...
SCOPE
1.1 This guide provides a consistent basis for assessing methane in the vadose zone, evaluating hazard and risk, determining the appropriate response, and identifying the urgency of the response.  
1.2 Purpose—This guide covers techniques for evaluating potential hazards associated with methane present in the vadose zone beneath or near existing or proposed buildings or other structures (for example, potential fires or explosions within the buildings or structures), when such hazards are suspected to be present based on due diligence or other site evaluations (see 6.1.1). Buildings in this context include normal below grade utilities associated with a building.  
1.3 Objectives—This guide: (1) provides a practical and reasonable industry standard for evaluating, prioritizing, and addressing potential methane hazards based on mass flow and (2) provides a tool for screening out low-risk sites.  
1.4 This guide offers a set of instructions for performing one or more specific operations. This guide cannot replace education or experience and should be used in conjunction with professional judgment. Not all aspects of this guide may be applicable in all circumstances. This guide is not intended to represent or replace the standard of care by which the adequacy of a given professional service should be judged, nor should this guide be applied without consideration of a project's many unique aspects. The word “Standard” in the title means only that the guide has been approved through the ASTM International consensus process.  
1.5 Not addressed by this guide are:  
1.5.1 Requirements or guidance or both with respect to methane sampling or evaluation in federal, state, or local regulations. Users are cautioned that federal, state, and local guidance may impose specific requirements that differ from those of this guide;  
1.5.2 Safety concerns, if any, associated with its use. It is the responsibility of the user of this sta...

General Information

• Status: Published
• Publication Date: 31-Aug-2023
• ICS: 13.080.05 - Examination of soils in general
• Technical Committee: E50 - Environmental Assessment, Risk Management and Corrective Action
• Drafting Committee: E50.02 - Real Estate Assessment and Management

Relations

• Replaces: ASTM E2993-16 - Standard Guide for Evaluating Potential Hazard as a Result of Methane in the Vadose Zone
• Effective Date: 01-Sep-2023
• Referred By: ASTM E3382-23 - Standard Guide for Developing Representative Background Concentrations at Sediment Sites - Framework Overview, Including Conceptual Site Model Considerations
• Effective Date: 01-Sep-2023
• Referred By: ASTM E3361-22 - Standard Guide for Estimating Natural Attenuation Rates for Non-Aqueous Phase Liquids in the Subsurface
• Effective Date: 01-Sep-2023
• Referred By: ASTM E3300-21 - Standard Guide for NAPL Mobility and Migration in Sediment- Evaluating Ebullition and Associated NAPL/Contaminant Transport
• Effective Date: 01-Sep-2023

Overview

ASTM E2993-23 is the Standard Guide for Evaluating Potential Hazard in Buildings as a Result of Methane in the Vadose Zone, established by ASTM International. The standard provides a comprehensive and consistent framework for assessing site-specific hazards and risks associated with subsurface methane-specifically in the vadose zone-beneath or near existing or proposed structures. This guide is primarily intended to help professionals evaluate potential hazards such as fire or explosion risks that methane migration can pose to buildings and occupants, offering a process for determining appropriate response actions and prioritizing risk management.

Methane is a flammable gas that can accumulate in confined spaces, posing serious safety hazards. ASTM E2993-23 helps users differentiate between low-risk situations and those requiring immediate mitigation or further investigation, promoting informed action and resource allocation.

Key Topics

• Comprehensive Hazard Evaluation
  The standard emphasizes the importance of considering multiple factors rather than relying solely on methane concentration levels for hazard assessment. It recommends taking into account mass flow, site-specific conditions, soil characteristics, and potential migration pathways.

• Three-Tiered Screening Approach
  A stepwise, tiered evaluation is provided to streamline site assessments. The process begins with a basic Conceptual Site Model (CSM) using existing data, followed by refined and, if necessary, specialized evaluations based on identified data gaps or site complexities.

• Decision Matrix for Risk Management
  Users are guided to employ a matrix combining methane concentrations, differential pressure measurements, and gas volume estimates to assess current and potential risks-helping to prioritize which sites or scenarios require urgent attention.

• Site-Specific Variables
  Methane hazard evaluations must consider land use, building types, subsurface utility lines, soil types, permeability, and environmental variables such as precipitation and barometric pressure.

• Limitations of the Guide
  The standard does not address hazards from other subsurface gases (such as hydrogen sulfide or VOCs), and does not substitute legal or regulatory requirements.

Applications

ASTM E2993-23 is essential for professionals involved in:

• Environmental site assessments during real estate transactions or prior to development
• Due diligence evaluations for property transfer, refinancing, or permitting
• Building safety and risk management for existing structures, utilities, and proposed developments
• Site remediation and mitigation planning, including the design of methane mitigation systems

Typical users include environmental consultants, engineers, risk assessors, and property managers tasked with identifying and reducing methane-related hazards in residential, commercial, or industrial contexts.

By standardizing the hazard evaluation process, ASTM E2993-23 helps:

• Quickly identify and screen out low-risk sites for efficient resource allocation
• Provide defensible documentation for decision-making and regulatory compliance
• Promote targeted investigation and mitigation where the risk to people and property is greatest
• Tailor safety responses proportional to the potential hazard and urgency

Related Standards

ASTM E2993-23 complements and references several related standards and guidance documents:

• ASTM D7663 – Practice for Active Soil Gas Sampling in the Vadose Zone
• ASTM E2600 – Guide for Vapor Encroachment Screening
• BSI BS 8576 & BS 8485 – British Standards for ground gas investigation and mitigation
• EPA & ITRC Vapor Intrusion Guidance
  Documents addressing general vapor intrusion pathways and corrective actions.

Users should also consider local, state, and federal regulations that may dictate specific methane assessment or mitigation requirements beyond what this guide provides.

For a robust methane risk management strategy, refer to ASTM E2993-23 alongside these supporting standards to ensure a comprehensive and compliant approach to subsurface methane hazard evaluation in buildings.

Standards Content (Sample)

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.
Designation: E2993 − 23
Standard Guide for
Evaluating Potential Hazard in Buildings as a Result of
Methane in the Vadose Zone
This standard is issued under the fixed designation E2993; 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.
1. Scope 1.5.2 Safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
1.1 This guide provides a consistent basis for assessing
priate safety and health practices and determine the applicabil-
methane in the vadose zone, evaluating hazard and risk,
ity of regulatory limitations prior to use;
determining the appropriate response, and identifying the
1.5.3 Emergency response situations such as sudden rup-
urgency of the response.
tures of gas lines or pipelines;
1.2 Purpose—This guide covers techniques for evaluating
1.5.4 Methane entry into an enclosure from other than
potential hazards associated with methane present in the
vadose zone soils (for example, methane evolved from well
vadose zone beneath or near existing or proposed buildings or
water brought into an enclosure; methane generated directly
other structures (for example, potential fires or explosions
within the enclosure; groundwater intrusion, methane from
within the buildings or structures), when such hazards are
leaking natural gas lines or appliances within the enclosure,
suspected to be present based on due diligence or other site
direct migration into buildings from mine entries, etc.);
evaluations (see 6.1.1). Buildings in this context include
1.5.5 Methane entry into an enclosure situated atop or
normal below grade utilities associated with a building.
immediately adjacent to a municipal solid waste (MSW)
1.3 Objectives—This guide: (1) provides a practical and
landfill or a Construction and Demolition (C&D) landfill;
reasonable industry standard for evaluating, prioritizing, and
1.5.6 Methane from oil & gas reservoirs, injection wells, or
addressing potential methane hazards based on mass flow and
other sources potentially under high pressures relative to
(2) provides a tool for screening out low-risk sites.
typical vadose zone pressures;
1.4 This guide offers a set of instructions for performing one
1.5.7 Methane risk during construction activities, work in
or more specific operations. This guide cannot replace educa-
trenches, and confined space work (which are all best ad-
tion or experience and should be used in conjunction with
dressed via real-time monitoring);
professional judgment. Not all aspects of this guide may be
1.5.8 Potential hazards from other gases and vapors that
applicable in all circumstances. This guide is not intended to
may also be present in the subsurface such as hydrogen sulfide,
represent or replace the standard of care by which the adequacy
carbon dioxide, and/or volatile organic compounds (VOCs);
of a given professional service should be judged, nor should
1.5.9 Anoxic conditions in enclosed spaces;
this guide be applied without consideration of a project’s many
1.5.10 The forensic determination of methane source; or
unique aspects. The word “Standard” in the title means only
1.5.11 Potential consequences of fires or explosions in
that the guide has been approved through the ASTM Interna-
enclosed spaces or other issues related to safety engineering
tional consensus process.
design of structures or systems to address fires or explosions.
1.5 Not addressed by this guide are:
1.6 Units—The values stated in SI units are to be regarded
1.5.1 Requirements or guidance or both with respect to
as the standard.
methane sampling or evaluation in federal, state, or local
1.6.1 Exception—Values in inch/pound units are provided
regulations. Users are cautioned that federal, state, and local
for reference.
guidance may impose specific requirements that differ from
those of this guide;
1.7 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
1 priate safety, health, and environmental practices and deter-
This guide is under the jurisdiction of ASTM Committee E50 on Environmental
Assessment, Risk Management and Corrective Action and is the direct responsibil-
mine the applicability of regulatory limitations prior to use.
ity of Subcommittee E50.02 on Real Estate Assessment and Management.
1.8 This international standard was developed in accor-
Current edition approved Sept. 1, 2023. Published November 2023. Originally
dance with internationally recognized principles on standard-
approved in 2016. Last previous edition approved in 2016 as E2993–16. DOI:
10.1520/E2993–23 ization established in the Decision on Principles for the
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2993 − 23
Development of International Standards, Guides and Recom- New South Wales (NSW) EPA. Assessment and Manage-
mendations issued by the World Trade Organization Technical ment of Hazardous Ground Gases. May 2020.
Barriers to Trade (TBT) Committee. 29 CFR 1910.146 Permit-Required Confined Spaces
2. Referenced Documents 3. Terminology
3.1 Definitions:
2.1 ASTM Standards:
D653 Terminology Relating to Soil, Rock, and Contained 3.1.1 This section provides definitions and descriptions of
terms used in or related to this guide. An acronym list is also
Fluids
D1356 Terminology Relating to Sampling and Analysis of included. The terms are an integral part of this guide and are
Atmospheres critical to an understanding of the guide and its use.
3.1.2 advection, n—transport of molecules along with the
D1946 Practice for Analysis of Reformed Gas by Gas
Chromatography flow of a greater medium as occurs because of differential
pressures.
D2216 Test Methods for Laboratory Determination of Water
(Moisture) Content of Soil and Rock by Mass
3.1.3 ambient air, n—any unconfined portion of the atmo-
D2487 Practice for Classification of Soils for Engineering
sphere; open air.
Purposes (Unified Soil Classification System)
3.1.4 barometric lag, n—time difference between changes in
D5088 Practice for Decontamination of Field Equipment
total atmospheric pressure (barometric pressure) and subse-
Used at Waste Sites
quent changes in total gas pressure measured at a specific point
D6725 Practice for Direct Push Installation of Prepacked
in the subsurface.
Screen Monitoring Wells in Unconsolidated Aquifers
3.1.4.1 Discussion—Atmospheric pressure variations in-
D7663 Practice for Active Soil Gas Sampling in the Vadose
clude routine diurnal highs and lows as well as changes
Zone for Vapor Intrusion Evaluations
associated with exceptional meteorological conditions
E2600 Guide for Vapor Encroachment Screening on Prop-
(weather fronts). The time lag means that differential pressure
erty Involved in Real Estate Transactions
between the surface and the subsurface point may be out of
F1815 Test Methods for Saturated Hydraulic Conductivity,
phase and may reverse (6 relative to zero) with resulting
Water Retention, Porosity, and Bulk Density of Athletic
reversals in soil gas flow direction over time between the
Field Rootzones
shallow subsurface and the surface.
2.2 Other Standards:
3.1.5 barometric pumping, n—variation in the ambient at-
British Standards Institution (BSI), Guidance on Investiga-
mospheric pressure that causes motion of vapors in, or into,
tions for Ground Gases, Permanent Gases, and Volatile
porous and fractured earth materials.
Organic Compounds (VOCs). BS8576. 2013
3.1.6 biogas, n—mixture of methane and carbon dioxide
British Standards Institution (BSI), Code of Practice for the
produced by the microbial decomposition of organic wastes,
Design of Protective Measures for Methane and Carbon
also known as microbial gas.
Dioxide Ground Gases for New Buildings. BS
3.1.6.1 Discussion— For the purposes of this standard,
8485:2015+A1:2019
biogas may arise from any organic material not specifically
California DTSC, Evaluation of Biogenic Methane for Con-
excluded in 1.5. The sources addressed include plant material,
structed Fills and Dairies Sites, March 28, 2012
soil organic carbon, and petroleum hydrocarbons from past
CL:AIRE Ground Gas Monitoring and “Worst Case” Con-
releases.
ditions. August 2018
County of Los Angeles Building Code, Volume 1, Title 26,
3.1.7 biogenic, adv—resulting from the activity of living
Section 110 Methane
organisms.
ITRC Document VI-1 Vapor Intrusion Pathway: A Practical
3.1.8 contaminant, n—substance not normally found in an
Guideline
environment at the observed concentration.
ITRC Document PVI-1 Petroleum Vapor Intrusion: Funda-
3.1.9 continuous monitoring, n—measurements of selected
mentals of Screening, Investigation, and Management
parameters performed at a frequency sufficient to define critical
trends, identify changes of interest, and allow for relationships
EPA 530-R-10-003 Conceptual Model Scenarios for the
with other attributes in a predictive capacity.
Vapor Intrusion Pathway
3.1.10 dead volume, n—total air-filled internal volume of
the sampling system.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
3.1.11 differential pressure, n—relative difference in pres-
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
sure between two measurement points (∆P).
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
3.1.11.1 Discussion—∆P measurements are typically the
Available from British Standards Institution (BSI), 389 Chiswick High Rd.,
differences between pressure at some depth in the vadose zone
London W4 4AL, U.K., http://www.bsigroup.com.
4 and pressure above ground at the same location (indoors or
Available from dpw.lacounty.gov.
Available from the Interstate Technology & Regulatory Council, http://
www.itrcweb.org/Documents/VI-1.pdf.
6 7
Available from the Interstate Technology & Regulatory Council, http:// Available from Occupational Safety and Health Administration (OSHA), 200
www.itrcweb.org/PetroleumVI-Guidance/ Constitution Ave., Washington, DC 20210, http://www.osha.gov.
E2993 − 23
outdoors), but also could refer to the difference in pressure advection, molecules are transported along with the flow of a
between two subsurface locations. A ∆P measurement repre- greater medium. With pressure-driven flow, the introduced gas
sents a pressure gradient between the two locations. is the medium.
3.1.24.2 Discussion—In the vadose zone, elevated pressures
3.1.12 diffusion, n—gas transport mechanism in which mol-
in a given volume of soil can occur as a result of biogas
ecules move along a concentration gradient from areas of
generation at that location. Therefore, whether or not a given
higher concentration toward areas of lower concentration;
site has active biogas generation is an important consideration
relatively slow form of gas transport.
in evaluating methane hazard.
3.1.13 effective porosity, n—amount of interconnected void
3.1.25 porosity, n—volume fraction of a rock or unconsoli-
space (within intergranular pores, fractures, openings, and the
dated sediment not occupied by solid material but usually
like) available for fluid movement: generally less than total
occupied by liquids, vapor, and/or air.
porosity.
3.1.25.1 Discussion—Porosity is the void volume of soil
3.1.14 flammable range, n—concentration range in air in
divided by the total volume of soil.
which a flammable substance can produce a fire or explosion
3.1.26 probe, n—device designed to investigate and collect
when an ignition source is present.
information from a remote location.
3.1.14.1 Discussion—The flammable range extends from
3.1.26.1 Discussion—As used in this guide, a point or
the lower flammable limit (LFL) to the upper flammable limit
methane test well used to collect information from within the
(UFL). See Appendix X1, X1.6.
vadose zone or subslab space of a building.
3.1.15 fracture, n—break in the mechanical continuity of a
3.1.27 purge volume, n—amount of air removed from the
body of rock or soil caused by stress exceeding the strength of
sampling system before the start of sample collection.
the rock or soil and includes joints and faults.
3.1.27.1 Discussion—This is usually referred to in terms of
3.1.16 groundwater, n—part of the subsurface water that is
number of dead volumes of probe (test well) casing or test well
in the saturated zone.
plus granular backfill total volume.
3.1.17 hazard, n—source of potential harm from current or
3.1.28 repressurization, n—unpressurized soil vapors can be
future methane exposures.
pressurized by phenomena such as rapidly rising groundwater.
3.1.18 indoor air, n—the mixture of gases within the habit- 3.1.29 risk, n—probability that something will cause injury
able spaces of a building or harm.
3.1.19 microbial, adv—pertaining to or emanating from a 3.1.30 saturated zone, n—zone in which all of the voids in
the rock or soil are filled with water at a pressure that is greater
microbe.
than atmospheric.
3.1.19.1 Discussion—The preferred term for
3.1.30.1 Discussion—The water table is the top of the
nonthermogenic, nonpetrogenic methane such as from anaero-
saturated zone in an unconfined aquifer.
bic activity in shallow soils or sanitary landfills is “microbial.”
3.1.31 soil gas, n—vadose zone atmosphere; soil gas is the
3.1.20 moisture content, n—amount of water lost from a soil
air existing in void spaces in the soil between the groundwater
upon drying to a constant weight expressed as the weight per
table and the ground surface.
unit weight of dry soil or as the volume of water per unit bulk
volume of the soil.
3.1.32 soil moisture, n—water contained in the pore spaces
in the vadose zone.
3.1.21 perched aquifer, n—lens of saturated soil above the
main water table that forms on top of an isolated geologic layer
3.1.33 subslab vapor sampling, v—collection of vapor from
of low permeability.
the zone just beneath the lowest floor slab of a building or
below paving or soil cap.
3.1.22 permeability, n—ease with which a porous medium
can transmit a fluid under a potential gradient. 3.1.34 thermogenic, adj—methane that is generated at depth
under elevated pressure and temperatures during and following
3.1.23 preferential pathway, n—migration route for chemi-
the formation of petroleum (for example, in oil fields).
cals of concern that has less constraint on gas transport than the
surrounding soil. 3.1.35 tracer, n—material that can be easily identified and
determined even at very low concentrations and may be added
3.1.23.1 Discussion—Preferential pathways may be natural
to other substances to enable their movements to be followed
(for example, vertically fractured bedrock where the fractures
or their presence to be detected.
are interconnected) or man-made (for example, utility conduits,
sewers, and dry wells).
3.1.36 tracer gas, n—gas used with a detection device to
determine the rate of air interchange within a space or zone or
3.1.24 pressure-driven flow, n—gas transport mechanism
between spaces or zones.
that occurs along pressure gradients resulting from introduction
of gas into the soil matrix.
3.1.37 vadose zone, n—hydrogeological region extending
from the soil surface to the top of the principal water table.
3.1.24.1 Discussion—The flow of gas is from the region of
high pressure to regions of lower pressure and continues until 3.1.37.1 Discussion—Perched groundwater may exist
the gas pressure is equal or the flowpath is blocked. With within this zone.
E2993 − 23
3.1.38 vapor intrusion, n—migration of a volatile chemi- evidence for a given site. A decision matrix is applied to get an
cal(s) from subsurface soil or water into an overlying or nearby initial prediction of hazard. For sites in which potentially
building or other enclosed space. significant data gaps are identified during the Tier 1 review, the
second tier consists of a refined site evaluation. Additional field
3.1.39 volatile organic compound, VOC, n—an organic
-2
work is performed to address the data gaps. The results are
compound with a saturation vapor pressure greater than 10
compared with the CSM and the CSM revised, as necessary.
kPa at 25°C (Terminology D1356-14).
The decision matrix is again applied to the new, expanded data
3.1.40 water table, n—top of the saturated zone in an
set to get an updated prediction of hazard. If it is determined
unconfined aquifer.
that more data are needed, the third tier consists of a special
3.2 Acronyms and Abbreviations:
case evaluation. For all three tiers, the path forward at any
3.2.1 ACH—air changes per hour
point should respect applicable regulatory guidance and con-
sider risk management principles, technical feasibility, and
3.2.2 CSM—conceptual site model
community concerns.
3.2.3 FID—flame ionization detector
4.2.1 The evaluation process is typically implemented in a
3.2.4 HVAC—heating, ventilation, and air conditioning
tiered approach involving increasingly sophisticated levels of
3.2.5 In. H O—inches of water, a measure of pressure
data collection, analysis, and evaluation. Users may choose to
exerted by a column of water 1 in. (2.54 cm) in height; 1 in.
proceed directly to the most sophisticated tier, to pre-emptive
H O equals approximately 250 Pa
mitigation, or to routine monitoring based on site-specific
circumstances. It is good practice to seal utility openings and
3.2.6 LEL—lower explosive limit (same as lower flammable
plug potential gas entry points for any site with potential for
limit)
methane.
3.2.7 Pa—Pascal, a measure of pressure
4.2.2 For some sites, a limited number of samples may not
3.2.8 ppmv—part per million on a volume basis
be sufficient to address potential hazard because there are (1) a
3.2.9 psi—pounds per square inch large volume (for example, >100 m ) of methane gas and/or
significant potential methane source(s) at or nearby the site (for
3.2.10 QA/QC—quality assurance/quality control
example, a large mass (for example, >500 m ) of buried
3.2.11 UEL—upper explosive limit (same as upper flam-
organic matter such as plants, wood, etc.) (2) high-permeability
mable limit)
preferential pathways present that may result in higher than
3.2.12 USEPA—U.S. Environmental Protection Agency
typical rates of vapor transport (for example, gravel trench for
3.2.13 VOC—volatile organic compound utility lines), (3) relatively high permeability soils (for
example, sand or gravel) with insufficient moisture to support
3.2.14 v/v—by volume, as in percent by volume (% v/v)
methanotrophic bacteria, or (4) changes in groundwater eleva-
tion over short time periods, which can create pressure gradi-
4. Summary of Guide
ents in the vadose zone. For such sites, presumptive mitigation
4.1 This guide describes site screening, testing, data
or Tier 3 evaluation (for example, continuous or regular
analysis, evaluation, and selection of mitigation alternatives.
monitoring) should be considered.
4.2 Three-Tiered Approach—This guide provides an ap-
4.3 Site Categorization—This guide is designed to promote
proach for assessing and interpreting site methane, evaluating
rapid site characterization so that low-risk sites can be identi-
hazard and risk, determining the appropriate response, and
fied and efficiently removed from further evaluation.
identifying the urgency of the response. The approach is based
Conversely, high-risk sites can be identified and appropriate
on understanding the potential mass flow at a site and not
follow-up actions taken promptly. This guide focuses on Tier 1
relying solely upon concentration measurements. A three-tiered
and 2 evaluations. Special case evaluations (Tier 3) are
approach is given that uses a decision matrix based on methane
generally outside the scope of this guide, but applicable tools
concentrations in the vadose zone and other factors such as
and considerations are described for information purposes.
indoor air concentrations, differential pressure measurements,
and estimates of the volume of methane within soil gas near a
5. Significance and Use
building to determine the potential hazard. Alternatively, rather
5.1 Several different factors should be taken into consider-
than using these indirect measures of gas transport potential,
ation when evaluating methane hazard, rather than, for
gas flowrates can be measured directly (see Appendix X4). For
example, use of a single concentration-based screening level as
highly permeable soils or fill materials, direct measurements of
a de-facto hazard assessment level. Key variables are identified
gas flowrates will provide better, more conservative assess-
and briefly discussed in this section. Legal background infor-
ments. The first tier consists of a site evaluation that can
mation is provided in Appendix X3. The Bibliography includes
typically be done using existing, available information. This
references where more detailed information can be found on
information is compiled, reviewed, and used to develop a
the effect of various parameters on gas concentrations.
conceptual site model (CSM). The CSM should describe and
summarize the source of any methane that is present, vadose 5.2 Application—This guide is intended for use by those
zone conditions (for example, depth to groundwater and soil undertaking an assessment of hazards to people and property as
type), size of impacted area, design and use of any existing a result of subsurface methane suspected to be present based on
buildings, exposure scenario, and other relevant lines of due diligence or other site evaluations (see 6.1.1).
E2993 − 23
5.2.1 This guide addresses shallow methane, including its 5.5.2.1 Near-Surface Advection Effects—Within buildings,
presence in the vadose zone; at residential, commercial, and across building foundations, and in the immediate subsurface
industrial sites with existing construction; or where develop- vicinity of building foundations, advective flow may be driven
ment is proposed. by temperature differences, the on-off cycling of building
ventilation systems, the interaction of wind and buildings,
5.3 This guide provides a consistent, streamlined process
and/or changes in barometric pressure. These mechanisms can
for deciding on action and the urgency of action for the
pump air back and forth between the soil and the interior of
identified hazard. Advantages include:
structures. The effects may be significant in evaluation of VOC
5.3.1 Decisions are based on reducing the actual risk of
or radon migration between buildings and the subsurface, but
adverse impacts to people and property.
generally are relatively minor factors in evaluation of methane
5.3.2 Assessment is based on collecting only the informa-
migration and hazard unless the source of methane is in very
tion that is necessary to evaluate hazard.
shallow soils.
5.3.3 Available resources are focused on those sites and
5.5.2.2 Source Zone Flow Effects—Biogenic (microbial) gas
conditions that pose the greatest risk to people and property at
generation (methanogenesis) results in a net increase in molar
any time.
gas volume near the generation source. The resulting increased
5.3.4 Response actions are chosen based on the existence of
gas pressure causes gas flow away from the source zone. This
a hazard and are designed to mitigate the hazard and reduce
gas flow typically originates near sources of buried organic
risk to an acceptable level.
matter. Pressure-driven flow can also result from pressurized
subsurface gas sources including leaks from natural gas distri-
5.3.5 The urgency of initial response to an identified hazard
bution systems, subsurface gas storage, or seeps from natural
is commensurate with its potential adverse impact to people
and property. gas reservoirs. The evaluation of pressurized sources of gas
themselves (for example, pipelines, reservoirs, or subsurface
5.4 Limitations—This guide does not address potential haz-
storage) is outside the scope of this guide (see 1.5.3 – 1.5.6).
ards from other gases and vapors that may also be present in
5.5.2.3 Subsurface soil gas pressure change can also occur
the subsurface such as hydrogen sulfide, carbon dioxide, and/or
in other instances, such as with a rapidly rising or falling water
volatile organic compounds (VOCs) that may co-occur with
table in a partially confined aquifer or barometric pumping of
methane. If the presence of hydrogen sulfide or other poten-
fractured bedrock or very coarse gravel. This effect may occur
tially toxic gases is suspected, the analytical plan should be
in conjunction with advection of either dilute or high-
modified accordingly.
concentration soil gases and may be irregular or intermittent.
5.4.1 The data produced using this guide should be repre-
Induced pressure driven flow in response to diurnal barometric
sentative of the soil gas concentrations in the geological
pressure changes is both upward and downward and there is no
materials in the immediate vicinity of the sample probe or well
net upward pressure gradient. The CSM should consider the
at the time of sample collection (that is, they represent
potential for induced pressure-driven flow (which is sometimes
point-in-time and point-in-space measurements). The degree to
referred to as repressurization).
which these data are representative of any larger areas or
(1) Significant gas flow due to barometric pressure fluctua-
different times depends on numerous site-specific factors. The
tions may occur for nearby subsurface gas void volumes
smaller the data set being used for hazard evaluation, the more
(nominal gas volumes of 4000 m or greater) in confined
important it is to bias measurements towards worst-case
coarse sand or gravel connected to a building or enclosure
conditions.
(2) Significant gas flow due to water table changes may
5.5 Variables and Site-Specific Factors that May Influence occur for changes of 10 cm/day or greater in confined coarse
sand or gravel connected to a building or enclosure.
Data Evaluation:
5.5.1 Gas Transport Mechanisms—Methane migration in 5.5.3 Effect of Land Use—Combustible soil gas is a concern
soil gas results from pressure-driven flow, advection and mostly for sites with confined habitable space because of the
diffusion. Advective transport (for example, biogas within a safety risk. Combustible soil gas can also be a concern at sites
soil gas matrix) and pressure-driven flow (for example, pure or with other types of confined spaces, such as manholes or buried
vaults where a source of ignition may be present. Proximity or
nearly pure biogas) has been associated with methane incidents
(for example, fires or explosions), whereas no examples are entry to such spaces may require consideration of hazards
associated with methane.
known of methane incidents resulting from diffusive transport
alone. Therefore, diffusion is not considered a key transport
5.5.4 Pathways—Pathways into buildings from the soil can
mechanism when evaluating methane hazard.
include cracks in slabs, unsealed space around utility conduit
5.5.1.1 The potential for significant rates of soil gas trans- penetrations, the annular space inside of dry utilities (electrical,
port can often be recognized by relatively high differential communications), elevator pits (particularly those with piston
pressures (for example, >500 Pa [2 in. H O]), high concentra- wells), basement sumps, sewer lines with dry water traps, and
tions of leaked or generated gas, and concurrent displacement other avenues.
of atmospheric gases (nitrogen, argon) from the porous soil
5.5.5 Effect of Hardscape and Softscape—Any capping of
matrix. Alternatively, gas flowrates can be measured directly
the ground surface can impede the natural venting of soil gas
(see Appendix X4).
with concrete being generally less permeable than asphalt.
5.5.2 Effect of Gas Transport Mechanisms: Hardscape and well irrigated softscape both present barrier
E2993 − 23
conditions. Existing hardscape/softscape conditions should be taken before any gas readings, as purging can reduce any
noted during soil gas investigations. Proposed hardscape/ existing pressure differential and steady-state conditions may
softscape conditions should be considered when formulating not be reestablished for some time afterwards. Soil gas
alternatives for action at sites where methane hazard is to be pressures and soil gas concentrations should also be measured
mitigated. The potential for future hardscape/softscape condi- after purging. The recovery, or change of pressure with time,
tions also should be taken into account when evaluating the may also be of interest. Gas pressure readings taken in
representativeness of methane and pressure data. groundwater monitoring wells may not be representative of
vadose zone pressures.
5.5.6 Effect of Soil Physical Properties—The diffusion of
gas through soil is controlled by the air-filled porosity of the
5.6 Applicability of Results—Instantaneous data from moni-
soil, whereas the advection and pressure-driven flow of gas
toring probes represent conditions at a point in space and time.
through soil is controlled by the permeability of the soil. Two
Worst-case, short-term impacts are of interest in a methane
soils can have similar porosities but different permeabilities
evaluation because of the acute risk posed by methane.
and vice-versa. The effective porosity of a soil may be different
Single-sampling events in which data are collected from a
than the total porosity depending on whether the soil pores are
number of points at different locations may be sufficient if there
connected or not. For methane transport, advective and
is a robust CSM (that is, accounting for worst-case conditions)
pressure-driven flow is of much more concern than diffusive
and the site is well understood. If site results are inconsistent
flow, so permeability is a more important variable than
with the CSM, additional data may be needed to address
porosity. Large spaces such as fractures in fine-grained soils
uncertainties and increase the statistical reliability and confi-
can impart a high permeability to materials that would other-
dence in the results.
wise have a low permeability. Soil moisture can reduce the
air-filled porosity of soil and the gas permeability thereby
6. Approach to Methane Hazard Evaluation
reducing both diffusive and advective flow of soil gas.
6.1 Decision Framework:
5.5.7 Effect of Environmental Variables—A number of en-
6.1.1 Investigations may be triggered by site-specific find-
vironmental variables can affect the readings taken in the field
ings (for example, observations of bubbling at ground surface
and can be important in interpreting the readings once taken.
or in water wells; measurement of methane in soil gas; odors;
The effect of environmental variables tends to be greatest for
or, in extreme cases, fire or explosion or both) or may result
very shallow measurements in the vadose zone and typically is
from planned studies (for example, methane evaluations pur-
of limited importance at depths of 1.5 m and greater.
suant to property transfer, property refinance, or during the
5.5.8 Atmospheric Pressures and Barometric Lag—A fall-
application process for a building permit). Investigation of
ing barometer may leave soil gas under pressure as compared
methane in soil may also follow detection during other
with building interiors enabling increased soil gas flux out of
investigations, such as in confined space screening (29 CFR
the soil and into structures. The interpretation of barometric lag
1910.146) or environmental investigation of chemical-
data should take into account the type of soil. Barometric lag is
impacted soils and groundwater. The general process is shown
most pronounced in tight (clayey) soils in which the flow of
in Fig. 1. The volume of gas that is important will depend on
gases is retarded; barometric lag is least pronounced in
the size of the building footprint. In general, the greater the
granular (sandy) soils that provide the greatest permeability for
spatial extent of soil gas with elevated methane, the greater the
the flow of gas. The potential for pressure-driven gas transport
potential for vapor intrusion of methane to be an issue. A
through soil is significant only for permeable soil pathways
single, isolated hot spot of 5 to 30 % methane is unlikely to
(that is, air-filled coarse sands and gravels).
result in an indoor air issue with the hazard dependent on the
5.5.9 Precipitation—Normal outdoor soil gas venting (that
volume of the hot spot relative to the volume of the indoor
is, emissions at soil surface) is impeded when moisture fills the
space and the lateral distance from the hot spot to the building.
surface soil pore space. Infiltrating rainwater may displace soil
6.1.2 Decision making uses a matrix of soil gas and indoor
gas and cause it to vent into structures. Increases in soil
air values to address both current risk and potential future risk
moisture following rain or other precipitation events can lead
(see Table 1). The matrix is a risk management approach that
to enhanced rates of biogas generation, which may be evalu-
uses conservative screening values for methane concentration
ated through repeated measurements.
and differential pressure to rank site hazard. The available
5.5.10 Effect of Sampling Procedures—Sampling probes volume of soil gas containing elevated levels of methane also
(test wells) typically are designed to identify soil gas pressures is a consideration. The volume of gas at a given methane
and maximum soil gas concentrations at the point of monitor- concentration and differential pressure generally should be
ing. The sequence of steps (for example, purging, pressure and assumed to be sufficient to pose a potential issue unless the
concentration readings, and so forth) can affect the results. For contrary can be demonstrated via the CSM and/or field
differential pressure measurements, gages capable of measur- measurements. It is important to recognize that the values are
ing 500 Pa (2 in. H O) may be used. Ideally, the gage or gages guidelines and not absolute thresholds. Concentrations and
should be capable of measurements over a range of pressures pressure need to be considered in terms of the CSM. The
(for example, 0 to 1,250 Pa (0 to 5 in. H O)) and have a decision matrix shown in Table 1 is a suggested starting point
resolution of at least 25 Pa (0.1 in. H O). See the Bibliography and should be adjusted as appropriate for site-specific condi-
for references on equipment for concentration and differential tions. The 500 Pa (2 in. H O) criterion for ∆P is based on
pressure measurements. Initial readings of pressure should be measurements in the vadose zone at a depth or interval of 1.5 m
E2993 − 23
FIG. 1 Tiered Evaluation Process
TABLE 1 Suggested Default Decision Matrix for Methane in Soil Gas and Indoor Air
D
NOTE 1—Table based on Eklund, 2011 (1) and Sepich, 2008 (2) . See also Appendix X2. Table is intended for sites with existing buildings. To address
future development, no further action is recommended if the shallow soil gas concentration of methane is <30% and ∆P <500 Pa. The potential for
conditions to change in the future should be considered. Sites that cannot be screened out merit further evaluation (see Section 6).
NOTE 2—If the combined soil gas concentrations of methane and carbon dioxide are ≥90%, biogas generation likely is recent or on-going and mitigation
should be considered.
NOTE 3—Soil gas outside the building footprint but within a radius of 60 m (200 ft) of the building may be of interest. The total mass of methane present
should be considered (that is, concentration × volume).
Indoor Air Concentration
A
Shallow Soil Gas Conc.
No Measurements Available <0.01% (that is <100 ppm) 0.01 to <1.25% >1.25%
B
<1.25% to 5% No further action No further action Identify indoor sources Immediately notify authorities,
recommend owner/operator
evacuate building
C,E B B
>5% to 30% No further action unless ∆P Mitigate gas entry points Mitigate gas entry points Immediately notify authorities,
B
>500 Pa recommend owner/operator
evacuate building
C,E
>30% Collect indoor air data Evaluate on case-by-case Evaluate on case-by-case Immediately notify authorities,
basis basis recommend owner/operator
evacuate building
A
Maximum methane soil gas value (% CH4) for area of building footprint. Shallow soil gas refers to soil gas in the vadose zone within the top 10 m (33 ft) of soil below
ground surface but at least 1 m from the building envelope. If methane exceeds 5 % within 1 m of the building, the default decision matrix is not applicable.
B
Landowner or building owner/manager should identify indoor sources and reduce/control emissions. If no sources are found, additional subsurface characterization and
continued indoor air monitoring should be considered. ∆P refers to pressure gradients in the subsurface at a depth or interval of 1.5 m. For sandy soils, gravel, or other
highly permeable matrices, direct measurement of methane flow may be appropriate (see Appendix X4).
C
The potential for pressure gradients to occur in the future at a given site should be considered.
D
The boldface numbers in parentheses refer to a list of references at the end of this standard.
E
Potential points of gas entry to the building should be identified and plugged or sealed. If P>500 Pa, further mitigation should be considered.
E2993 − 23
(for example, difference between pressure measurements 1.5 m 6.2.1 Source—Methane is produced by two primary mecha-
below ground surface and ambient air). For measurements at nisms: thermogenic and microbial (see Appendix X1). Ther-
1.5 m or greater, temporal variability is typically not signifi- mogenic methane consists primarily of methane with relatively
cant. Measurements at shallower depths (for example, sub-
small amounts of ethane, propane, and higher molecular weight
slab) also may be useful, but recognize that there is gr eater
hydrocarbons. Thermogenic or “fossil” methane typically
potential for temporal variability for shallower measurements
originates from petroleum deposits at depths generally far
or measurements at sites with highly permeable matrices.
below the vadose zone. Natural gas is largely thermogenic
6.1.3 The screening values for methane concentration are, in
methane and may occur in coal mines, oil and gas fields, and
most cases, derived from the lower flammable limit for
other geological formations. Thermogenic methane, once
methane in air, that is, 5 %, since methane hazard is related to
produced, is carried in natural gas transmission and distribution
flammability rather than toxicity. Concentration, pressure, and
lines. Microbial or “biogenic” methane typically is generated at
volume should be taken into account. Physical and toxicologi-
relatively shallow depths by the recent microbial decomposi-
cal characteristics of methane are summarized in Appendix X1.
tion of organic matter in soil. The “biogas” produced is
Additional discussion of the screening values is provided in
essentially all methane and carbon dioxide, present at roughly
Appendix X2. Note that for soil gas, methane concentration
equal percentages. If CH + CO approach 100 %, the gas is
4 2
alone is insufficient to evaluate potential hazard. Information
said to be “whole” or “undiluted.” Microbial methane is a
on pressures and volumes is also essential.
product of decomposition of organic matter in both natural (for
6.1.4 Screening values are location specific. That is, soil gas
example, wetlands and river and lake sediments) and man-
screening values should be used for comparison with site soil
made settings (for example, sewer lines, septic systems, and
gas results and indoor air/confined space screening values
manure piles). A given mass of organic carbon will have a fixed
should be compared only with indoor air/confined space results
volume of biogas it can potentially generate. For a given
(for example, Table 1).
organic material, the rate at which this gas generation takes
6.1.5 Volume of methane in the vadose zone for a given site
place will largely be a function of the soil moisture. Once all
will always be an estimate. The uncertainty in the estimate can
the carbon has been degraded, the site is said to be “gassed
be reduced by characterizing the spatial variability. A minimum
out.” Note that the organic matter can be degraded without
sampling interval for a developed property is typically one
methane generation if other terminal electron receptors (for
sub-slab measurement per 500 m of building footprint, plus
example, oxygen, nitrate, iron, sulfate) are present or are
one shallow (for example, 5 ft.) soil vapor sample per 1000 m
introduced. Methane can also be effectively “trapped” in the
of property area, and one deeper cluster of soil vapor samples
ground and be immobile. Methane adsorbs onto organic
(for example, 5 ft., 10 ft., and 20 ft.) per 2000 m of property
material in the ground and desorbs into monitoring wells when
area. If readings of 30 % or higher methane are found, it may
they are installed. In fine grained cohesive organic soils such as
be helpful to better characterize the relative “hot spot” by
Alluvium the gas can be adsorbed, trapped in soil pores or
collecting additional data to provide a 3-dimensional concen-
dissolved in pore water and does not cause hazardous emis-
tration map in the immediate area.
sions at the ground surface.
6.2 Develop Conceptual Site Model (CSM)—The user is
6.2.2 Transport—Methane will migrate along pressure gra-
required to identify the potential primary sources of methane in
dients from areas where it is present at higher pressures to areas
the subsurface, potential receptor points, and significant likely
where it is present at lower pressures, or along concentration
transport pathways from the primary sources to the receptors.
gradients, also from high to low. The primary mechanism for
Various vapor intrusion guidance documents describe the
significant methane migration in subsurface unsaturated soils is
development of CSMs (ITRC Document VI-1 and PVI-1 and
pressure-driven flow. Diffusion also occurs but at rates too low
EPA/OSWER), though not for methane sites. Guidance spe-
to result in unacceptable indoor air concentrations under
cific to methane is available (NSW, 2020; BSI, 2013) along
reasonably likely scenarios. Soils can be a significant sink for
with guidance for determining “worst case” conditions
methane, with aerobic biodegradation also an important fate
(CLAIRE, 2018). The CSM provides a framework for the
and transport consideration.
process of evaluating methane hazard. The CSM summarizes
6.2.3 Receptors—Residential, commercial, and industrial
what is known about the site in terms of source, depth to
buildings, and the individuals therein, are the primary receptors
groundwater, geology, data trends, receptors, building design
of interest. Buildings typically have roughly 0.5 to 1 air
and operation, and so forth. The CSM should consider reason-
changes per hour (ACH) and a relatively high rate of vapor
able worst-case conditions such as falling and low relative
intrusion is necessary for the indoor atmosphere to approach
barometric pressure conditions or potential soil gas repressur-
the lower flammability limit for methane of 5 %. Therefore,
ization. The potential for conditions to change in the future
portions of the buildings with lower rates of air exchange are
should be considered (for example, increase or decrease in
of most interest, such as closed cabinets beneath sinks, closets,
impervious cover). The results of any further investigations are
and stagnant areas of basements. Utility vaults and other small,
compared with the CSM to see whether or not the results are
poorly ventilated subsurface structures may be viewed as
consistent with the expectations derived from the CSM. If the
results are found to differ in material ways from these receptors or as worst-case indicators of potential conditions in
expectations, the CSM will require modification. nearby buildings.
-
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E2993 − 16 E2993 − 23
Standard Guide for
Evaluating Potential Hazard in Buildings as a Result of
Methane in the Vadose Zone
This standard is issued under the fixed designation E2993; 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.
1. Scope
1.1 This guide provides a consistent basis for assessing site methane in the vadose zone, evaluating hazard and risk, determining
the appropriate response, and identifying the urgency of the response.
1.2 Purpose—This guide covers techniques for evaluating potential hazards associated with methane present in the vadose zone
beneath or near existing or proposed buildings or other structures (for example, potential fires or explosions within the buildings
or structures), when such hazards are suspected to be present based on due diligence or other site evaluations (see 6.1.1). Buildings
in this context include normal below grade utilities associated with a building.
1.3 Objectives—This guide: (1) provides a practical and reasonable industry standard for evaluating, prioritizing, and addressing
potential methane hazards based on mass flow and (2) raises awareness of the key variables needed to properly evaluate such
hazards.provides a tool for screening out low-risk sites.
1.4 This guide offers a set of instructions for performing one or more specific operations. This guide cannot replace education or
experience and should be used in conjunction with professional judgment. Not all aspects of this guide may be applicable in all
circumstances. This guide is not intended to represent or replace the standard of care by which the adequacy of a given professional
service should be judged, nor should this guide be applied without consideration of a project’s many unique aspects. The word
“Standard” in the title means only that the guide has been approved through the ASTM International consensus process.
1.5 Not addressed by this guide are:
1.5.1 Requirements or guidance or both with respect to methane sampling or evaluation in federal, state, or local regulations. Users
are cautioned that federal, state, and local guidance may impose specific requirements that differ from those of this guide;
1.5.2 Safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate
safety and health practices and determine the applicability of regulatory limitations prior to use;
1.5.3 Emergency response situations such as sudden ruptures of gas lines or pipelines;
1.5.4 Methane entry into an enclosure from other than vadose zone soils (for example, methane evolved from well water brought
into an enclosure; methane generated directly within the enclosure; groundwater intrusion, methane from leaking natural gas lines
or appliances within the enclosure, direct migration into buildings from mine entries, etc.);
This guide is under the jurisdiction of ASTM Committee E50 on Environmental Assessment, Risk Management and Corrective Action and is the direct responsibility
of Subcommittee E50.02 on Real Estate Assessment and Management.
Current edition approved March 15, 2016Sept. 1, 2023. Published May 2016November 2023. Originally approved in 2016. Last previous edition approved in 2016 as
E2993–16. DOI: 10.1520/E2993–1610.1520/E2993–23
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2993 − 23
1.5.5 Methane entry into an enclosure situated atop or immediately adjacent to a municipal solid waste (MSW) landfill or a
Construction and Demolition (C&D) landfill;
1.5.6 Methane from oil & gas reservoirs, injection wells, or other sources potentially under high pressures relative to typical
vadose zone pressures;
1.5.7 Methane risk during construction activities, work in trenches, and confined space work (which are all best addressed via
real-time monitoring);
1.5.8 Potential hazards from other gases and vapors that may also be present in the subsurface such as hydrogen sulfide, carbon
dioxide, and/or volatile organic compounds (VOCs);
1.5.9 Anoxic conditions in enclosed spaces;
1.5.10 The forensic determination of methane source; or
1.5.11 Potential consequences of fires or explosions in enclosed spaces or other issues related to safety engineering design of
structures or systems to address fires or explosions.
1.6 Units—The values stated in SI units are to be regarded as the standard.
1.6.1 Exception—Values in inch/pound units are provided for reference.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and healthsafety, health, and environmental practices and determine
the applicability of regulatory limitations prior to use.
1.8 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.
2. Referenced Documents
2.1 ASTM Standards:
D653 Terminology Relating to Soil, Rock, and Contained Fluids
D1356 Terminology Relating to Sampling and Analysis of Atmospheres
D1946 Practice for Analysis of Reformed Gas by Gas Chromatography
D2216 Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass
D2487 Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)
D5088 Practice for Decontamination of Field Equipment Used at Waste Sites
D6725 Practice for Direct Push Installation of Prepacked Screen Monitoring Wells in Unconsolidated Aquifers
D7663 Practice for Active Soil Gas Sampling in the Vadose Zone for Vapor Intrusion Evaluations
E2600 Guide for Vapor Encroachment Screening on Property Involved in Real Estate Transactions
F1815 Test Methods for Saturated Hydraulic Conductivity, Water Retention, Porosity, and Bulk Density of Athletic Field
Rootzones
2.2 Other Standards:
British Standards Institution (BSI), Guidance on Investigations for Ground Gases, Permanent Gases, and Volatile Organic
Compounds (VOCs). BS8576. 2013
British Standards Institution (BSI), Code of Practice for the Design of Protective Measures for Methane and Carbon Dioxide
Ground Gases for New Buildings. BS 8485:2015+A1:2019
California DTSC, Evaluation of Biogenic Methane for Constructed Fills and Dairies Sites, March 28, 2012
CL:AIRE Ground Gas Monitoring and “Worst Case” Conditions. August 2018
County of Los Angeles Building Code, Volume 1, Title 26, Section 110 Methane
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from dpw.lacounty.gov.British Standards Institution (BSI), 389 Chiswick High Rd., London W4 4AL, U.K., http://www.bsigroup.com.
Available from dpw.lacounty.gov.
E2993 − 23
ITRC Document VI-1 Vapor Intrusion Pathway: A Practical Guideline
ITRC Document PVI-1 Petroleum Vapor Intrusion: Fundamentals of Screening, Investigation, and Management
EPA 530-R-10-003 Conceptual Model Scenarios for the Vapor Intrusion Pathway
New South Wales (NSW) EPA. Assessment and Management of Hazardous Ground Gases. May 2020.
29 CFR 1910.146 Permit-Required Confined Spaces
3. Terminology
3.1 Definitions:
3.1.1 This section provides definitions and descriptions of terms used in or related to this guide. An acronym list is also included.
The terms are an integral part of this guide and are critical to an understanding of the guide and its use.
3.1.2 advection, n—transport of molecules along with the flow of a greater medium as occurs because of differential pressures.
3.1.3 ambient air, n—any unconfined portion of the atmosphere; open air.
3.1.4 barometric lag, n—time difference between changes in total atmospheric pressure (barometric pressure) and subsequent
changes in total gas pressure measured at a specific point in the subsurface.
3.1.4.1 Discussion—
Atmospheric pressure variations include routine diurnal highs and lows as well as changes associated with exceptional
meteorological conditions (weather fronts). The time lag means that differential pressure between the surface and the subsurface
point may be out of phase and may reverse (6 relative to zero) with resulting reversals in soil gas flow direction over time between
the shallow subsurface and the surface.
3.1.5 barometric pumping, n—variation in the ambient atmospheric pressure that causes motion of vapors in, or into, porous and
fractured earth materials.
3.1.6 biogas, n—mixture of methane and carbon dioxide produced by the microbial decomposition of organic wastes, also known
as microbial gas.
3.1.6.1 Discussion—
For the purposes of this standard, biogas may arise from any organic material not specifically excluded in 1.5. The sources
addressed include plant material, soil organic carbon, and petroleum hydrocarbons from past releases.
3.1.7 biogenic, adv—resulting from the activity of living organisms.
3.1.8 contaminant, n—substance not normally found in an environment at the observed concentration.
3.1.9 continuous monitoring, n—measurements of selected parameters performed at a frequency sufficient to define critical trends,
identify changes of interest, and allow for relationships with other attributes in a predictive capacity.
3.1.10 dead volume, n—total air-filled internal volume of the sampling system.
3.1.11 differential pressure, n—relative difference in pressure between two measurement points (∆P).
3.1.11.1 Discussion—
∆P measurements are typically the differences between pressure at some depth in the vadose zone and pressure above ground at
the same location (indoors or outdoors), but also could refer to the difference in pressure between two subsurface locations. A ∆P
measurement represents a pressure gradient between the two locations.
3.1.12 diffusion, n—gas transport mechanism in which molecules move along a concentration gradient from areas of higher
concentration toward areas of lower concentration; relatively slow form of gas transport.
Available from the Interstate Technology & Regulatory Council, http://www.itrcweb.org/Documents/VI-1.pdf.
Available from the Interstate Technology & Regulatory Council, http://www.itrcweb.org/PetroleumVI-Guidance/
Available from Occupational Safety and Health Administration (OSHA), 200 Constitution Ave., Washington, DC 20210, http://www.osha.gov.
E2993 − 23
3.1.13 effective porosity, n—amount of interconnected void space (within intergranular pores, fractures, openings, and the like)
available for fluid movement: generally less than total porosity.
3.1.14 flammable range, n—concentration range in air in which a flammable substance can produce a fire or explosion when an
ignition source is present.
3.1.14.1 Discussion—
The flammable range extends from the lower flammable limit (LFL) to the upper flammable limit (UFL). See Appendix X1, X1.6.
3.1.15 fracture, n—break in the mechanical continuity of a body of rock or soil caused by stress exceeding the strength of the rock
or soil and includes joints and faults.
3.1.16 groundwater, n—part of the subsurface water that is in the saturated zone.
3.1.17 hazard, n—source of potential harm from current or future methane exposures.
3.1.18 indoor air, n—the mixture of gases within the habitable spaces of a building
3.1.19 microbial, adv—pertaining to or emanating from a microbe.
3.1.19.1 Discussion—
The preferred term for nonthermogenic, nonpetrogenic methane such as from anaerobic activity in shallow soils or sanitary
landfills is “microbial.”
3.1.20 moisture content, n—amount of water lost from a soil upon drying to a constant weight expressed as the weight per unit
weight of dry soil or as the volume of water per unit bulk volume of the soil.
3.1.21 perched aquifer, n—lens of saturated soil above the main water table that forms on top of an isolated geologic layer of low
permeability.
3.1.22 permeability, n—ease with which a porous medium can transmit a fluid under a potential gradient.
3.1.23 preferential pathway, n—migration route for chemicals of concern that has less constraint on gas transport than the
surrounding soil.
3.1.23.1 Discussion—
Preferential pathways may be natural (for example, vertically fractured bedrock where the fractures are interconnected) or
man-made (for example, utility conduits, sewers, and dry wells).
3.1.24 pressure-driven flow, n—gas transport mechanism that occurs along pressure gradients resulting from introduction of gas
into the soil matrix.
3.1.24.1 Discussion—
The flow of gas is from the region of high pressure to regions of lower pressure and continues until the gas pressure is equal or
the flowpath is blocked. With advection, molecules are transported along with the flow of a greater medium. With pressure-driven
flow, the introduced gas is the medium.
3.1.24.2 Discussion—
In the vadose zone, elevated pressures in a given volume of soil can occur as a result of biogas generation at that location.
Therefore, whether or not a given site has active biogas generation is an important consideration in evaluating methane hazard.
3.1.25 porosity, n—volume fraction of a rock or unconsolidated sediment not occupied by solid material but usually occupied by
liquids, vapor, and/or air.
3.1.25.1 Discussion—
Porosity is the void volume of soil divided by the total volume of soil.
3.1.26 probe, n—device designed to investigate and collect information from a remote location.
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3.1.26.1 Discussion—
As used in this guide, a point or methane test well used to collect information from within the vadose zone or subslab space of
a building.
3.1.27 purge volume, n—amount of air removed from the sampling system before the start of sample collection.
3.1.27.1 Discussion—
This is usually referred to in terms of number of dead volumes of probe (test well) casing or test well plus granular backfill total
volume.
3.1.28 repressurization, n—unpressurized soil vapors can be pressurized by phenomena such as rapidly rising groundwater.
3.1.29 risk, n—probability that something will cause injury or harm.
3.1.30 saturated zone, n—zone in which all of the voids in the rock or soil are filled with water at a pressure that is greater than
atmospheric.
3.1.30.1 Discussion—
The water table is the top of the saturated zone in an unconfined aquifer.
3.1.31 soil gas, n—vadose zone atmosphere; soil gas is the air existing in void spaces in the soil between the groundwater table
and the ground surface.
3.1.32 soil moisture, n—water contained in the pore spaces in the vadose zone.
3.1.33 subslab vapor sampling, v—collection of vapor from the zone just beneath the lowest floor slab of a building or below
paving or soil cap.
3.1.34 thermogenic, adj—methane that is generated at depth under elevated pressure and temperatures during and following the
formation of petroleum (for example, in oil fields).
3.1.35 tracer, n—material that can be easily identified and determined even at very low concentrations and may be added to other
substances to enable their movements to be followed or their presence to be detected.
3.1.36 tracer gas, n—gas used with a detection device to determine the rate of air interchange within a space or zone or between
spaces or zones.
3.1.37 vadose zone, n—hydrogeological region extending from the soil surface to the top of the principal water table.
3.1.37.1 Discussion—
Perched groundwater may exist within this zone.
3.1.38 vapor intrusion, n—migration of a volatile chemical(s) from subsurface soil or water into an overlying or nearby building
or other enclosed space.
-2
3.1.39 volatile organic compound, VOC, n—an organic compound with a saturation vapor pressure greater than 10 kPa at 25°C
(Terminology D1356-14).
3.1.40 water table, n—top of the saturated zone in an unconfined aquifer.
3.2 Acronyms and Abbreviations:
3.2.1 ACH—air changes per hour
3.2.2 CSM—conceptual site model
3.2.3 FID—flame ionization detector
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3.2.4 HVAC—heating, ventilation, and air conditioning
3.2.5 In. H O—inches of water, a measure of pressure exerted by a column of water 1 in. (2.54 cm) in height; 1 in. H O equals
2 2
approximately 250 Pa
3.2.6 LEL—lower explosive limit (same as lower flammable limit)
3.2.7 Pa—Pascal, a measure of pressure
3.2.8 ppmv—part per million on a volume basis
3.2.9 psi—pounds per square inch
3.2.10 QA/QC—quality assurance/quality control
3.2.11 UEL—upper explosive limit (same as upper flammable limit)
3.2.12 USEPA—U.S. Environmental Protection Agency
3.2.13 VOC—volatile organic compound
3.2.14 v/v—by volume, as in percent by volume (% v/v)
4. Summary of Guide
4.1 This guide describes site screening, testing, data analysis, evaluation, and selection of mitigation alternatives.
4.2 Three-Tiered Approach—This guide provides an approach for assessing and interpreting site methane, evaluating hazard and
risk, determining the appropriate response, and identifying the urgency of the response. The approach is based on understanding
the potential mass flow at a site and not relying solely upon concentration measurements. A three-tiered approach is given that uses
a decision matrix based on methane concentrations in the vadose zone and other factors such as indoor air concentrations,
differential pressure measurements, and estimates of the volume of methane within soil gas near a building to determine the
potential hazard. Alternatively, rather than using these indirect measures of gas transport potential, gas flowrates can be measured
directly (see Appendix X4). For highly permeable soils or fill materials, direct measurements of gas flowrates will provide better,
more conservative assessments. The first tier consists of a site evaluation that can typically be done using existing, available
information. This information is compiled, reviewed, and used to develop a conceptual site model (CSM). The CSM should
describe and summarize the source of any methane that is present, vadose zone conditions (for example, depth to groundwater and
soil type), size of impacted area, design and use of any existing buildings, exposure scenario, and other relevant lines of evidence
for a given site. A decision matrix is applied to get an initial prediction of hazard. For sites in which potentially significant data
gaps are identified during the Tier 1 review, the second tier consists of a refined site evaluation. Additional field work is performed
to address the data gaps. The results are compared with the CSM and the CSM revised, as necessary. The decision matrix is again
applied to the new, expanded data set to get an updated prediction of hazard. If it is determined that more data are needed, the third
tier consists of a special case evaluation. For all three tiers, the path forward at any point should respect applicable regulatory
guidance and consider risk management principles, technical feasibility, and community concerns.
4.2.1 The evaluation process is typically implemented in a tiered approach involving increasingly sophisticated levels of data
collection, analysis, and evaluation. Users may choose to proceed directly to the most sophisticated tier, to pre-emptive mitigation,
or to routine monitoring based on site-specific circumstances. It is good practice to seal utility openings and plug potential gas entry
points for any site with potential for methane.
4.2.2 For some sites, a limited number of samples may not be sufficient to address potential hazard because there are (1) a large
volume (for example, >100 m ) of methane gas and/or significant potential methane source(s) in the vicinity of the at or nearby
the site (for example, a large mass (for example, >500 m ) of buried organic matter such as plants, wood, etc.) (2)
high-permeability preferential pathways present that may result in higher than typical rates of vapor transport (for example, gravel
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trench for utility lines), (3) relatively high permeability soils (for example, sand or gravel) with insufficient moisture to support
methanotrophic bacteria, or (4) changes in groundwater elevation over short time periods, which can create pressure gradients in
the vadose zone. For such sites, presumptive mitigation or Tier 3 evaluation (for example, continuous or regular monitoring) should
be considered.
4.3 Site Categorization—This guide is designed to promote rapid site characterization so that low-risk sites can be identified and
efficiently removed from further evaluation. Conversely, high-risk sites can be identified and appropriate follow-up actions taken
promptly. This guide focuses on Tier 1 and 2 evaluations. Special case evaluations (Tier 3) are generally outside the scope of this
guide, but applicable tools and considerations are described for information purposes.
5. Significance and Use
5.1 Several different factors should be taken into consideration when evaluating methane hazard, rather than, for example, use of
a single concentration-based screening level as a de-facto hazard assessment level. Key variables are identified and briefly
discussed in this section. Legal background information is provided in Appendix X3. The Bibliography includes references where
more detailed information can be found on the effect of various parameters on gas concentrations.
5.2 Application—This guide is intended for use by those undertaking an assessment of hazards to people and property as a result
of subsurface methane suspected to be present based on due diligence or other site evaluations (see 6.1.1).
5.2.1 This guide addresses shallow methane, including its presence in the vadose zone; at residential, commercial, and industrial
sites with existing construction; or where development is proposed.
5.3 This guide provides a consistent, streamlined process for deciding on action and the urgency of action for the identified hazard.
Advantages include:
5.3.1 Decisions are based on reducing the actual risk of adverse impacts to people and property.
5.3.2 Assessment is based on collecting only the information that is necessary to evaluate hazard.
5.3.3 Available resources are focused on those sites and conditions that pose the greatest risk to people and property at any time.
5.3.4 Response actions are chosen based on the existence of a hazard and are designed to mitigate the hazard and reduce risk to
an acceptable level.
5.3.5 The urgency of initial response to an identified hazard is commensurate with its potential adverse impact to people and
property.
5.4 Limitations—This guide does not address potential hazards from other gases and vapors that may also be present in the
subsurface such as hydrogen sulfide, carbon dioxide, and/or volatile organic compounds (VOCs) that may co-occur with methane.
If the presence of hydrogen sulfide or other potentially toxic gases is suspected, the analytical plan should be modified accordingly.
5.4.1 The data produced using this guide should be representative of the soil gas concentrations in the geological materials in the
immediate vicinity of the sample probe or well at the time of sample collection (that is, they represent point-in-time and
point-in-space measurements). The degree to which these data are representative of any larger areas or different times depends on
numerous site-specific factors. The smaller the data set being used for hazard evaluation, the more important it is to bias
measurements towards worst-case conditions.
5.5 Variables and Site-Specific Factors that May Influence Data Evaluation:
5.5.1 Gas Transport Mechanisms—Methane migration in soil gas results from pressure-driven flow, advection and diffusion.
Advective transport (for example, biogas within a soil gas matrix) and pressure-driven flow (for example, pure or nearly pure
biogas) has been associated with methane incidents (for example, fires or explosions), whereas no examples are known of methane
incidents resulting from diffusive transport alone. Therefore, diffusion is not considered a key transport mechanism when
evaluating methane hazard.
5.5.1.1 The potential for significant rates of soil gas transport can often be recognized by relatively high differential pressures (for
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example, >500 Pa [2 in. H O]), high concentrations of leaked or generated gas, and concurrent displacement of atmospheric gases
(nitrogen, argon) from the porous soil matrix. Alternatively, gas flowrates can be measured directly (see Appendix X4).
5.5.2 Effect of Gas Transport Mechanisms:
5.5.2.1 Near-Surface Advection Effects—Within buildings, across building foundations, and in the immediate subsurface vicinity
of building foundations, advective flow may be driven by temperature differences, the on-off cycling of building ventilation
systems, the interaction of wind and buildings, and/or changes in barometric pressure. These mechanisms can pump air back and
forth between the soil and the interior of structures. The effects may be significant in evaluation of VOC or radon migration
between buildings and the subsurface, but generally are relatively minor factors in evaluation of methane migration and
hazard.hazard unless the source of methane is in very shallow soils.
5.5.2.2 Source Zone Flow Effects—Biogenic (microbial) gas generation (methanogenesis) results in a net increase in molar gas
volume near the generation source. The resulting increased gas pressure causes gas flow away from the source zone. This gas flow
typically originates near sources of buried organic matter. Pressure-driven flow can also result from pressurized subsurface gas
sources including leaks from natural gas distribution systems, subsurface gas storage, or seeps from natural gas reservoirs. The
evaluation of pressurized sources of gas themselves (for example, pipelines, reservoirs, or subsurface storage) is outside the scope
of this guide (see 1.5.3 – 1.5.51.5.6).
5.5.2.3 Subsurface soil gas pressure change can also occur in other instances, such as with a rapidly rising or falling water table
in a partially confined aquifer or barometric pumping of fractured bedrock or very coarse gravel. This effect may occur in
conjunction with advection of either dilute or high-concentration soil gases and may be irregular or intermittent. Induced pressure
driven flow in response to diurnal barometric pressure changes is both upward and downward and there is no net upward pressure
gradient. The CSM should consider the potential for induced pressure-driven flow (which is sometimes referred to as
repressurization).
(1) Significant gas flow due to barometric pressure fluctuations may occur for nearby subsurface gas void volumes (nominal
gas volumes of 4000 m or greater) in confined coarse sand or gravel connected to a building or enclosure
(2) Significant gas flow due to water table changes may occur for changes of 10 cm/day or greater in confined coarse sand or
gravel connected to a building or enclosure.
5.5.3 Effect of Land Use—Combustible soil gas is a concern mostly for sites with confined habitable space because of the safety
risk. Combustible soil gas can also be a concern at sites with other types of confined spaces, such as manholes or buried vaults
where a source of ignition may be present. Proximity or entry to such spaces may require consideration of hazards associated with
methane.
5.5.4 Pathways—Pathways into buildings from the soil can include cracks in slabs, unsealed space around utility conduit
penetrations, the annular space inside of dry utilities (electrical, communications), elevator pits (particularly those with piston
wells), basement sumps, sewer lines with dry water traps, and other avenues.
5.5.5 Effect of Hardscape and Softscape—Any capping of the ground surface can impede the natural venting of soil gas. gas with
concrete being generally less permeable than asphalt. Hardscape and well irrigated softscape both present barrier conditions.
Existing hardscape/softscape conditions should be noted during soil gas investigations. Proposed hardscape/softscape conditions
should be considered when formulating alternatives for action at sites where methane hazard is to be mitigated. The potential for
future hardscape/softscape conditions also should be taken into account when evaluating the representativeness of methane and
pressure data.
5.5.6 Effect of Soil Physical Properties—The diffusion of gas through soil is controlled by the air-filled porosity of the soil,
whereas the advection and pressure-driven flow of gas through soil is controlled by the permeability of the soil. Two soils can have
similar porosities but different permeabilities and vice-versa. The effective porosity of a soil may be different than the total porosity
depending on whether the soil pores are connected or not. For methane transport, advective and pressure-driven flow is of much
more concern than diffusive flow, so permeability is a more important variable than porosity. Large spaces such as fractures in
fine-grained soils can impart a high permeability to materials that would otherwise have a low permeability. Soil moisture can
reduce the air-filled porosity of soil and the gas permeability thereby reducing both diffusive and advective flow of soil gas.
5.5.7 Effect of Environmental Variables—A number of environmental variables can affect the readings taken in the field and can
be important in interpreting the readings once taken. The effect of environmental variables tends to be greatest for very shallow
measurements in the vadose zone and typically is of limited importance at depths of 1.5 m and greater.
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5.5.8 Atmospheric Pressures and Barometric Lag—A falling barometer may leave soil gas under pressure as compared with
building interiors enabling increased soil gas flux out of the soil and into structures. The interpretation of barometric lag data should
take into account the type of soil. Barometric lag is most pronounced in tight (clayey) soils in which the flow of gases is retarded;
barometric lag is least pronounced in granular (sandy) soils that provide the greatest permeability for the flow of gas. The potential
for pressure-driven gas transport through soil is significant only for permeable soil pathways.pathways (that is, air-filled coarse
sands and gravels).
5.5.9 Precipitation—Normal outdoor soil gas venting (that is, emissions at soil surface) is impeded when moisture fills the surface
soil pore space. Infiltrating rainwater may displace soil gas and cause it to vent into structures. Increases in soil moisture following
rain or other precipitation events can lead to enhanced rates of biogas generation, which may be evaluated through repeated
measurements.
5.5.10 Effect of Sampling Procedures—Sampling probes (test wells) typically are designed to identify soil gas pressures and
maximum soil gas concentrations at the point of monitoring. The sequence of steps (for example, purging, pressure and
concentration readings, and so forth) can affect the results. For differential pressure measurements, gages capable of measuring 500
Pa (2 in. H O) may be used. Ideally, the gage or gages should be capable of measurements over a range of pressures (for example,
0 to 1,250 Pa (0 to 5 in. H O)) and have a resolution of at least 25 Pa (0.1 in. H O). See the Bibliography for references on
2 2
equipment for concentration and differential pressure measurements. Initial readings of pressure should be taken before any gas
readings, as purging can reduce any existing pressure differential and steady-state conditions may not be reestablished for some
time afterwards. Soil gas pressures and soil gas concentrations should also be measured after purging. The recovery, or change of
pressure with time, may also be of interest. Gas pressure readings taken in groundwater monitoring wells may not be representative
of vadose zone pressures.
5.6 Applicability of Results—Instantaneous data from monitoring probes represent conditions at a point in space and time.
Worst-case, short-term impacts are of interest in a methane evaluation because of the acute risk posed by methane. Single-sampling
events in which data are collected from a number of points at different locations may be sufficient if there is a robust CSM (that
is, accounting for worst-case conditions) and the site is well understood. If site results are inconsistent with the CSM, additional
data may be needed to address uncertainties and increase the statistical reliability and confidence in the results.
6. Approach to Methane Hazard Evaluation
6.1 Decision Framework:
6.1.1 Investigations may be triggered by site-specific findings (for example, observations of bubbling at ground surface or in water
wells; measurement of methane in soil gas; odors; or, in extreme cases, fire or explosion or both) or may result from planned studies
(for example, methane evaluations pursuant to property transfer, property refinance, or during the application process for a building
permit). Investigation of methane in soil may also follow detection during other investigations, such as in confined space screening
(29 CFR 1910.146) or environmental investigation of chemical-impacted soils and groundwater. The general process is shown in
Fig. 1. The volume of gas that is important will depend on the size of the building footprint. In general, the greater the spatial extent
of soil gas with elevated methane, the greater the potential for vapor intrusion of methane to be an issue. A single, isolated hot spot
of 5 to 30 % methane is unlikely to result in an indoor air issue.issue with the hazard dependent on the volume of the hot spot
relative to the volume of the indoor space and the lateral distance from the hot spot to the building.
6.1.2 Decision making uses a matrix of soil gas and indoor air values to address both current risk and potential future risk (see
Table 1). The matrix is a risk management approach that uses conservative screening values for methane concentration and
differential pressure to rank site hazard. The available volume of soil gas containing elevated levels of methane also is a
consideration. The volume of gas at a given methane concentration and differential pressure generally should be assumed to be
sufficient to pose a potential issue unless the contrary can be demonstrated via the CSM and/or field measurements. It is important
to recognize that the values are guidelines and not absolute thresholds. Concentrations and pressure need to be considered in terms
of the CSM. The decision matrix shown in Table 1 is a suggested starting point and should be adjusted as appropriate for
site-specific conditions. The 500 Pa (2 in. H O) criterion for ∆P is based on measurements in the vadose zone at a depth or interval
of 1.5 m (for example, difference between pressure measurements 1.5 m below ground surface and ambient air). For measurements
at 1.5 m or greater, temporal variability is typically not significant. However, for Measurements at shallower depths (for example,
sub-slab) also may be useful, but recognize that there is gr eater potential for temporal variability for shallower measurements or
measurements at sites with highly permeable matrices, the potential for temporal variability warrants further consideration.ma-
trices.
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FIG. 1 Tiered Evaluation Process
6.1.3 The screening values for methane concentration are, in most cases, derived from the lower flammable limit for methane in
air, that is, 5 %, since methane hazard is related to flammability rather than toxicity. Concentration, pressure, and volume should
be taken into account. Physical and toxicological characteristics of methane are summarized in Appendix X1. Additional
discussion of the screening values is provided in Appendix X2. Note that for soil gas, methane concentration alone is insufficient
to evaluate potential hazard. Information on pressures and volumes is also essential.
6.1.4 Screening values are location specific. That is, soil gas screening values should be used for comparison with site soil gas
results and indoor air/confined space screening values should be compared only with indoor air/confined space results (for
example, Table 1).
6.1.5 Volume of methane in the vadose zone for a given site will always be an estimate. The uncertainty in the estimate can be
reduced by characterizing the spatial variability. A minimum sampling interval for a developed property is typically one sub-slab
2 2
measurement per 500 m of building footprint, plus one shallow (for example, 5 ft.) soil vapor sample per 1000 m of property
area, and one deeper cluster of soil vapor samples (for example, 5 ft., 10 ft., and 20 ft.) per 2000 m of property area. If readings
of 30 % or higher methane are found, it may be helpful to better characterize the relative “hot spot” by collecting additional data
to provide a 3-dimensional concentration map in the immediate area.
6.2 Develop Conceptual Site Model (CSM)—The user is required to identify the potential primary sources of methane in the
subsurface, potential receptor points, and significant likely transport pathways from the primary sources to the receptors. Various
vapor intrusion guidance documents describe the development of CSMs (ITRC Document VI-1 and PVI-1 and EPA/OSWER),
though not for methane sites. Guidance specific to methane is available (NSW, 2020; BSI, 2013) along with guidance for
determining “worst case” conditions (CLAIRE, 2018). The CSM provides a framework for the process of evaluating methane
hazard. The CSM summarizes what is known about the site in terms of source, depth to groundwater, geology, data trends,
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TABLE 1 Suggested Default Decision Matrix for Methane in Soil Gas and Indoor Air
D
NOTE 1—Table based on Eklund, 2011 (1) and Sepich, 2008 (2) . See also Appendix X2. Table is intended for sites with existing buildings. To address
future development, no further action is recommended if the shallow soil gas concentration is <30% and ∆P <500 Pa.
NOTE 2—If the combined soil gas concentrations of methane and carbon dioxide are ≥90%, mitigation should be considered.
NOTE 3—Soil gas outside the building footprint but within a radius of 60 m (200 ft) of the building may be of interest. The total mass of methane present
should be considered (that is, concentration × volume).
Indoor Air Concentration
A
Shallow Soil Gas Conc.
No Measurements Available <0.01% (that is <100 ppm) 0.01 to <1.25% >1.25%
B
<1.25% to 5% No further action No further action No further action Immediately notify authorities,
recommend owner/operator
evacuate building
C
>5% to 30% No further action unless ∆P No further action unless ∆P No further action unless ∆P Immediately notify authorities,
B B B
>500 Pa >500 Pa >500 Pa recommend owner/operator
evacuate building
C
>30% Collect indoor air data Evaluate on case-by-case Evaluate on case-by-case Immediately notify authorities,
basis basis recommend owner/operator
evacuate building
TABLE 1 Suggested Default Decision Matrix for Methane in Soil Gas and Indoor Air
D
NOTE 1—Table based on Eklund, 2011 (1) and Sepich, 2008 (2) . See also Appendix X2. Table is intended for sites with existing buildings. To address
future development, no further action is recommended if the shallow soil gas concentration of methane is <30% and ∆P <500 Pa. The potential for
conditions to change in the future should be considered. Sites that cannot be screened out merit further evaluation (see Section 6).
NOTE 2—If the combined soil gas concentrations of methane and carbon dioxide are ≥90%, biogas generation likely is recent or on-going and mitigation
should be considered.
NOTE 3—Soil gas outside the building footprint but within a radius of 60 m (200 ft) of the building may be of interest. The total mass of methane present
should be considered (that is, concentration × volume).
Indoor Air Concentration
A
Shallow Soil Gas Conc.
No Measurements Available <0.01% (that is <100 ppm) 0.01 to <1.25% >1.25%
B
<1.25% to 5% No further action No further action Identify indoor sources Immediately notify authorities,
recommend owner/operator
evacuate building
C,E B B
>5% to 30% No further action unless ∆P Mitigate gas entry points Mitigate gas entry points Immediately notify authorities,
B
>500 Pa recommend owner/operator
evacuate building
C,E
>30% Collect indoor air data Evaluate on case-by-case Evaluate on case-by-case Immediately notify authorities,
basis basis recommend owner/operator
evacuate building
A
Maximum methane soil gas value (% CH4) for area of building footprint. Shallow soil gas refers to soil gas in the vadose zone within the top 10 m (33 ft) of soil below
ground surface.surface but at least 1 m from the building envelope. If methane exceeds 5 % within 1 m of the building, the default decision matrix is not applicable.
B
Landowner or building owner/manager should identify indoor sources and reduce/control emissions. If no sources are found, additional subsurface characterization and
continued indoor air monitoring should be considered. ∆P refers to pressure gradients in the subsurface at a depth or interval of 1.5m. For gravel 1.5 m. For sandy soils,
gravel, or other highly permeable matrices, use of a more conservative criterion less than 500 Pa (2 in. Hdirect measurement of methane flow O) may be
appropriate.appropriate (see Appendix X4).
C
The potential for pressure gradients to occur in the future at a given site should be considered.
D
The boldface numbers in parentheses refer to a list of references at the end of this standard.
E
Potential points of gas entry to the building should be identified and plugged or sealed. If P>500 Pa, further mitigation should be considered.
receptors, building design and operation, and so forth. The CSM should consider reasonable worst-case conditions such as falling
and low relative barometric pressure conditions or potential soil gas repressurization. The potential for conditions to change in the
future should be considered (for example, increase or decrease in impervious cover). The results of any further investigations are
compared with the CSM to see whether or not the results are consistent with the expectations derived from the CSM. If the results
are found to differ in material ways from these expectations, the CSM will require modification.
6.2.1 Source—Methane is produced by two primary mechanisms: thermogenic and microbial (see Appendix X1). Thermogenic
methane consists primarily of methane with relatively small amounts of ethane, propane, and higher molecular weight
hydrocarbons. Thermogenic or “fossil” methane typically originates from petroleum deposits at depths generally far below the
vadose zone. Natural gas is largely thermogenic methane and may occur in coal mines, oil and gas fields, and other geological
formations. Thermogenic methane, once produced, is carried in natural gas transmission and distribution lines. Microbial or
“biogenic” methane typically is generated at relatively shallow depths by the recent microbial decomposition of organic matter in
soil. The “biogas” produced is essentially all methane and carbon dioxide. dioxide, present at roughly equal percentages. If CH
+ CO approach 100 %, the gas is said to be “whole” or “undiluted.” Microbial methane is a product of decomposition of organic
matter in both natural (for example, wetlands and river and lake sediments) and man-made settings (for example, sewer lines, septic
systems, and manure piles). A given mass of organic carbon will have a fixed volume of biogas it can potentially generate. For
E2993 − 23
a given organic material, the
...