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PART III MITIGATION
7.0 INTRODUCTION
7.1 OBJECTIVES
Provide useful, non-prescriptive considerations that will help implement successful
mitigation of vapor intrusion impacts to indoor air.
Generally focus on engineering controls as defined by the American Society for Testing
and Materials (ASTM) as: “Physical modifications to a site or facility to reduce or
eliminate the potential for exposure to chemicals of concern” (ASTM 2005).
8.0 SOURCE REMEDIATION
This section presents an overview of remediation/abatement of the source(s) of soil vapor
(e.g., soil and/or groundwater) intruding into buildings. Generally, source remediation is
considered a long term solution to reducing concentrations to below risk-based standards,
including impacts from concentrations of contaminants in soil vapor impacting indoor air.
The building control remedies discussed in other sections under this portion of the
guidance are generally considered interim measures that are mitigating soil vapor until
source remediation is accomplished.
Remediation of source contamination will ultimately mitigate vapor intrusion impacts.
Examples of source remediation include soil removal, groundwater treatment and in-situ
technologies to remediate soil and groundwater.
A complete vapor intrusion pathway has been defined as having three components:
contaminants in soil gas, an entry route for the soil gas to enter the building and pressure
or diffusion gradients that draw the soil gas into the building. Mitigation of the vapor
intrusion pathway necessitates removing one of these components.
Ultimately, removal of the source of contaminants in soil gas is the long term solution.
Realistically, removal of the source of contaminants in the soil gas could take decades in
some situations. Therefore, interim mitigation approaches that prevent soil gas from
entering the building by short-circuiting the pressure gradients and/or diffusion gradients
that draw soil gas into the building may be necessary. Another approach may be to
remove contaminants once they have entered the building. (EPA Engineering Issue,
Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008;
p.2-3)”.
Below is a list of references that provide additional information regarding source
remediation:
http://www.clu-in.org/remed1.cfm
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http://www.epa.gov/superfund/policy/ic/index.htm
http://www.brownfieldstsc.org/pdfs/Roadmap.pdf
8.1 Source Remediation Under the MCP
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Insert a brief review of source remediation in the context of the MCP: under what
response actions might source remediation occur: IRAs, RAMs, CSAs.
Discuss Engineered Barriers vs vapor membranes discussed later.
Discuss Department roles.
Discuss Feasibility Evaluation/Considerations.
MCP Citations.
8.2 Contaminants of Concern and Conceptual Site Model
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Suggest re-stating the obvious – volatiles; but could discuss pre-dominance of
organic compounds, review classes of VOCs and sources: petroleum USTs,
chemical USTs, chemical dumping, improper disposal, drywells, leaching fields,
illegal land disposal.
Present specific links to Part II Assessment.
If not covered in Assessment, introduce and refer to appendices with Look Up
tables of Chemical/Compound properties and characteristics.
Overview of the limited inorganic volatiles of interest (Mercury, H2S, CN-?), and
potential non-MCP volatiles (methane, H2S), etc.
Discuss the elements of the CSM that would relate to the sources of the volatiles,
and how those elements would support decisions on source remediation.
8.3 Mitigation and Abatement Technologies
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Develop an objective for this subsection, i.e., to present an overview and directory
of references for a large array of technologies and methods. Note references to
DEP documents, US EPA documents, and technical literature.
Matrix or “thumbnail descriptions”, as below for SVE. Include definitions.
8.3.1 Soil vapor extraction
Soil vapor extraction (SVE), which is generally considered a technology used to
remediate source contamination in the vadose zone away from the building, may be
designed to mitigate vapor intrusion impacts in the indoor air space. The use of an SVE
system to mitigate vapor intrusion impacts is only applicable if it can be demonstrated
that the SVE system adequately depressurizes beneath the entire building foundation.
(EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R08-115, October 2008; p.17)”.
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Discuss major design and implementation topics.
Refer to EPA and other manuals on SVE
8.3.2 In-Situ Chemical Oxidation
In-situ chemical oxidation (ISCO) is a set of technologies that may be applied in the
remediation of contaminated soil and groundwater, which may be the source of volatile
contaminants contributing in whole or part to indoor air impacts. ISCO typically
involves injecting or applying chemical oxidants and, potentially, related chemicals
directly into the source zone and/or downgradient portions of a plume. The objective is
for the oxidants to react with the contaminants of concern producing non-toxic, common
end products such as carbon dioxide, chlorides, water and minerals. Many chemical
reaction steps may be involved and chemical reaction intermediates may be produced.
Inducing the chemical reactions to go to their endpoints and/or controlling and mitigating
chemical intermediates are important considerations in the implementation of ISCO.
Primary advantages to the use of ISCO, as described in the technical literature include:
 In-situ destruction of the COCs, rather than transfer to another media for disposal;
 Limiting off-site disposal and generation of wastes;
 Relatively rapid cleanup.
Potential dis-advantages to the use of ISCO, as described in the technical literature
include:
 Incomplete destruction of the COCs, resulting in generation of chemical
intermediates and the potential toxicities of those intermediates;
 Potential short-term generation of fumes and gases during reactions;
 Safety issues with some oxidants, which can be dangerous to handle in high
strengths; and
 Relatively high costs.
Four major groups of oxidants are currently in use or development for the remediation of
commonly encountered soil and groundwater contamination:
1. Peroxides, most commonly hydrogen peroxide or activated (catalyzed)
hydrogen peroxide;
2. Permanganates, sodium and potassium;
3. Persulfates; and
4. Ozone.
Various mixtures of the classes of oxidants are also in use or development, including
many proprietary compounds developed specifically for remediation.
Most organic contaminants encountered at Massachusetts release sites are amenable to
destruction/treatment by ISCO, including broad range petroleum hydrocarbons, BTEX,
MTB, many chlorinated solvents, organic pesticides, PCBs (according to the literature
and company claims) and phenols.
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(Technical and Regulatory Guidance for In Situ Chemical Oxidation of Contaminated
Groundwater & Soil, ITRC, Second Edition, 2005)
[many other references could be listed, including EPA and web-sites.]
8.3.3 Biodegradation
text to be developed. Lots of information available
8.3.4 Groundwater Pump and Treat
[Should not be overlooked for migration control, and, therefore, to interrupt a pathway to
a downgradient receptor. ]
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Insert general description, including in what situations it may be applicable –
groundwater contamination (versus vadose zone soil contamination), appropriate
aquifer characteristics, etc.
 Note that it is not generally considered an effective remediation technology, but
can be effective for migration control - interrupt a critical pathway to a
downgradient receptor.
 Discuss general design and implementation topics: aquifer characteristics,
evaluate if plume migration be effectively stopped, etc.
 Discuss treatment requirements, permitting, typical technologies.
Discuss discharge options, permitting, practical issues – recharge vs discharge
8.3.5 Removal and Off-Site Disposal or Recycling/Beneficial Reuse
[Can be cost effective and with recycling/beneficial re-use can meet 21E objectives.]
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Insert general description, including in what situations it may be applicable or
most appropriate – petroleum soils versus solvents.
Obviously not an on-site destructive technology, but with recycling/beneficial reuse (typically in-state) can meet 21E objectives
Discuss implementation topics, including specific technical and Department
information/policies regarding under what conditions would Federal or Mass
hazardous waste be generated.
Cost effectiveness: very different for petroleum soils versus listed or characteristic
haz. waste soils.
8.3.6 Soil Pile Treatment
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Pile treatment by various means – chemical, VE, etc.]
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Need clarification of current Department policy/regulation and EPA
regulation and guidance on what contaminants, sites and pile locations are
suitable for this approach (i.e., under what conditions would Federal or Mass
hazardous waste be generated, or is not generated in construction of the pile.)
Criteria for onsite re-use, off-site disposal. Specifics and policy references.
9.0 INDOOR AIR PATHWAY MITIGATION
9.1 Introduction
“In general, after conducting any necessary emergency response activities (e.g., building
venting), further mitigative remedial measures should generally proceed in an iterative
fashion, starting with the least invasive/least costly, and progressing as needed to more
and more invasive and costly measures, until remedial endpoints are achieved (see
Flowchart II-3).(Indoor Air SOP, 2007; p.15)”
To prevent the entry of the contaminants into the building, one must do one of the
following: eliminate the entry routes or ; remove or reverse the driving forces ( the
negative pressure or diffusion gradients) that induce the contaminants into the building or
provide a preferential pathway to divert contaminants away from the structure.
The two general approaches to eliminating the entry routes are to seal the individual
routes or to create a barrier that isolates the internal building spaces from the soil gas or
prevents migration through all entry routes of the soil gas.
[I think this is a good, very short summary. But, it contrasts with the very effective
division of the rest of this section into Active and Passive mitigation strategies. So, I
suggest the following:]
These general approaches correlate approximately to passive mitigation strategies,
discussed in subsection 9.4 and active mitigation strategies presented in subsection 9.5.
In general, after conducting any necessary emergency response activities (e.g., building
venting), further mitigative remedial measures should generally proceed in an iterative
fashion, starting with the least invasive/least costly, and progressing as needed to more
and more invasive and costly measures, until remedial endpoints are achieved (see
Flowchart II-3). (SOP pg. 16)
Sub-slab depressurization system is the preferred method of mitigation for structures with
basement slab or slab on grade foundation (NYSDOH 2006, p. 58).
Building controls are essentially the prevention of contaminants into the building.
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Eliminate contaminant entry routes. In existing buildings this typically includes sealing
cracks and voids in the slab or foundation. In new construction this typically includes the
installation of a barrier, usually a membrane, that impedes the flow of soil gas into the
building.
Short-circuit the negative pressure or diffusion gradients forcing contaminants into the
building. Advective flow is generally accepted as the predominant mechanism of soil gas
infiltration and will be the focus of this guidance. Although, methods of mitigating
advective flow of soil gas into the building will generally mitigate diffusive flow as well.
Mitigating the advective flow of soil gas into indoor air may be accomplished by creating
a negative pressure in sub-slab soil gas or creating a positive pressure throughout the
building. Sub-slab depressurization systems are the most common and proven method of
mitigating soil vapor intrusion. Sub-slab depressurization systems use a pipe installed
beneath the slab connected to a mechanically powered fan to draw soil gas from directly
beneath the slab and vent to the atmosphere. (EPA Engineering Issue, Indoor Air Vapor
Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008; p.4)”.
Contaminants that have entered the building need to be removed. This is often the case
immediately following the identification of contaminants in indoor air (e.g., during
immediate response actions). Increased ventilation is one method of removing
contaminants from the indoor air space. Forced ventilation using a fan to blow air into or
out of a building is generally used to dilute contaminants in indoor air as an interim
measure. Additional information and considerations regarding ventilation and
contaminant removal is contained in sections _____ and _____ respectively. (EPA
Engineering Issue, Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R-08115, October 2008; p.6)”. )”. [Exhausting air needs to be carfully balanced with
appropriate “make up” air to minimize potentially for backdrafting of appliances and/or
extinguishing pilot lights…etc (Wiley)]
Mitigation of vapor intrusion at existing and new construction.
Typical reasons for mitigation include data identifying unacceptable risk posed by
contaminants in soil vapor that have infiltrated the indoor air. The method of mitigation is
based on site specific factors that include timing of response actions necessary to mitigate
unacceptable concentrations of contaminants in indoor air. For example, an immediate
response action may incorporate active venting of a basement as an interim measure until
the source is abated and/or a sub-slab depressurization system can be installed.
The pressure gradient that drives advective flow into the building can be neutralized or
reversed by inducing a positive pressure in the building or a negative pressure in the subslab soil gas. Sub-slab ventilation/depressurization systems are the most common method
for producing a negative pressure in the sub-slab soil gas. Sub-slab ventilation may also
significantly reduce the diffusion gradient across the foundation “. (EPA Engineering
Issue, Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R-08-115, October
2008; p.6)
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If contaminants have entered the building, it is necessary to remove them. Increased
ventilation accomplished by opening windows, doors and vents is one means of removing
contaminants. Mechanical ventilation may be through the use of a fan to blow air into or
out of the building may also be used to remove contaminants. Exhausting air from a
building may contribute to a negative pressure in the building which may result in
increased infiltration of soil gas.] (EPA Engineering Issue, Indoor Air Vapor Intrusion
Mitigation Approaches, EPA/600/R-08-115, October 2008; p.6)”.
9.2 Vapor Intrusion Pathway Mitigation Under the MCP
Once a pathway of relevance is confirmed, it should be evaluated with respect to risk
issues and, in cases of homes and schools, Critical Exposure Pathways:
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in all cases, an Imminent Hazard condition must be promptly addressed
and eliminated. Eventually, a condition of No Significant Risk must be
achieved; and
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to the extent feasible, Critical Exposure Pathways must be eliminated or
mitigated, with feasible defined as benefit vs. cost. Note that feasible in
this context is not synonymous with possible, but must consider such
issues as effectiveness, costs, fate of contaminant, access, and logistical
issues. If the owner of an owner-occupied home does not want a sub-slab
depressurization system installed or operated in his/her home, and if a
condition of No Significant Risk exists, that measure would be considered
infeasible, with respect to the need to address a Critical Exposure
Pathway.
[end of 9.2. Expand it to bring in more MCP considerations, as per Section 1.2 of
the current WSC Policy #02-430?]
Address releases to building interior with respect to indoor air contamination?
9.3 Vapor Intrusion Mechanisms
[In my opinion, there should be a review or summary of the basic mechanisms of vapor
intrusion that are presented in the early subsections of Section II of the DEP SOP, even if
that material is already presented in the Assessment part of the new document. If it is not
covered in detail the Assessment part, than it should be here, in my opinion.]
9.4
PASSIVE CONTROLS
Available Passive mitigation strategies include:
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Passive sub-slab venting
Sealing building cracks, gaps and voids or installation of vapor barriers in new
construction
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Modification of the building foundation
Increasing natural ventilation by opening windows, doors, and vents
Selective placement of building location to avoid contact with contaminated soil
vapor
Pier construction
Selective placement of occupance spaces in the building that are impacted by
vapor intrusion
Specialized building designs that reduce stack effect, building orientation with
respect to prevailing winds, incorporation of additional windows or increased
ventilation on the lowest level (e.g., garage on lowest level)
Combination of mitigation methods may be necessary based on foundation type
(NYSDOH, 2006; p.58) or timing of mitigation measures.
Active systems necessary to produce large decreases in vapor intrusion.
Passive sub-slab mitigation systems have been shown to range between 30% to 90%
efficient.
There have been few studies designed to evaluate the effectiveness of passive systems
over long periods of time.
Passive depressurization systems may be less effective during warm seasons (Air
conditioning reduces temperature gradient and reduces stack effect).
Many passive systems may be designed so that the addition of a fan could transform the
system from passive to an active depressurization system if additional negative pressure
beneath the slab is necessary to mitigate the intrusion of soil vapor to the structure. (EPA
Engineering Issue, Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R-08115, October 2008; p.10)”.
9.4.1
Characterization of Building Foundation
An inspection of the building foundation should be conducted, with particular attention paid
to identifying all potential entry routes for VOC contaminated soil gases, such as cracks in
concrete walls or slabs, gaps in fieldstone walls, construction joints between walls and slabs,
annular space around utility pipes, open sumps, etc. These potential entry points should be
surveyed with a portable PID or FID meter; it is often possible to find discrete "hits" (>1
ppmV) [(ppbV) reference to ppbv Rae. These devices are valuble tools during soil vapor
intrusion evalutations. Wiley] at particular points where vapor intrusion is occurring.
All possible entry routes should be sealed off, if possible, to prevent the entrance of soil gas,
and enhance the sub-slab negative pressure field when the SSD system is in operation.
Sealing/caulking materials should not contain significant amounts of VOC's. Buildings with
no slabs should have an impermeable barrier installed before considering SSD.
A particularly problematic feature of commercial and school buildings is the presence of
floor drains in lavatories and other areas. Often, the water seal within the plumbing trap of
these drains is ineffective, as the water either leaks out or evaporates. This provides a
vehicle for soil gases and/or sewer gases [MCP issue?] to discharge into these areas
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(especially true in lavatories with fans or vents which create a negative pressure within these
rooms). In such cases, efforts should be made to periodically add water to these traps, or to
install a Dranjer type seal. (see http://www.dranjer.ca/) (SOP, Toolbox 4)
[It is noted that sealing of sumps that work by collected groundwater/water flowing off the top of the floor
and into the sump at a low point, can prevent the sump from functioning and caused flooding of the
basement (e.g. water cannot get into the sump becauses it is covered) Wiley
Foundation types are classified as:
 Basements (with concrete slabs or dirt floors)
 Slab on grade
 Slab below grade
 Foundation/crawlspace (foundation may be wood, stone, brick or block masonry,
poured in place concrete or precast concrete panels)
 Footings/piers
 Mobile home
Typically slabs are supported beneath the load bearing walls or foundation walls by block
or concrete footings or a thicker section of poured concrete.
Cinder block wall can be a significant entry route of soil vapor due to the permeability of
the concrete.
Some of the most common entry routes of vapor intrusion have been identified as: the
seams between construction materials (expansion and other joints), utility penetrations
and sumps, elevator shafts, and cracks. (EPA Engineering Issue, Indoor Air Vapor
Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008; p.6)”.
http://www.clu-in.org/conf/tio/vapor
9.4.2 Sealing of Cracks, Sumps, Utility Conduit Penetrations
Small Cracks and Joints
Accessible cracks and joints up to 1/8th inch (0.125”) in width and depth should be sealed
with an elastomeric sealant (e.g., caulking) in accordance with the manufacturer’s
instructions. These sealants must be specifically designed to seal concrete, have low
odor, low VOC content (i.e., less than 100 grams VOCs per Liter), and must not contain
ingredients known to cause cancer, birth defects, or other reproductive harm, per the state
of California “Proposition 65” (this information/statement should be provided in the
product’s MSDS). Surfaces to be sealed must be clean, dry, and free of soil, decomposed
concrete, dust, grease and debris. [there are a lot of other products, including the many
foams used in the insulation industry.]
Large Cracks and Joints
Accessible cracks and joints larger than 1/8th inch (0.125”) shall be either (i) sealed in
accordance with the provisions of Section 7.1, utilizing a foam backer rod or other
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comparable filler material as per the manufacturer’s instruction; and/or (ii) filled with
non-shrinking cementitious material.
Sumps
The presence of a sump in a basement can provide a significant short-circuiting vehicle to
the establishment of a subslab negative pressure field. In such cases, an air tight cover
should be installed over the sump; if a sump pump is present, the cover should be equipped
with appropriate fittings or grommets to ensure an air tight seal around piping and wiring,
and the cover itself should be fitted with a gasket to ensure an air-tight seal to the slab while
facilitating easy access to the pump. Note that it is also possible to use the sump as a soil
gas extraction point (where appropriate); a number of manufacturers make equipment for
just such applications. (SOP, Toolbox 4)
The contractor shall ensure that sumps do not drain or pump to a sanitary sewerage
system. Exceptions or uncertainties in this regard shall be reported to the MassDEP
Project Manager.
Drainage sumps shall be covered by a lid constructed of durable plastic or other rigid
material that produces an airtight seal. If a sump pump is present, the lid must be easily
removable by the owner/occupant of the building. Electrical wiring and water ejection
piping penetrating the lid shall be made airtight by the use of grommets and/or sealants.
If necessary to prevent short-circuiting of the SSD system, a check valve or water trap
shall be installed on the sump drain/ejection piping.
Floor Drains
Floor drains that are not in use shall be sealed with concrete or grout, but only if
permission is obtained from the building owner. Floor drains that need to be maintained
shall be inspected and evaluated with respect to the potential to short-circuit the SSD
system. Floor drains that do not have an acceptable water trap shall be so modified or
retrofitted with a Dranjer or equivalent insert. (SOP, Toolbox 5)
Utility Conduit Seals
“ Seals should be retrofit at the termination of all utility conduits to reduce the portencial
for gas migration along the conduit of the interior of the building. These seals should be
constructed of closed cell polyurethane foam, or other inert gas-impermeable material,
extending a minimum of six conduit diameters or six inches, whichever is greater, into
the conduit. Wye seals should not be used for main electrical feed lines”. (EPA
Engineering Issue, Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R-08115, October 2008; p.14)”. [More permeable utility pipe bedding can be a preferential pathway for
vapor migration into a structure. Mitigation in these instances can include venting of the utility bedding
itself if sealing of penetrations is not feasible or is ineffective. Wiley]
9.4.3 Passive Barriers
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Passive barriers include sheet membranes and poured/cure-in-place products. Clay
barriers have also been used in new construction. Sheet membranes used as passive
barriers typically require 40-60 mil high-density polyethylene (HDPE). Polyvinyl
chloride or EPDM (ethylene propylene diene monomer) rubber may also be used. In
order for passive membranes to be effective, they must be durable enough to withstand
damage during the installation of the slab including placement of reinforcing steel,
concrete, and crushed stone. Therefore, sheet membranes less than 30 mil thickness are
not recommended. Fluid applied membranes are spray applied to the required thickness
(e.g., 40 mil). (EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation
Approaches, EPA/600/R-08-115, October 2008; p.22, Section 3.3)”.
[Proper installation and QAQC of seams, joints, and welds is critical to the performance
of a passive barrier…eg. Smoke/pressure testing, testing of heat welds. Etc…. Wiley]
Passive barriers are primarily suited to use in new construction or renovations where it is
possible to remove and replace existing floor slab or to install a barrier system and “false
floor” over the existing floor.
California gas barrier requirements listed here: (EPA Engineering Issue, Indoor Air
Vapor Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008; p.31)
In subterranean basements, must ensure walls located below ground are evaluated as
conduits of contaminated soil vapor. Specifically, but not exclusively, block and
fieldstone foundations which tend to have cracks, gaps, or are exceptionally porous.
Cracks in slab or openings near utility penetrations should be sealed.
Expansion gaps between the slab and basement wall.
Soil vapor has been observed to migrate through the cores in block walls or pores in the
blocks themselves.
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Discuss vapor barriers in building code?
Seams and variety of methods to join membranes are important considerations.
Lower cost vapor barriers may be protected with sand.
9.4.4 Passive Ventilation
Natural ventilation occurs in all buildings and is increased by opening doors, windows
and vents. Increased ventilation allows outdoor air to enter the structure, mixing with
indoor air and reducing/diluting indoor concentrations of contaminants. In a typical
building in which a “stack effect” is occurring, opening a window only in an upper story
can exacerbate the stack effect, increasing the flow of soil gas into the structure, which is
counterproductive. Once the windows, doors and vents are closed, concentrations that
were reduced by natural ventilation return to previously observed concentrations within
about 12 hours. Natural ventilation should be regarded solely as a temporary solution to
reduce concentrations of contaminants in indoor air because the cost of heating or air
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conditioning will require windows, doors and vents to be closed. Ventilation of the
occupied space without pressurization has been shown to achieve only partial reductions
in concentrations (50-75 percent) Additional increases in ventilation rates can become
uncomfortable for occupants (CIRIA, 1994).(EPA Engineering Issue, Indoor Air Vapor
Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008; p.23/24)”.
9.4.5 Passive Venting
“EPA has defined a passive sub-slab depressurization system as “ A system designed to
achieve lower sub-slab air pressure relative to indoor air pressure by use of a vent pipe
routed through the conditioned space of a building and venting to the outdoor air, thereby
relying solely on the convective flow of air upward in the vent to draw air from beneath
the slab” (http://www.epa.gov/radon/pubs/newconst.html)”. This includes systems that
have a wind-driven turbine to enhance convective flow. (EPA Engineering Issue, Indoor
Air Vapor Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008; p.18)”.
[These systems have limited effectiveness for exisiting buildings, unless void spaces or
highly permeable subsurface material are present. Designs for these systems should
consider the potential need to convert them to active if necessary.] Wiley
Ideally, the vent pipe routed through the conditioned space of a building reduces the
pressure zone beneath the building and prevents contaminants in the soil gas from
entering the building. Because mechanical devices are not used to reduce the pressure
zone within the system, understanding the parameters that impact the effectiveness of
passive systems, such as wind and stack height, is important for proper performance of
the system. (EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation Approaches,
EPA/600/R-08-115, October 2008; p.18)”.
The primary factor determining performance of the passive venting system is the
buoyancy of the air that is warmed as it passes through the heated indoor air space.
Because this advective force is small, it is recommended that piping be large diameter (4
inch diameter) and rise vertically from the collection points with as few bends in the
piping as possible. During the cooling season, it is possible that flow may be reduced and
minor reverse stack effects may be observed. (EPA Engineering Issue, Indoor Air Vapor
Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008; p.18)”.
EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R08-115, October 2008; p. 19 explains why passive less effective than active mitigation.
The equipment used for an active sub-slab depressurization (SSD) system is similar to
passive sub-slab ventilation systems.
Radon mitigation systems are usually designed to produce a sub-slab pressure field that
compensates for the depressurization within the building. The average range of
soil/building depressurization is approximately 4-10 Pa. Therefore, a mitigation system
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that compensates for a minimum of 4-10 Pa everywhere beneath the slab should mitigate
the vapor intrusion. Actual demonstration of the mitigation systems ability to reduce
indoor air concentrations to below the required (EPA Engineering Issue, Indoor Air
Vapor Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008; p.10)”.
“Proper sealing of penetrations and entryways is especially important for a passive
system because minor leaks in buildings can offset the small pressure differentials that
passive systems rely on.”
(EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R08-115, October 2008; p.14)”.
9.5
ACTIVE CONTROLS
Available active mitigation strategies include:
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Sub-slab depressurization systems
Drain-tile depressurization
Block wall depressurization
Sub-membrane depressurization
Soil vapor extraction could be designed so that the radius of influence creates a
negative pressure beneath the slab
Indoor air purification using adsorbtion technology (e.g., carbon adsorbtion)
Heat recovery ventilation technology
Modifications to building HVAC systems to induce a sustained positive pressure
within the structure.
Active injection of air beneath a building to enhance venting?
9.5.1
Subslab Ventilation Systems
Sub-slab ventilation is achieved by producing adequate air flow beneath the slab to
sufficiently dilute VOCs diffusing from soil or groundwater. (EPA Engineering Issue,
Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008;
p.14)”.
Adequate performance of an SSV system is more difficult to determine than an SSD
system because the flows necessary for dilution of VOCs beneath the slab is difficult to
determine. Measuring negative pressures beneath the slab provides some indication of
system efficacy. However, this measurement is less useful in determining SSV
effectiveness than SSD effectiveness because the pressure/rate of ventilation required to
provide a working margin of safety is difficult to determine. (EPA Engineering Issue,
Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008;
p.15)”.
9.5.2
Subslab Depressurization Systems
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EPA defines SSD technology as: “a system designed to achieve a lower sub-slab air
pressure relative to indoor air pressure by use of a fan-powered vent drawing air from
beneath the slab”. (EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation
Approaches, EPA/600/R-08-115, October 2008; p.14)”.
“A sub-slab depressurization (SSD) system is a proven technique to eliminate or mitigate
vapor intrusion into impacted structures (See Figure 4-1). Based upon traditional radonmitigation technology, this approach creates a negative
pressure field beneath a structure of concern, inducing the
flow of VOC vapors to one or more collection points, with
subsequent discharge up a stack into the ambient air. In
essence, the system “short circuits” the subsurface VOC
vapor migration pathway, eliminating or reducing exposures
to building occupants (SOP, 2007; p.20)”
“Importantly, this is a somewhat invasive, energy &
maintenance intensive remedial measure, and therefore an
option of secondary resort. Moreover, there are certain site
and building conditions (e.g., high groundwater table) that
may preclude or limit its application. Therefore, before
pursuing this option, it is essential that conclusive evidence
exist documenting the presence of a subsurface VOC source
and/or migration pathway, and that less invasive steps be Figure 4-1: SSD System
initially considered and/or implemented. Where appropriate,
this effort should include investigations to identify possible
source/source areas, and source control or mitigation measures (SOP, 2007; p.20)
While SSD systems are considered a remedial activity and measure under the MCP, they are
typically not a component of a site-wide (soil and groundwater) remediation approach.
Rather, their design objective is to prevent soil gases from infiltrating a building… in most
cases SSD systems will not have an appreciable impact on site contaminant levels (SOP,
2007; p20).”
9.5.2.1 Description of the SSD System
“A sub-slab depressurization system basically consists of a fan or blower that draws air from
the soil beneath a building and discharges it to the atmosphere through a series of collection
and discharge pipes. One or more holes are cut through the building slab so that the
extraction pipe(s) can be placed in contact with sub grade materials, in order for soil gas to
be drawn in from just beneath the slab. In some cases the system may require horizontal
extraction point(s) through a foundation wall, although in most cases the pressure field from
an extraction point in the slab will extend upward adjacent to the foundation walls.
SSD systems are generally categorized as Low Pressure/High Flow or High Pressure/Low
Flow. Site conditions dictate which approach and system is most appropriate.
15
Some buildings have pervious fill/soil materials beneath the slab. Soil gas/air movement through
such materials is rapid, and only a slight vacuum will create high flowrates. In such cases, the SSD
system should utilize a low pressure/high flow fan. Other building slabs are underlain by less
pervious materials, and common fan units will not be able to draw the appropriate level of vacuum.
In these cases, a high pressure/low flow blower unit is required, capable of creating high vacuum
levels.
Low Pressure/High Flow systems generally use 3-4 inch diameter piping; High Pressure/Low Flow
systems may use smaller diameter piping. This piping is generally run from the extraction point(s)
through an exterior wall to the outside of the building. The piping is connected to a fan/blower,
which is mounted either on the outside of the building or in the attic. Placement of the fan/blower in
this manner ensures that a pressurized discharge pipe is not present within occupied spaces (in case
of leakage). Exhaust piping is run so that the discharge is above the roofline (SOP, 2007; p21-22).”
9.5.3 Submembrane Depressurization
In buildings with a crawlspace foundation or dirt floor basement, it may be appropriate to
use a membrane to install a sub-membrane depressurization (SMD) system. A cross
laminated polyethylene membrane or equivalent flexible sheeting membrane with a
recommended minimum thickness of 9 mil is recommended to withstand damage in
heavily trafficked areas. The flexible sheeting membrane should cover the entire floor
area and be sealed at the seams and floor/wall interface. The sheeting should not be
pulled tight during installation. When the depressurization system is turned on the
membrane will be drawn down, potentially straining the seals at the seams and floor/wall
interface. (EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation Approaches,
EPA/600/R-08-115, October 2008; p.22, Section 3.3)”.
9.5.4
Subslab Pressurization
In situations where both sub-slab depressurization (SSD) and sub-slab ventilation (SSV)
have proven to be ineffective, sub-slab pressurization (SSP) may be possible. SSP may be
effective in situations where the permeability of the soil is too high to allow adequate
pressure to be created for the SSD and the fan does not generate enough flow for
effective SSV. In this situation the fan may be installed so that it blows into the sub-slab
environment, creating a flow away from the slab. (EPA Engineering Issue, Indoor Air
Vapor Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008; p.15)”. (See
EPA 1993b and ITRC 2007)
9.5.5
Building Pressurization/ HVAC Modification
In certain situations, it is possible to modify the HVAC system to create positive pressure
within at least the lower level of the structure to effectively mitigate vapor intrusion.
Postive pressure within the building must be consistently maintained so that advective
flow of subsurface soil into the structure is not occurring. This approach may not be
suitable to older buildings since they may not be as air tight as newer buildings, which
16
would make this approach more costly. Heating and air conditioning systems may need to
be modified from running on an as-needed basis to running continuously. Although this
approach may be capable of reducing advective forces, diffusive flow may continue.
(EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R08-115, October 2008; p.23)”.
9.5.6 Off-Gas Controls and Treatment
“In accordance with DEP Policy #WSC-94-150, off-gas control systems are not required for
SSD systems, provided that the system will emit less than 100 pounds/year of VOCs and
will not cause air pollution/odor problems in the surrounding area (SOP, 2007; p.31)”

9.6
Discuss in detail and/or by reference with a summary, the current Department
policy on off-gas controls WSC-94-150.
Indoor Air Treatment
[This could be a separate section. It could include discussion of this technology/strategy
as a temporary measure. Could also discuss localized, mobile treatment units versus
full-building systems.]
Indoor air treatment refers to equipment used to remove contaminants that are already
present in indoor air. Indoor air treatment equipment typically employ zeolite and carbon
sorbtion, ozone oxidation or photocatalytic oxidation to remove contaminants.
Technologies that rely on injecting ozone into the indoor air cannot be recommended
because ozone is a criteria pollutant. Mitigation methods that incorporate adsorbtion such
as zeolites and carbon generate waste that must be regenerated or disposed of properly.
Mitigation systems that incorporate sorbtion filters generally had better removal
efficiencies with filters that have more surface area and better air-to-sorbent contact. Use
of indoor air treatment devices may be a good alternative at locations where a high
groundwater table precludes the installation of a sub-slab mitigation system. (EPA
Engineering Issue, Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R-08115, October 2008; p.25)
10.0 MITIGATION SYSTEM SELECTION AND DESIGN CONSIDERATIONS
10.1 General Mitigation System and Design Considerations
The objective of the mitigation technology must be clearly defined and quantified.
The most important input to selecting a mitigation technology should be whether the
technology is capable of achieving the necessary reduction in contaminants.
17
The mitigation system must reliably reduce indoor air contaminant concentrations to
acceptable levels in the short and long term, if applicable.
Considerations with respect to mitigation system reliability include: the systems’ ability
to consistently reduce concentrations of indoor air contaminants below the target
concentrations; the system should be resistant to failure and if break downs occur they
should generally be easily identified and corrected; the system should be “resistant to
harm from reasonably foreseeable events occurring around it”.
Another consideration is the interaction of building occupants with the mitigation system.
The mitigation system may be modified/turned off if it is noisy, windows and vents
promoting ventilation may be closed, etc. Therefore, building use and occupants must be
taken into consideration with respect to the ancillary affects/impacts of the mitigation
system. In addition, information dissemination and training of building occupants may
minimize the likelihood of mitigation system disturbance. (EPA Engineering Issue,
Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008)”.
Type of system depends on the reduction in soil vapor concentrations and necessary
reductions in risk. Acute risks must also be mitigated more quickly. This could be
accomplished by implementing one technology on a short term basis (e.g., active venting)
while another, more long-term technique is implemented (e.g., sub-slab depressurization).
System reliability (engineering redundancy) higher for acute risk rather than chronic risk.
Concentrations representing acute risks are usually much higher than concentrations
representing chronic risk. (EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation
Approaches, EPA/600/R-08-115, October 2008; p.36)
Elaborate on these Ideas?
Costs – Capital cost, installation cost, operation and maintenance cost, monitoring cost
Ease of public acceptance
Other characteristics of indoor air including appropriate levels of humidity, carbon
dioxide, carbon monoxide, temperature, particulates, dust, mold, allergens, and airflow
must be maintained. (EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation
Approaches, EPA/600/R-08-115, October 2008; p.26-27)”.
Moisture concerns should be evaluated. (EPA Engineering Issue, Indoor Air Vapor
Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008; Section 4.3.1 p.28)
10.2 New and Existing Buildings
10.2.1 New Buildings
18
Siting of new construction as a tool in preventing vapor intrusion. Sites that may likely
cause indoor air contamination from soil vapor intrusion may be better left as green
space.
Expansion joint is often an entry point for soil vapor.
Monolithic pour eliminates the expansion joint that exists in construction where the slab
is floating on a foundation.
The installation of sub-slab VOC collection and vent piping and a membrane system is a
popular method for mitigating potential vapor intrusion issues in new construction. In
situations where relatively small differences in sub-slab pressures are necessary to
mitigate the intrusion of sub-slab vapors, the system may be operated as a passive subslab depressurization system. The existing sub-slab VOC collection and vent piping can
be converted to an active system if greater negative pressures are necessary in the subslab or more reliability is required.
Design of new buildings, such as an open parking garage on the ground floor of a
building, can short-circuit a potential vapor intrusion pathway.
New construction has the advantage of being able to create a permeable layer (e.g.,
gravel, drainage mat) beneath the slab to enhance the influence of negative pressure
beneath the slab.
(EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R08-115, October 2008; p.31)”.
Massachusetts requirements for passive barriers (gas barriers, membranes)?
10.2.2 Existing Buildings
For existing structures, the building characteristics and subsurface conditions determine
the type of mitigation system that is most appropriate. In most cases, existing structures
or slabs can be modified to incorporate a vapor intrusion mitigation system cost
effectively.
Site specific factors such as soil type and density and groundwater level may limit some
mitigation system alternatives.
ASTM (2005) section X2.3.2.2(d)
Crawlspaces – passive or active venting of crawlspaces may be sufficient to mitigate
vapor intrusion in the short term or when minor reductions in soil vapor contaminant
concentrations are necessary. Ventilation may lead to pipes freezing, not cost efficient
during cold months.
Building Size ?
19
10.3 Building Use
Building use may be categorized as follows:
 Residential – single family or multi-family
 Commercial or mixed use (commercial/residential structures)
 Industrial
 Educational
 Religious/Institution
Building use is an important consideration with respect to mitigating vapor intrusion
issues for a variety of reasons. The building type suggests the amount of time people
occupy the building. This consideration is important with respect to identifying exposure
scenarios and characterizing risk which is discussed in Section ___ of this document. A
change in building use after a mitigation system has been installed would require a
reevaluation of the mitigation system objectives and exposure scenarios.
Buildings used for different purposes are generally constructed differently (i.e.,
residential structures v.s. a manufacturing facility) and therefore have different factors
influencing the air exchange rate (AER) within the buildings. (EPA Engineering Issue,
Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008;
p.6)”.
Buildings used for different purposes may have different standards for acceptable
concentrations of contaminants in indoor air. For example, a commercial dry cleaner
must adhere to the OSHA standard for perchloroethylene (PCE) in indoor air within the
facility to ensure worker protection. The OSHA standard for PCE is not considered
protective for the same concentration of PCE in the indoor air of a residence.
(See Indoor Air SOP , 2007, p. 16-17)
Brownfield Considerations?
With respect to development or redevelopment of contaminated Sites, it is important to
identify actual or potential vapor intrusion issues as soon as possible in the planning
process. Early incorporation of engineering designs to mitigate or obviate indoor air
contamination resulting from soil vapor intrusion may result in significant cost savings.
10.4 Soil Type
Soil type and density are especially important considerations during the installation of a
sub-slab mitigation system in existing structures. This includes structures, such as
residences with basements or slab-on-grade construction. Information detailing material
beneath the slab should be collected before system design. Specifically, this information
should include soil type, density (porosity), and moisture content; the presence of
aggregate drainage layers; the presence of moisture barriers; and typical range of
groundwater depth.
20
“Knowledge/information on the fill/soil conditions beneath the slab is desirable. Small diameter
test holes can be drilled through the slab at various representative locations to collect sub-slab
material for visual inspection. Test holes should be installed above the groundwater table and
should not be deeper than one foot. A general evaluation of the material's permeability should be
made. Test holes and visual inspection of sub-slab materials are not essential, however, as system
design is based primarily on the results of pressure testing (SOP, 2007; p.23).”
10.5 Water Table Depth
“The depth to groundwater should be ascertained. In general, the groundwater table should
be at least 6 inches below the building slab for an SSD system to be effective. Seasonal
changes in groundwater elevation should be considered when evaluating the feasibility of
SSD systems(SOP, 2007; p.23).”
10.6 Conceptual Model of Building Air Exchange
Advection occurs when pressure in the ground floor (basement, slab, crawl space) is
lower than the pressure in the soil beneath the building. This pressure difference between
the soil below the building and the space in the lowest portion within the building
promotes the advective movement of soil gas through cracks and openings into the indoor
air.
Factors contributing to the negative pressure often observed in buildings are winds,
barometric pressure changes, and the stack effect. Stack effect refers to the phenomenon
of warmer air rising within a building which is being replaced by and actually drawing
cooler air into the building from various locations including the air within the interstitial
soil pores beneath the building.
Stack effect may be less severe during the summer months as many commercial buildings
and some residences utilize cooling systems. (couldn’t stack effect in the summer be
more significant for residences and buildings without cooling systems?)
Empirical observations indicate that even with cooling systems in operation during
summer months, stack effect is still a valid model of air movement within buildings, at
least over a 24 hour period (e.g., radon problems still observed in Florida where most
buildings are equipped with cooling systems).
Soil vapor temperature still may be lower than indoor air temperature in summer
time.(find data to support this claim. If gw any indication, may be around 50 degrees
farenheit).
Advection is the primary mechanism of soil vapor intrusion to indoor air.
Diffusion is ancillary mechanism of soil vapor intrusion to indoor air.
Diffusion is a separate distinct phenomenon in which molecules move from areas of high
concentration to areas of low concentration.
Diffusion becomes a more important mechanism when advective flow controlled by
sealing cracks and voids in the slab and/or foundation walls. However, diffusion typically
only becomes a significant pathway when soil vapor concentrations beneath the slab are
exceptionally high or the concrete slab is not equipped with a vapor barrier and the slab is
thin and porous.
21
Cinder block walls are designed to be light, thin and porous and therefore may be more
susceptible to diffusive infiltration in the absence of cracks or voids that would promote
advective flow of soil vapor.
Other factors that may enhance negative pressures in buildings are mechanical heating
and cooling systems, exhaust fans (e.g., fans serving grilles and ovens), clothes dryers,
central vacuums combustion devices and fireplaces.
Impact of clothes dryers, exhaust fans, etc. on vapor intrusion occurs when these devices
exhaust outside.
Ventilation fans serving specific rooms (e.g., bathrooms, kitchens, utility rooms) are
capable of removing large volumes of air and could potentially depressurize these rooms
enough to enhance vapor intrusion if the rooms are located in the basement or directly on
the slab. (EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation Approaches,
EPA/600/R-08-115, October 2008; p.3-5)”.
Building heating, cooling and ventilation methods should be evaluated. Considering how
the heating, cooling, and ventilation within a structure directs the air flow and impacts the
AER is important with respect to the influence of these systems on vapor intrusion (e.g.
identifying net infiltration and net exfiltration to determine positive and negative pressure
changes within the structure. Due to the complexity of determining this information in
certain structures, it may be practical to include the expertise of a qualified and licensed
HVAC professional to aid in collection of this data (EPA Engineering Issue, Indoor Air
Vapor Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008; p.7).
Air exchange rate (AER) can be measured using either tracer gases (ASTM Method
E741) or by the blower door method (ASTM methods E779 or E1827).
Methods of using tracer gases:
Inject tracer gas to the subsurface and measure AER from the steady state condition of
the tracer gas and the known emission rate of the source.
Inject a puff of tracer gas and measure the rate of decay of the tracer over time.
Soil gas entry rates can be determined by directly measuring a contaminant unique to soil
gas such as radon (not found in home, although the issue of offgassing of radon from
granite countertops may be of concern with this technique). When the AER is measured
in a building under positive pressure with indoor air measurements over time, soil gas or
indoor sources can be identified. If under positive pressure contaminants in indoor air do
not degrade over time, an indoor source is suggested. (EPA Engineering Issue, Indoor Air
Vapor Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008; p.39)”.
EPA Engineering Issue 2.2.1
Useful Links:
Stack effect – http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd104_ethml
ASTM E741-00 Standard Test Method for Determining Air Change in a Single Zone by
Means of Tracer Gas Dilution
Dietz and Cote, 1982
22
10.7 Pre-Mitigation System Installation Diagnostic Evaluation
“The airflow characteristics and capacity of the material(s) beneath the slab should be quantitatively
determined by diagnostic testing. This is the most important step in the SSD design process, and
should always be performed prior to the design and installation of an SSD system (SOP, 2007;
p.23).”
Formulate a clear understanding of the problem to be solved (e.g., contaminant limit that
must be attained in indoor air).
Quality Assurance Project Plan (QAAP) process that includes the development of data
quality objectives (DQO) is recommended. The development of a QAAP and DQOs
ensures that the objectives of the project have been identified and that data collection will
verify that the objectives are being achieved.
Establishing pre-mitigation system performance baseline to which system performance is
compared.
Vapor intrusion has been observed to be seasonally and temporally variable. Acurately
defining the performance baseline may require multiple sampling events over several
seasons and weather conditions. Therefore, defining the performance baseline can present
a dilemma from a risk perspective. In many cases it would be unacceptable to delay
installation of a mitigation system due to concentrations of soil vapor contaminants that
present an unacceptable risk. However, not establishing an accurate performance baseline
may result in a fundamental miscalculation of the actual risk observed. Designing
mitigation systems and barriers conservatively may account for undercalculation of the
performance baseline.
Measurements of soil vapor contaminants can be confounded by the presence of the same
contaminants from sources other than soil vapor (e.g., consumer products, hobby
materials, drycleaning, cosmetics, cigarettes). The presence of these contaminants from
sources other than soil vapor can confound interpretation of indoor air data and
establishment of an accurate performance baseline and establishing an accurate
background concentration. Evaluation of sub-slab soil gas samples and/or use of an
independent tracer for sub-slab soil gas may be useful to distinguish the contribution of
contaminants in indoor air resulting from soil vapor intrusion. (EPA Engineering Issue,
Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008;
p.____)”
There is an inherent variability in the AER within structures that is influenced by changes
in temperature, wind load, and barometric pressure. This variability is enhanced by
activities within the structure such as operation of an HVAC system, opening or closing
windows, or turning on an oven exhaust fan. These activities add additional variables that
influence the establishment of an accurate performance baseline.
Implementing a consistent set of building constraints (reducing building variables) (e.g.,
keeping windows and doors closed) enables data from various seasons or studies to be
23
more easily compared. (EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation
Approaches, EPA/600/R-08-115, October 2008; p.38)”.
See MassDEP, 2002 document for more on this.
Communication tests/pressure field extension tests commonly used in designing sub-slab
depressurization systems to ensure the negative pressure field extends beneath the entire
slab and foundation. In general, communication tests involve applying vacuum to a drill
hole through and in the central portion of the slab. Typically, a micromanometer is used
to measure the pressure differential at holes drilled in other locations throughout the slab.
Probe locations across the slab could be used later to evaluate system performance.
A lack of pressure differential indicates flow issues (e.g., moist soil near the slab or a
footing separating probe locations).
Potential performance of a sub-slab depressurization system may be based upon the
system’s ability to extend an adequate pressure field beneath the entire slab. (EPA
Engineering Issue, Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R-08115, October 2008; p.40)”.
(See SOP, 2007; Section 4.4; p.23-24) Would it make more sense to:
 include specific considerations in the text
 to reference this information in the SOP;
 to include this information in the appendix and reference the appendix within the
text
(See ITRC discussion of pros/cons to communication tests)
“Issues regarding piping routes, fan location, vibration and noise concerns, etc., should be
discussed with the building owners and occupants. The local municipal Building
Department should also be contacted to determine if any permits are required (SOP, 2007;
Toolbox 4; p.29).”
http://www.epa.gov/quality/qa docs.html DQO info
http://www.cdc.gov/niosh/idlh/intridl4.html - NIOSH
http://www.atsdr.cdc.gov/mrls/index.html - Agency for Toxic Substances and Disease
Registry (ATSDR)
10.8 Mitigation System Installation Testing and Modifications
Mitigation system performance must be verified once the system has been installed.
Performance verification of the mitigation system should include direct measurements of
contaminants in indoor air. Secondary performance metrics such as the measurement of
differential pressures beneath the entire slab indicating maintenance of an adequate
pressure field beneath the slab may also be indicators of system performance.
(Active vs. passive systems).
24
Use manometers or magnehelic to measure pressure beneath the slab.
System operation testing must include an evaluation of backdrafting of combustion or
vented appliances (e.g., woodstoves, fireplaces, clothes dryers) to ensure dangerous
combusution gases such as carbon monoxide are not building up within the structure
(NYSDOH, 2005).
(EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R08-115, October 2008; p.41)”.
Mitigation system should be clearly labeled to avoid accidental modifications to the
system that could reduce its effectiveness. (NYSDOH, 2006; p.63).
“All SSD systems should be designed in conformance with standard engineering principles
and practices. As the work will likely be conducted in close proximity to building
inhabitants, safety concerns are a priority. Attempts should be made to minimize noise,
dust, and other inconveniences to occupants. Attempts should also be made to minimize
alterations in the appearance of the building, by keeping system components as
inconspicuously located as practicable.”
“The installation of an SSD system should be conducted under the direct supervision of a
competent professional with specific experience in building vapor mitigation, site
remediation, and/or environmental engineering practices. There are many firms that
specialize in installing SSD systems for residential radon mitigation, as the same processes
described above apply to the intrusion of radon into buildings (SOP, 2007; p.22)”.
“The creation of an effective sub-slab negative pressure field should result in the reduction
of VOC concentrations in the indoor air within the building. After SSD system startup,
indoor air quality sampling data should be collected to confirm that concentrations of VOCs
in indoor air are reduced (e.g. to levels at or below typical indoor air concentrations).
Generally, this confirmatory monitoring should be done 2 to 4 weeks after system startup.
Subsequent to this initial evaluation, consideration should be given to conducting one
additional indoor air sampling effort during the "worst case" months of January or February
(unless, of course, the initial evaluation is conducted during these months). This is
especially true if non-winter SSD negative pressure conditions were marginal.
If indoor air quality data continues to indicate elevated concentrations of VOCs, further evaluation
would be necessary to determine if (1) the SSD system is functioning properly, but levels of
contaminants in the building exceed typical indoor air concentrations, or (2) the SSD system
requires modification or expansion. To make such a determination, a Lines of Evidence approach
should be applied, looking at all available information and data (e.g., soil gas data; contaminant
concentration trends in basement and first floor; chemical forensics, etc.) Short-circuiting problems
are of particular concern, where cracks, holes, sumps, or annular spaces in the building
foundation/slab disrupt a negative pressure field.
Once an adequate demonstration of SSD system effectiveness has been made, as long as an
adequate negative pressure is maintained at the extraction point(s), indoor air quality should
25
be acceptable. For single-family residential structures, it is generally not necessary to
institute a regular or long term indoor air-monitoring program, although periodic checks
(every 1-3 years) are advisable. More frequent and/or systematic monitoring programs are
advisable for larger and more complex buildings, such as schools (SOP, 2007; Toolbox 4;
p.30.”
“The presence of a sump in a basement can provide a significant short-circuiting vehicle to
the establishment of a subslab negative pressure field. In such cases, an air tight cover
should be installed over the sump; if a sump pump is present, the cover should be equipped
with appropriate fittings or grommets to ensure an air tight seal around piping and wiring,
and the cover itself should be fitted with a gasket to ensure an air-tight seal to the slab while
facilitating easy access to the pump. Note that it is also possible to use the sump as a soil
gas extraction point (where appropriate); a number of manufacturers make equipment for
just such applications (SOP, 2007; Toolbox 4; p.29)”
“At buildings where establishment of a negative pressure field is difficult, steps can be taken
to improve the effectiveness of the SSD system by reducing the degree of
underpressurization occurring within the basement. These include: Ducting make-up air
from outside the building for combustion and drafting; and/or overpressurizing the basement
by using fans to direct air from the rest of the building into the basement, or an air/air heat
exchanger to direct outside air into the basement (SOP, 2007; Toolbox 4; p.29).”
“Start-up of the system should not occur until several hours after the extraction hole has
been grouted, to allow the grout to cure. Otherwise, the fan/blower could draw moisture
from the wet grout and cause the patch to shrink and crack (SOP, 2007; Toolbox 4; p.29).”
“The contractor designing and installing the SSD system should be required to guarantee
and demonstrate that the system will effectively prevent the intrusion of VOCs into the
building. The specific requirements for demonstrating that performance standards have
been met can be set on a case-by-case basis. There are two levels of performance standards
for SSD systems: confirmation of pressure field and achievement of indoor air quality goals
(SOP, 2007; Toolbox 4; p.29)”
10.9 Mitigation System Hazards
It is important to evaluate the effects of the pressure and/or ventilation changes in
structures with combustion equipment (e.g., heating systems, clothes dryers, cooking
appliances).
Combustion equipment usually draws combustion air from the indoor air space.
Therefore, it is important to consider how installation of the mitigation system will
influence combustion equipment within the structure and whether backdrafting is
occurring or has the potential to occur.
Backdraft tests = setting appliances, HVAC systems for worst case negative pressure that
will be “anticipated” for the building. Carbon monoxide or flow visualization test
26
performed at each stack serving a combustion device. (EPA Engineering Issue, Indoor
Air Vapor Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008; p.29)”.
Impact of mitigation systems on ambient air quality.
This may be a concern when concentrations of soil gas vented to the atmosphere are
extremely high (more specific identification of “high concentrations”), acutely toxic,
dispersion of vented soil gas is poor due to localized wind conditions, if multiple systems
installed in a densely populated area, low stack height.
Outlets from a vented mitigation system should not be located near a window or
ventilation system intakes. Care should be taken to ensure vented soil gas does not
reenter the building (SSTM E 2121, ASTM E 1465-92). (EPA Engineering Issue, Indoor
Air Vapor Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008; p.29)”.
http://www.p2pays.org/ref/07/06430.pdf Air Emissions from the Treatment of Soils
Contaminated with Petroleum Fuels and Other Substances
http://www.epa.gov/ttn/atw/siterm/fr08oc03.pdf National Emission Standards for
Hazardous Air Pollutants for Site Remediation
Does condensation from a leaking SSDS fan represent a possible fire hazard?
Installation of CO detectors to warn occupants in the event backdrafting occurs.
“Electrical work for the fan installation will generally require the utilization of a licensed
electrician. At locations where extremely high concentrations of combustible VOCs are
expected, explosion-proofed equipment must be used (SOP, 2007; Toolbox 4; p.29).”
11.0 SYSTEM OPERATION, MAINTENANCE AND MONITORING
11.1 General Operation and Maintenance Considerations
11.2 Mitigation System Monitoring
Because indoor sources unrelated to soil gas can degrade indoor air quality, sampling
performed to evaluate mitigation systems should include updates to indoor air quality
surveys (e.g., chemical inventories). This will help identify inconsistencies in collected
data and update changes that have occurred since the characterization phase.
Concurrent sampling of indoor air, ambient outdoor air and soil gas is recommended to
identify contaminants present in indoor air that may not be attributable to soil gas.
(EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R08-115, October 2008; p.38)”.
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Substantial decreases in contaminant concentrations beneath the slab is an indication that
the sub-slab depressurization system is working properly. However, even if contaminants
beneath the slab have not decreased substantially as a result of mitigation system
operation, the system may still be performing properly because the pressure differential
has been reversed.
Sub-slab probes can be used to directly measure differential pressures beneath the slab
and monitor system performance.
Periodic, long-term monitoring may consist of direct measurement of contaminants in
indoor air, measurement of contaminants in sub-slab soil vapor, measurement of pressure
differentials, inspection of equipment and materials (ASTM, 2005/2008?; NYSDOH,
2005).
Periodic monitoring should evaluate if maintenance or modifications to the system are
necessary. Monitoring results should be comprehensive enough to indicate if
modifications or maintenance to the system is necessary (ASTM, 2005/2008?)
When direct measurement of indoor air concentrations is included in periodic monitoring
of mitigation systems, sampling during the heating season should be included to evaluate
indoor air during “worst case” conditions.
Direct measurement of indoor air concentrations at new construction sites should take
into account off-gassing of new building materials (e.g., carpets, paints, other?, etc.).
When monitoring indicates the mitigation system is not performing effectively,
diagnostic testing should be performed to identify design, installation, or other problems
(e.g., occupant behavior impacting system operation – opening windows, doors, or other
activities that may impact pressures etc.) (EPA, 1993a, pgs. 5-5 to 5-10).
Routine inspection of mitigation system: inspection of visible components of system and
collection points. Inspection should identify significant changes in Site condition and
damage/ “degradation” of mitigation system. Monitor pressure and flow rates in vent
risers to ensure adequate pressure/flow. Ensure moisture is draining correctly and that
condensation is not a problem. Conduct routine maintenance, calibration and diagnostic
testing in accordance with the manufacturers specifications. Periodic direct measurement
of contaminant concentrations in indoor air to ensure there are no significant increases in
soil vapor concentrations in indoor air. Periodic evaluation of pressure differentials across
the entire slab should be performed to ensure the system is functioning properly.
NYSDOH, 2005 (Chapter 4)
Mitigation system failure warning devices should be installed on active mitigation
systems (are these available for passive systems? e.g., flows not maintained) to monitor
system effectiveness. The device should provide a visual or audible alarm when system
performance metrics (e.g., pressure or flow) are not maintained.
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The warning device should be easy to read and understand and be located in a frequently
trafficked location where it can be easily seen or heard.
Ease of calibration of warning device.
Effort should be made to ensure building occupants understand failure warning devices.
Occupants whose first language is not English should be provided instructions in their
native language.
Proper installation and operation of the warning device should be verified upon
installation and monitored regularly.
Telemetry should be considered. (how easy/costly is this feature for SSD systems?)
(EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R08-115, October 2008; p.42).
Installation of CO detectors to warn occupants in the event backdrafting occurs.
Types of mitigation system warning devices include: manometers, sounding alarms, light
indicators, a needle display gauge (NYSDOH, 2006; p.64)
11.3 Sampling Type
Sample placement is typically in the breathing zone. Sample duration usually for 24
hours to average over the diurnal cycle.
Sample durations that are large multiples of 24 hours may be used to allow for variations
that occur over a longer period of time.
Identify zones that can be considered well-mixed (e.g., auditoriums, reception areas, and
living spaces of residential areas). Open zones with uniform temperatures can be
expected to have contaminants well mixed within the zone. Strong drafts, temperature
gradients, or flow restrictions may indicate that the air within the zone is not
homogeneous.
When establishing primary performance metric or indoor air concentrations or evaluating
the performance of a mitigation system in a complex building, it may be appropriate to
organize the conceptual site model as a group of interacting zones. Depending on the
complexity of the building being evaluated, it may be beneficial to incorporate the
expertise of a licensed HVAC professional during the sampling program to establish an
accurate conceptual site model of the AER within the building and between different
zones. Having an accurate model of AER will help establish a more representative
sampling plan to identify contaminants and ultimately a more effective mitigation
approach.
“Evaluation of health risks requires long-term estimates of indoor air concentrations
applied to an exposure scenario”
Primary performance metric for evaluating soil vapor intrusion mitigation systems is the
measurement of concentrations of contaminants in indoor air.
Engineering parameters such as pressure differentials and air exchange rate (AER) are
considered secondary performance indicators. (EPA Engineering Issue, Indoor Air Vapor
Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008; p.37)”.
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Most commonly used indoor air sampling methods include EPA Methods TO-14A and
15 (requiring stainless steel summa canister). EPA Method TO-17 uses sorbent tubes to
collect air samples. (EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation
Approaches, EPA/600/R-08-115, October 2008; p.38)”.
Sub-slab Sampling
It is impossible to state the number and location of sub-slab monitoring locations.
Considerations with respect to planning a sub-slab sampling plan include:
Determining the buildings footing and slab design. The area beneath the slab can
sometimes be sub-divided by footings, influencing the distribution of vapors beneath the
building.
The location of utility corridors should be evaluated for safety and because they could
serve as a preferential pathway for soil vapor.
Identification of the source location, major soil vapor entry routes, and routes of
contaminant migration.
“Distinct occupied areas that differ in ways likely to influence AER with the sub-slab
environment, then samples from each distinct area would be practical”
If applicable, HVAC system should be operating for at least 24 hours to establish
consistent indoor temperature.
Multiple rounds of testing advised to account for variability of sub-slab contaminant
concenrations. Variability of sub-slab contaminant concentrations may be influenced by
temperature, variations in wind and barometric pressure, occupant activities such as
opening and closing doors and windows, and HVAC operation. (EPA Engineering Issue,
Indoor Air Vapor Intrusion Mitigation Approaches, EPA/600/R-08-115, October 2008;
p.40)”.
MassDEP, 2002
NYSDOH, 2005
NJDEP, 2005
11.4 Sampling Frequency
ASTM (2005 (2008?) recommends sampling to evaluate mitigation system performance
[shortly…define?] after start up.
Reducing sampling frequency after system reliability and effectiveness have been
demonstated. (how are “reliability” and “effectiveness” defined?)
Regular monitoring and maintenance intervals are required with frequency being a
function of the susceptibility of mitigation system failure and the effect of system failure
with respect to receptors (immediate vs. delayed impact, sensitivity of receptors, e.g.,
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commercial setting with reduced exposure durations vs. residence with children with 24
hour exposure durations).
It may be appropriate to vary monitoring frequency over the life of the system. If system
effectiveness has been demonstrated throughout a variety of weather conditions and
seasons and during worst-case conditions (presumably winter months), reduced
monitoring may be efficient and protective.
DTSC, 2004
NYSDOH, 2005
11.5 Sampling Duration
11.6 Closure Evaluations
Approval for mitigation system termination may be appropriate if data suggests that soil
vapor intrusion is no longer resulting in a vapor intrusion pathway (CEPwithout the
operation of the mitigation system or that the vapor intrusion pathway (CEPs?) (MCP
Citation applicable here?). Lines of evidence to support this assertion must include direct
measurement of contaminants in indoor air during worst case conditions. Due to the
sensitive nature of receptors, it may be necessary to have several sampling events to
ensure a false negative indoor air sample is not the basis of system shutdown. Additional
data may include measurement of contaminants in soil gas.
For complex buildings with zones that may have varying impacts from vapor intrusion, it
may be necessary to evaluate individual zones within the building.
Stakeholders/building occupants may want to run mitigation system after vapor intrusion
pathway has been determined to be incomplete (no significant risk?) for peace of mind to
mitigate low concentrations below (NSR), reduce mold and mildew, protect against radon
gas. (EPA Engineering Issue, Indoor Air Vapor Intrusion Mitigation Approaches,
EPA/600/R-08-115, October 2008; p.41)”.
NJDEP, 2005
ITRC (2007), Section 4.5
11.7 Institutional Controls
In situations where mitigation is required to address indoor air impacts and site-wide
remediation is not expected to eliminate the vapor intrusion hazard in the near future,
institutional controls may be required on an interim or permanent basis. When the risks
associated with uncontrolled use of the property are unacceptable, legal actions in the
form of activity and use limitations, (zoning or requirements that mitigation systems
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(passive or active) be implemented for new or existing building or due to changes in use
as appropriate….Wiley), or excavation prohibitions may be instituted to limit uses
associated with unacceptable health risks.
[I suggest: 1 ) adding a detailed discussion of how AULs generally and specifically may
and may not be used to address the vapor pathway and vapor intrusion, and 2) inclusion
of specific references to the MCP and current and updated Department AUL guidance
and policies.-RT]