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.htmhttp://www.brownfieldstsc.org/pdfs/Roadmap.pdf

8.1 Source Remediation Under the MCP

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

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

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/R-08-115, 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. ]

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.]

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 re-use (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

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-08-115, 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 sub-slab 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:

in all cases, an Imminent Hazard condition must be promptly addressed and eliminated. Eventually, a condition of No Significant Risk must be achieved; and

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:

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-08-115, 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

9.4.2Sealing 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-08-115, 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]

9.4.3Passive 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.

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.

9.4.4Passive 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)”.

9.4.5Passive 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/R-08-115, 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/R-08-115, October 2008; p.14)”.

9.5 ACTIVE CONTROLS

Available active mitigation strategies include:

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?

9.5.1Subslab 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)”.

9.5.2Subslab 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 radon-mitigation 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 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.

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Figure 4-1: SSD System

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).”

9.5.3Submembrane 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)”.

9.5.4Subslab 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)

9.5.5Building 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

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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/R-08-115, 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)”

Discuss in detail and/or by reference with a summary, the current Department policy on off-gas controls WSC-94-150.

9.6 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-08-115, 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.

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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 costEase 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

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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 sub-slab 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 sub-slab 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/R-08-115, 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 ?

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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.

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“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.

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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 2.2.1Useful 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 DilutionDietz and Cote, 1982

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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

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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-08-115, 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 infohttp://www.cdc.gov/niosh/idlh/intridl4.html - NIOSHhttp://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).

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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/R-08-115, 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

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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

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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 Substanceshttp://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/R-08-115, 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/R-08-115, 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 SamplingIt 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, 2002NYSDOH, 2005NJDEP, 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, 2004NYSDOH, 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, 2005ITRC (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]

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