Vapor Intrusion

Mitigation

The preferred long-term response1 to vapor intrusion (VI) is to eliminate or substantially reduce the underlying sources of vapor through a removal action or by remediating the contaminated soil or groundwater source contributing to soil or conduit VI (EPA, 2015). Because these responses can take considerable time, building mitigation to prevent human exposure in the interim may be warranted2. At some sites, it may be appropriate to implement mitigation pre-emptively before all lines of evidence have been developed to sufficiently evaluate the vapor intrusion pathway for an existing building or a building under construction. Pre-emptive mitigation can be taken if sufficient data indicate that vapor intrusion is: 1) occurring or may occur due to subsurface contamination; or 2) is posing or may pose a health concern to building occupants (EPA, 2015).

Mitigation of VI involves engineered exposure controls that decrease the mass flux of vapor entry into buildings or ameliorate the accumulation of vapors in occupied spaces (e.g., through treatment or ventilation). Choosing an appropriate mitigation approach relies, in part, upon distinguishing whether soil vapor intrusion and/or conduit vapor intrusion are pertinent mechanisms in a building.

Mitigation approaches can involve:

• Eliminating the entry routes to the building.
• Removing or reversing the force that drives the contaminants into the building.
• Ventilating occupied spaces in buildings.
• Providing a pathway to divert contaminants away from the building
• Treating indoor air using air purifying units (e.g., adsorption using granular activated carbon).
• Repairing or sealing damaged pipes and other conduits leading to the building.

A combination of approaches is commonly used.

The following are examples of mitigation that address the soil vapor intrusion, conduit vapor intrusion, and groundwater vapor intrusion mechanisms. Links to Resources provide additional information.

Mitigation Options for the Soil Vapor Intrusion Mechanism

• Sealing of entryways: Cracks and openings in the building foundation can be the primary routes of vapor entry. If this is the case, sealing cracks in the floors, walls, and gaps around utility conduits, sumps, and elevator shafts can be an important first step in preventing vapor intrusion. Sealing cracks and gaps may also be necessary when used with other mitigation strategies, such as sub-slab depressurization, to ensure system efficiency. Sealants include materials such as synthetic rubbers, asphalt, and swelling cement.
  Resources
• Passive barriers: Passive barriers are materials or structures (e.g., geomembrane or polyethylene plastic) installed below a building to block the entry of vapors (Figure 1). Barriers are typically installed during construction, but can be installed in existing buildings with a crawl space, if needed.
  Resources


Figure 2. Gravel base and passive ventilation pipes.

• Passive venting: Passive venting systems are often combined with passive barriers to safeguard against vapor intrusion. Typically, perforated collection pipes are installed in a layer of permeable sand or gravel to direct vapors to the edges of the foundation (Figure 2). Passive systems rely on wind currents to induce vapor flow through the pipes, so may be less effective at removing vapors on non-windy days.
  Resources
• Crawlspace ventilation: Air flow under buildings with a sealed crawlspace can be increased by opening vents, if present, or connecting an exterior-mounted fan to piping extended into the crawlspace (ITRC, 2020).


Figure 3. Exhaust pipe and fan for a sub-slab depressurization exhaust vapors at the top of a house.

• Depressurization below building foundations: There are several types of systems to depressurize the area below a building, including sub-slab depressurization and sub-membrane depressurization systems. In most instances, mitigation of soil vapor intrusion to residential structures requires a sub-slab depressurization system (Mosley, 2005), which can be installed in houses with basements or slab-on-grade construction. Sub-slab depressurization systems include fans to reduce the pressure under the building relative to the indoor space (Figure 3). Sub-membrane systems are installed in buildings with dirt-floor crawlspaces or basements where a depressurization mechanism covered by a durable membrane can be installed on the dirt floor.
  Resources
• Building pressurization: Building pressurization involves adjusting the building's heating, ventilation, and air-conditioning (HVAC) system or installing a new system to maintain a positive pressure indoors relative to the sub-slab area. This approach can be the most cost effective if the existing HVAC system already maintains a positive pressure (ITRC, 2007).
  Resources
• Indoor Air Treatment: Indoor air is a mitigation option that addresses vapors that have intruded into a building. Commercially available indoor air cleaners, which include both in-duct models and stand-alone air cleaners operate on various principles, such as zeolite and carbon sorption and photocatalytic oxidation (EPA, 2008). The most common vapor intrusion air cleaning technology employs a sorbent bed or sorbent layer, usually composed of carbon, to remove vapor phase contaminants from the air. The use of air treatment units is typically used when a temporary reduction of vapors in indoor air is needed while a longer-term mitigation or cleanup is implemented (EPA, 2017).

Mitigation Options for the Conduit Vapor Intrusion Mechanism

Mitigation methods that address the soil vapor intrusion pathway may not effectively address conduit vapor intrusion. Vapor intrusion from a sewer or other conduits is mitigated either by preventing contaminants from intruding into the conduit or by preventing vapors in the conduit from intruding into indoor air. Mitigation methods for conduit vapor intrusion described in Birn Nielson and Hvidberg, 2017 include:

• Repairing damaged pipes or lining the piping to prevent the contamination from intruding into the conduit.
• Sealing a sewer system in the building to prevent contamination from the sewer system from intruding to indoor air.
• Venting manholes. Passive venting of manholes near a building can lower the concentrations of vapors entering a building via a sewer connection.
• Depressurizing the conduit to ensure the pressure gradient is altered away from the building and toward the sewer. Depressurizing a sewer involves installing a pump with a regulator in a manhole as close to the building as possible (Birn Nielsen and Hvidberg, 2017). The pump pulls air from the sewer through a ventilator and carbon filter before exhausting it outside.
• Indoor Air Treatment. (See description above.)

References

Birn Nielsen, K. and B. Hvidberg, 2017. Remediation Techniques for Mitigating Vapor Intrusion from Sewer Systems to Indoor Air. Remediation. 27, 6 pp.

EPA, 2017. Adsorption-Based Treatment Systems for Removing Chemical Vapors from Indoor Air. EPA/600/R-08-115. 107 pp. August.

EPA, 2015. OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air. Office of Solid Waste and Emergency Response Publication 9200.2-154, 267 pp, June.

EPA, 2008. Indoor Air Vapor Intrusion Mitigation Approaches. EPA/600/R-08-115, 49 pp, October.

ITRC (Interstate Technology and Regulatory Council), 2020. Technical Resources for Vapor Mitigation Training. Vapor Intrusion Mitigation Team, 266 pp, December.

Navy Alternative Restoration Technology Team, 2011.Vapor Intrusion Mitigation in Existing Buildings Fact Sheet. NFEC, 6 pp.

Resources

The following articles, reports, and weblinks address various mitigation technologies. The resources in each section are listed chronologically from newest to oldest, although undated resources are listed first.

• Multiple Mitigation Technologies
• Sealing of Entryways
• Passive Barriers
• Passive Venting
• Depressurization Below Building Foundations
• Conduit Depressurization
• Building Pressurization
• Indoor Air Treatment

Multiple Mitigation Technologies

Community Guide to Vapor Intrusion Mitigation
EPA 542-F-21-025, 2021

The Community Guide series (formerly Citizen's Guides) is a set of two-page fact sheets describing cleanup methods used at Superfund and other hazardous waste cleanup sites. Each guide answers six questions about the method: 1) What is it? 2) How does it work? 3) How long will it take? 4) Is it safe? 5) How might it affect me? 6) Why use it?

Vapor Intrusion Mitigation (VIM)
Interstate Technology and Regulatory Council online documentation.

This online document is designed for state and federal environmental staff, as well as others (including stakeholders, project managers, and decision makers), to provide a working knowledge of vapor mitigation and practice. Various active and passive approaches are discussed. ITRC plans to periodically update the document as significant new information and regulatory approaches.

Technical Resources for Vapor Mitigation Training
Interstate Technology and Regulatory Council (ITRC) Vapor Intrusion Mitigation Team 266 pp, 2020.

Online documentation includes fact sheets and other resources describing rapid responses to VI and various approaches for building mitigation of VI.

ASTM E2435 - 05(2020) Standard Guide for Application of Engineering Controls to Facilitate Use or Redevelopment of Chemical-Affected Properties

Provides a general overview of the procedures for evaluating and selecting engineering controls for use in property development or reuse. Intended for use at properties that are presently developed or proposed for development but have contaminated soil, groundwater, air, or other environmental media, that may pose a risk to human health.

Demonstration/Validation of More Cost-Effective Methods for Mitigating Radon and VOC Subsurface Vapor Intrusion to Indoor Air
McAlary, T., W. Wertz, and D. Mali.
ESTCP Project ER-201322, 1346 pp, 2018.

New lines of evidence and mathematical modeling were developed to aid in the design and performance monitoring of sub-slab venting systems for mitigation of radon and VOC vapor intrusion to protect the health of building occupants from inhalation exposures. Results indicate that system performance depends strongly on the relative permeability of the floor slab and the material below the floor slab, and that both parameters can be determined with reasonable confidence using tests and analysis that are rapid and affordable. Where coarse granular fill is present below a high-quality floor slab, the radius of influence may extend to considerable distances, which reduces the number of suction points required and the associated capital cost. Where a system already exists, mass emission monitoring may help reduce the operation, maintenance, and monitoring costs. Additional information: Executive Summary

OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air
EPA, Office of Solid Waste and Emergency Response Publication 9200.2-154, 267 pp, June 2015.

Chapter 8.0 of this EPA guidance describes the respective roles of building mitigation and subsurface remediation and identifies the need for accompanying long-term monitoring and operation and maintenance of the systems.

Vapor Intrusion Mitigation in Existing Buildings Fact Sheet
Navy Alternative Restoration Technology Team, NFESC, 6 pp, 2011.

This fact sheet provides a brief overview of methods that can be used to mitigate VI in existing buildings along with important considerations for selecting and designing an appropriate mitigation system for your site. The methods discussed include sub-slab depressurization, submembrane depressurization, building pressurization, and indoor air treatment. More detailed information on VI mitigation systems for existing buildings can be found in the resources listed at the end of this fact sheet.

Vapor Intrusion Mitigation Advisory
Department of Toxic Substances Control, California EPA. 71 pp, October 2011.

Guide contains sections on vapor intrusion mitigation methods, evaluation of mitigation approaches, design of mitigation systems, and considerations for implementation.

Indoor Air Vapor Intrusion Mitigation Approaches
EPA/600/R-08-115, U.S. Environmental Protection Agency, 49 pp, October 2008.

Engineering Issue Paper explains the range of vapor intrusion mitigation technologies available. It also provides information on selecting appropriate technologies in consultation with qualified engineering and risk management professionals. It provides information for making appropriate selections and evaluating the recommendations of mitigation contractors and engineers.

Vapor Intrusion Pathway: A Practical Guide
Interstate Technology & Regulatory Council, 172 pp, January 2007.

Step 13 of ITRC's 13-step approach to evaluating vapor intrusion is deciding whether mitigation is warranted. The guidance summarizes several approaches including passive barriers, passive venting, sub-slab and sub-membrane depressurization, sub-slab and building pressurization, indoor air treatment, and sealing the building envelope.

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Sealing of Entryways

Technical Resources for Vapor Mitigation Training
Interstate Technology and Regulatory Council (ITRC) Vapor Intrusion Mitigation Team 266 pp, 2020

Online documentation includes fact sheets and other resources describing rapid responses to VI and various approaches for building mitigation of VI.

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Passive Barriers

Sustainable Vapor Intrusion Controls - Designing an Effective Passive System
Ash, James, Mark Ensign, and William Simons, in Proceedings of the Air & Waste Management Association's Vapor Intrusion 2010 Conference, 8 pp, 2010.

Describes some of the conditions and design requirements for implementing successful passive vapor intrusion control systems (generally consisting of vapor barrier membranes combined with ventilation piping) at both residential and commercial buildings. It also presents monitoring data collected at residential properties and an office building where passive systems were successfully implemented.

Designing Efficient Sub Slab Venting and Vapor Barrier Systems for Schools and Large Buildings
Hatton, T. E. 2010 International Radon Symposium, Columbus, OH.

Paper covers the basic components of designing efficient sub-slab radon venting and vapor barrier systems for schools and large buildings.

Vapor Intrusion/Indoor Air Guidance Survey
Massachusetts Department of Environmental Protection, 215 pp, July 2010.

Presents the results of a survey of state and agency guidance and of vapor barrier research.

Accelerating the Redevelopment of a Vapor-Impacted Property Based on Data-Informed Verification of Vapor Barrier Technology
Lowe, John, Jessica Raphael, Loren Lund, and Robert Casselberry, in Proceedings of the Air & Waste Management Association's Vapor Intrusion 2009 Conference. 11 pp, 2009.

Describes a cold spray applied vapor barrier equipped with an overlying monitoring layer that was installed as part of an integrated commercial redevelopment, demonstrating protection of human health without the need for indoor air sampling.

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Passive Venting

Sustainable Vapor Intrusion Controls - Designing an Effective Passive System
Ash, James, Mark Ensign, and William Simons, in Proceedings of the Air & Waste Management Association's Vapor Intrusion 2010 Conference, 8 pp, 2010.

Describes some of the conditions and design requirements for implementing successful passive vapor intrusion control systems (generally consisting of vapor barrier membranes combined with ventilation piping) at both residential and commercial buildings. It also presents monitoring data collected at residential properties and an office building where passive systems were successfully implemented.

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Depressurization Below Building Foundations

Peace of Mind Against Unwanted Intruders: Case Study on Sub-Slab Vapour Intrusion Protection Measures
Patel, P. | Smart Remediation, 4 February, virtual, 27 slides, 2021.

One of the many contaminant management strategies to prevent soil vapor intrusion in a building involves installing a sub-slab depressurization system (SSDS). A Cupolex® SSDS was installed at a multi-tenant residential property when the presence of groundwater containing elevated concentrations of PCE and TCE had an impact on the air quality within the unoccupied basement.

Steps Needed to Operate and Maintain the Sub-Slab Depressurization System Installed by EPA at the Chem-Fab Property
U.S. EPA Region 3, Document ID 2242795, 28 pp, 2017.

EPA installed a vapor mitigation system in the main commercial building at 300 N. Broad Street, Doylestown, Pennsylvania (part of the Chem-Fab Superfund Site) in response to the potential for an imminent and substantial endangerment to human health presented by the vaporous migration of benzene and chlorinated VOCs from soils beneath the building into office suites. The system draws sub-slab vapors from beneath the foundation using a system of fans and forces the vapors through PVC pipes into the air outside the building, where the vapors will not concentrate into unacceptable levels as they would in confined office space.

Assessment of Mitigation Systems on Vapor Intrusion: Temporal Trends, Attenuation Factors, and Contaminant Migration Routes under Mitigated and Non-Mitigated Conditions
Truesdale, R., C. Lutes, B. Cosky, B. Munoz, R. Norberg, H. Hayes, and B. Hartman.
EPA 600-R-13-241, 608 pp, 2015.

In 2011, researchers began an investigation into the general principles of how vapors enter into a single residence study site, a highly instrumented pre-1920 residential duplex located in Indianapolis, Indiana. This report, the second in a series of reports based on that research, examines the efficiency of a sub-slab depressurization system to prevent and remove radon and VOCs with reference to (a) subsurface conditions that influence the movement of VOCs and radon into the home; (b) system effects on VOC and radon concentrations; and (c) the influence of a winter capping event on vapor movement into the home.

Important Information about Vapor Mitigation Systems and Power Outages, New Jersey Department of Environmental Protection
1 p, November 2012.

Fact sheet describes actions to reduce exposure to organic vapors during a power outage for property owners with subsurface depressurization systems.

Pneumatic Testing, Mathematical Modeling and Flux Monitoring to Assess and Optimize the Performance and Establish Termination Criteria for Sub-Slab Depressurization Systems (PowerPoint presentation)
McAlary, T., D. Bertrand, P. Nicholson, S. Wadley, D. Rowlands, G. Thrupp, and R. Ettinger. Presented at the U.S. EPA workshop, "Addressing Regulatory Challenges in Vapor Intrusion: A State-of-the-Science Update Focusing on Chlorinated VOCs," held at the Association for Environmental Health and Sciences 21st Annual Meeting and West Coast Conference on Soils, Sediments, and Water - Workshop: Addressing Regulatory Challenges in Vapor Intrusion, San Diego, California, March 15, 2011.

High Vacuum, High Airflow Blower Testing and Design for Soil Vapor Intrusion Mitigation in Commercial Buildings
Brodhead, William and Thomas Hatton in Proceedings of the Air & Waste Management Association' Vapor Intrusion 2010 Conference, 22 pp, 2010.

Describes method used to design and optimize a soil vapor intrusion mitigation system for a medical center heavily contaminated by benzene and chlorinated solvents. The system uses high-output radial blowers.

Overview of Two Large-Scale Residential Sub-Slab Depressurization System Installation Programs
Lucas Hellerich, et al., in Proceedings of the Annual International Conference on Soils, Sediments, Water and Energy, 11(1), 21 pp, January 11, 2010.

Describes case studies of two large-scale residential sub-slab depressurization systems designed and installed to mitigate intrusion of soil gas that contained low levels of volatile organic compounds, into over 100 houses and several buildings in a multi-structure condominium complex.

Subsurface Depressurization Systems, New Jersey Department of Environmental Protection
1 p, June 2005.

Fact sheet describes the two most common subsurface depressurization systems: the sub-slab depressurization system and the sub-membrane depressurization system.

Design, Effectiveness, and Reliability of Sub-Slab Depressurization Systems for Mitigation of Chlorinated Solvent Vapor Intrusion
David Folkes presented in a series of EPA seminars on vapor intrusion at the rollout of the 2002 draft OSWER guidance, 5 pp, 2002 and 2003.

Describes the design, installation, and performance of the Redfield Site sub-slab depressurization and sub-membrane depressurization systems, the modifications required in approximately 30% of the homes to achieve the State of Colorado interim action level for 1,1-dichloroethene, and factors that may affect long-term system reliability.

Designing Commercial Sub-Slab Depressurization Systems
B. Brodhead. 2002 International Radon Symposium Proceedings, March 2002, 9 pp.

Describes the design of a sub-slab depressurization system for an 18,000 square-foot school building in New Jersey and a three-story, stone-faced building at Lehigh University in Pennsylvania.

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Conduit Depressurization

Remediation Techniques for Mitigating Vapor Intrusion from Sewer Systems to Indoor Air
Birn Nielsen, K. and B. Hvidberg.
Remediation, vol 27, pp 67-73, 2017.

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Building Pressurization

Verification of Building Pressure Control as Conducted by GSI Environmental, Inc. for the Assessment of Vapor Intrusion: Environmental Technology Verification Report
MacGregor, I., M. Prier, D. Rhoda, A. Dindal, and J. McKernan. ETV Advanced Monitoring Systems Center, 148 pp, December 2011.

The building pressure-control technique was implemented in autumn 2010 at each of two buildings over the course of 3.5 days. The effectiveness of the method to support decision-making was evaluated through three different metrics. 1) Can building pressure be decreased, controlled, and subsequently elevated and controlled at each of the two buildings under induced negative and positive pressure (NP and PP) conditions, respectively? 2) Does inspection of the mass discharge of radon from subsurface sources show whether VI was enhanced under NP and reduced (or stopped) under PP? 3) How well does the technique allow calculation of the fractional contribution of VI for different concentrations of indoor COCs? While pressure control was achieved at both buildings, the magnitude of the induced pressure gradients varied, likely due to differences in building characteristics, such as HVAC systems.

HVAC Influence on Vapor Intrusion in Commercial and Industrial Buildings
Shea, David, Claire Lund, and Bradley Green, in Proceedings of the Air & Waste Management Association's Vapor Intrusion 2010 Conference, 12 pp, 2010.

Presents an overview of common HVAC components and how they influence indoor air quality. Several case studies are presented describing the role of HVAC operations in vapor intrusion assessment and mitigation. Favorable and unfavorable effects of HVAC operations on vapor intrusion are provided.

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Indoor Air Treatment

Adsorption-Based Treatment Systems for Removing Chemical Vapors from Indoor Air
EPA/600/R-17/276 U.S. Environmental Protection Agency, 107 pp, August 2017.

Engineering Issue Paper summaries the state of the science on selecting and using air treatment units. Performance data and specifications, monitoring to verify performance, and challenges and limitations are also discussed.

Indoor Air Vapor Intrusion Mitigation Approaches
EPA/600/R-08-115, U.S. Environmental Protection Agency, 49 pp, October 2008.

Engineering Issue Paper explains the range of vapor intrusion mitigation technologies available. It also provides information on selecting appropriate technologies in consultation with qualified engineering and risk management professionals. It provides information for making appropriate selections and evaluating the recommendations of mitigation contractors and engineers.

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