United States Environmental Protection Agency (EPA) Final Vapor Intrusion Technical Guide
Vapor intrusion (VI) is the migration of volatile chemicals in soil gas into the indoor air of overlying occupied buildings. In June 2015, the EPA released final guidance on assessment/mitigation of VI.
- OSWER Technical Guide For Assessing and Mitigating the Vapor Intrusion Pathway From Subsurface Vapor Sources to Indoor Air (OSWER VI Guide)
- Technical Guide for Addressing Petroleum Vapor Intrusion at Leaking Underground Storage Tank Sites (PVI Guide)
The OSWER VI Guide replaces EPA's 2002 Draft VI Guidance. The OSWER VI Guide and PVI Guide are substantively similar to their April 2013 draft versions (and two prior 2012 drafts of the OSWER VI Guide), with some reorganization and additional technical detail. The guides apply to Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), and Resource Conservation and Recovery Act (RCRA) sites, as well as underground storage tank sites.
The OSWER VI Guide and PVI Guide provide direction on developing VI conceptual site models, detailed VI investigation, risk assessment/management, VI mitigation, and risk communication. A key theme in the OSWER VI Guide is collecting and weighing multiple lines of evidence to assess potential VI health risks. This same approach was advocated by the Interstate Technology & Regulatory Council (ITRC, 20071), and before that by the Electric Power Research Institute, Inc. (EPRI, 20052). Many states have adopted the multiple lines of evidence approach in developing their own VI guidance. GHD has been applying the multiple lines of evidence approach to evaluate VI over the last decade consistent with the earlier ITRC and EPRI guidance, and state VI guidance.
Key items in the OSWER VI Guide and PVI Guide that are significant to stakeholders interested in contaminated or potentially contaminated property include:
- Preemptive mitigation is a recognized alternative to lengthy drawn-out VI assessments to alleviate exposure and protect human health. However, addressing the source causing VI remains a requirement under CERCLA/RCRA, and terminating VI mitigation is contingent on demonstrating the source of vapors (e.g., contaminated soil and/or groundwater) has been treated. Also, post-construction monitoring, such as measuring pressure differentials between the building interior and subsurface, tracer testing, and potentially indoor air quality monitoring, is recommended to confirm the mitigation measure operates appropriately and effectively.
- EPA recommends using human health risk assessment methods consistent with EPA guidance and current chemical toxicity to evaluate potential risks to building occupants due to a complete or potentially complete VI pathway.
- EPA does not recommend the use of Occupational Safety and Health Administration (OSHA) permissible exposure limits (PELs) to assess human health risk posed to workers by VI or to support a no further action determination for VI in nonresidential buildings. However, in GHD's experience, EPA has agreed to use PELs for VI assessment at OSHA compliant facilities where chemicals of concern are used in facility processes and workers are informed/trained regarding the relevant chemicals.
- EPA frequently is raising the need for VI assessments during five-year reviews of Superfund sites, particularly in situations where VI was not considered in the previous site assessment and remedy selection, or where site conditions have changed. Even where VI was considered previously, EPA is identifying the need for updated VI assessments to ensure consistency with current VI guidance, chemical toxicity, and assessment of human health risks. Some state regulatory agencies (e.g., NYSDEC) are taking similar actions and reopening previously closed sites where VI may not have been fully addressed or addressed at all.
- The OSWER VI Guide states that it is not intended to offer recommendations for VI assessments that stakeholders may choose to conduct as part of real estate transactions. However, parties dealing in real estate transactions should ensure VI is appropriately considered during due diligence, even for properties that previously received regulatory closure, and give consideration to potential future migration of volatile contaminants from nearby impacted properties.
- EPA recognizes that rapid biodegradation of petroleum hydrocarbons can occur over short distances, even with light non-aqueous phase liquid (LNAPL) present, when sufficient oxygen and soil moisture exist to support adequate microbial activity.
- EPA establishes inclusion zones, or horizontal and vertical separation distances beyond which VI is not of concern, except where preferential vapor migration pathways exist.
- EPA recommends time-integrated measurements from multiple sampling events be used to assess VI exposures to account for variability in soil gas and indoor air concentrations that can occur due to changing weather and building operation conditions.
- EPA requires evaluation of acute or short-term exposures in the assessment of VI (e.g., explosion hazards or short-term exposures posing unacceptable health effects warranting prompt action) as well as chronic or long-term exposures.
- Mathematical modeling of VI alone is not considered sufficient to demonstrate the presence or absence of human health risks. Modeling is appropriate in conjunction with other lines of evidence and when calibrated to site-specific conditions.
- EPA recommends community engagement and involvement to communicate VI risk-related information and findings, which will facilitate building trust and gaining access to conduct VI investigations (and potentially mitigation).
- EPA's Vapor Intrusion Screening Level (VISL) Calculator provides VISLs for groundwater, soil gas, and indoor air for use in evaluating potential VI concerns.
1 Interstate Technology & Regulatory Council (ITRC), 2007. Vapor Intrusion Pathway: A Practical Guide
2 Electric Power Research Institute, Inc. (EPRI), 2005. Reference Handbook for Site-Specific Assessment of Vapor Intrusion to Indoor Air
How can GHD help?
GHD provides comprehensive VI assessment/mitigation services using the multiple lines of evidence approach and defensible scientific and engineering design principles. Services include sampling of soil gas, sub-slab soil gas, and indoor air using active, passive, or real-time sampling approaches; building inspection; background source evaluation; human health risk assessment; risk communication; design/implementation of VI mitigation systems for existing/new buildings; mitigation system monitoring; developing vapor management plans; and litigation support/expert testimony. GHD can provide webinars and/or in-person presentations on our VI services. Contact us to learn how GHD can assist with your VI needs.