USEPA Releases Vapor Intrusion Technical Guidance

Jul 7th, 2015 | By | Category: Environmental Management


USEPA OSWER Vapor Intrusion Technical Guide

From the USEPA OSWER Vapor Intrusion Technical Guide

On June 11, 2015, The U.S. Environmental Protection Agency (USEPA) released its long awaited vapor intrusion guidance documents:

These documents present a recommended framework for addressing the vapor intrusion pathway at USEPA Resource Conservation and Recovery Act (RCRA), Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), or Brownfield sites. Of the two documents, the VI Technical Guide is much more substantial and applies to many more sites.  The PVI Guidance is limited to releases from Underground Storage Tanks (USTs), while the VI Technical Guide applies to all other sites including petroleum hydrocarbon releases that are mixed with other volatile chemicals or are the result of releases from sources other than USTs.  The remainder of this article discusses the VI Technical Guide.



The overall stated purpose of the VI Technical Guide is to promote national consistency in assessing vapor intrusion. The VI Technical Guide provides an overarching approach; however, USEPA indicates that because assessing vapor intrusion will vary from site to site, the VI Technical Guide provides a framework for planning and conducting a VI investigation rather than prescriptive steps.


Vapor Intrusion Pathways

The VI Technical Guide defines the vapor intrusion pathway as complete if the following conditions are met under current conditions for one or more buildings.

1. A subsurface source of vapor-forming chemicals is present (e.g., in soil or groundwater) underneath or near the building.

2. Vapors form and have a route along which to migrate toward the building.

3. The building is susceptible to soil gas entry, which means openings exist for the vapors to enter the building, and driving “forces” exist to draw the vapors from the subsurface through the openings into the building.

4. Vapor-forming chemical(s) in the subsurface are also present in the indoor environment.

5. The building is occupied by one or more individuals when the vapor-forming chemical(s) are present indoors.

The VI Technical Guide does clarify that the vapor intrusion pathway is incomplete if one or more of these conditions is currently absent and is reasonably expected to be absent in the future.  As presented throughout the document, a determination of an incomplete pathway is likely to require significant data. Also note that the determination of a complete vapor intrusion pathway does not consider whether vapors are present at concentrations below risk-based or background levels.


Vapor Intrusion Investigation Process

The VI Technical Guide identifies two general levels of analysis:

  • preliminary analysis; and
  • detailed investigation.

Preliminary analysis is the initial data gathering stage. The overall goal of this step is to determine if vapor-forming chemicals were or are being used, stored, or handled at or near the site and whether any releases to the subsurface may have occurred.  This information should be used to develop an initial conceptual site model (CSM).  In addition, the preliminary analysis can identify conditions that warrant prompt actions (e.g., odors, wet basement). The preliminary analysis is not expected to eliminate many sites due to the conservative nature of the VI Technical Guide including the need to find conclusive evidence that the pathway is incomplete. Historical data or data of questionable quality may not be used to eliminate the vapor intrusion pathway.

USEPA recommends using a 100 foot distance as the inclusion distance for VI, but also indicates that it should be site specific and may need to be larger for some sites with pressure driven flow or preferential pathways.

Detailed investigation is needed if the preliminary analysis finds the presence of (or potential for) subsurface contamination with vapor-forming chemicals under or near buildings. Figure 6-1 (in the VI Technical Guide) provides an overview of the process. USEPA indicates the following for a detailed analysis:

  • Data collection should proceed in a step-wise fashion to develop an understanding of the site, based on the CSM, which is critical to determining the degree to which vapor intrusion is a pathway of concern. Subsequent data collection should focus on filling data gaps to enhance the CSM.
  • Due to temporal and spatial variability, multiple samples should be collected to fully evaluate shallow groundwater, exterior soil gas, sub-slab soil gas and indoor air. USEPA further adds that site-specific conditions may result in large variations (within a day and between days and seasons) in indoor air concentrations in an individual residential building. As a result, a single sample is insufficient to characterize variability and to ensure that final risk management decisions are based upon a reasonable maximum vapor intrusion condition.
  • Although the need for multiple samples is clearly stated, the VI Technical Guide does not provide any specific recommendations on the number of samples, thereby leaving interpretation and implementation up to the individual preferences of the Regulator.
  • Buildings can be prioritized depending upon site-specific factors, such as: strength, proximity, and extent of the vapor source; the potential for attenuation due to geologic, hydrologic, or biochemical conditions; and type(s), characteristics, and structural condition of the overlying building(s). However, USEPA also states that because so many factors affect vapor intrusion, it may be difficult to identify representative worst-case buildings.
  • To focus vapor intrusion assessments, it may be appropriate to limit the analytical list to just those chemicals determined to be site-related.
  • Collecting indoor air data over a longer period (i.e., 24 hours or more) is recommended. As a result, passive samplers that can be deployed for longer periods of time may be useful for many sites.
  • When collecting indoor air samples, one sample from each floor is sufficient to evaluate buildings less than 1,500 square feet. However, for sub-slab soil gas, USEPA recommends collecting three samples for the same size buildings.
  • The collection of pressure differential data several days before sampling can provide valuable information in understanding whether buildings are susceptible to vapor entry.
  • Deeper soil gas samples collected in the vadose zone immediately above a source will tend to be more suitable than shallow soil gas samples for assessing vapor concentrations that may be in contact with the building’s slab.


Evaluating the VI Pathway

The data evaluation focuses on using multiple lines of evidence (MLE) and risk assessment to evaluate results. The VI Technical Guide identifies the following categories to include in an MLE evaluation:

  • subsurface vapor sources;
  • vapor migration and attenuation in the vadose zone;
  • building foundation assessment including susceptibility to soil gas entry;
  • interior assessment; and
  • indoor and outdoor sources of vapor-forming chemicals.

USEPA indicates that the specific lines of evidence to be used will depend on site-specific factors as identified in the CSM. Under ideal conditions all lines of evidence will unambiguously support decisions.  However, if the weight of evidence does not support a confident decision, USEPA recommends re-evaluating the CSM or potentially collecting additional data. The MLE approach is identified as particularly important for reaching a no further action decision considering both current and future building uses and construction. To ensure the protection of future buildings, the VI Technical Guide includes recommendations for response actions at sites where subsurface vapor sources remain into the future, but do not pose unacceptable human health risk under current conditions.

The risk-based screening and risk assessment should be conducted consistent with CERCLA and RCRA principles.  The primary objective of risk-based screening is to identify sites or buildings unlikely to pose a health concern through the vapor intrusion pathway.  As part of the risk evaluation, USEPA acknowledges that background sources may be present in buildings and that the Agency does not clean up to concentrations below natural or anthropogenic background levels. The VI Technical Guide indicates that if subsurface concentrations are less than VISLs (vapor intrusion screening levels), then, generally, no further action or study is needed.  If subsurface concentrations exceed a VISL, this would not automatically trigger mitigation or subsurface remediation; however, it does suggest the need for further evaluation. When using the VISLs, Table 6-1 in the VI Technical Guide provides recommended attenuation factors (AFs).  The following AFs are now recommended:


Exposure Pathway Recommended Attenuation Factor
Groundwater to indoor 0.0005 (was previously 0.001) for fine grained soils
Sub-slab to indoor air 0.03 (was previoulsy 0.1)
Near source soil gas to indoor air 0.03


The VI Technical Guide states that considerable scientific and professional judgment will likely be needed when weighing lines of evidence to determine whether the vapor intrusion pathway is complete or incomplete.  This includes determining whether it is appropriate to use VISLs or whether an exceedance of a VISL is a VI concern or risk to human health.  A complete pathway does not necessarily indicate a human health risk.

Finally, in a clear deviation from previous policy, USEPA asserts that the U.S. Occupational Safety and Health Administration (OSHA) Permissible Exposure Limits (PELs) and ACGIH Threshold Limit Values should not be used to evaluate indoor air concentrations at sites where the same chemical is present in the subsurface. USEPA cites the OSHA website, which notes that many of the PELs are outdated.  This change calls into question not only the evaluation of vapor intrusion at operating facilities, but the overall use of the OSHA values at any facility that uses chemicals as part of its operation.



Throughout the VI Technical Guide, USEPA refers to pre-emptive mitigation (PEM) and mitigation as important steps in addressing vapor intrusion.  PEM based on limited but credible subsurface and building data is identified as an appropriate (and potentially cost effective) approach to begin to quickly implement actions.  If mitigation is necessary, the VI Technical Guide discusses options for determining the type of mitigation as well as monitoring and OMM requirements.  For example, for subsurface depressurization (SSD) systems, USEPA recommends initially monitoring slab pressure quarterly to verify system operation as well as periodic indoor air monitoring. A reduced monitoring frequency may be appropriate after one year of successful operation of the mitigation system.

If a mitigation system is put in place, USEPA does provide information on system termination and exit strategy.  Although it is a rare occurrence to turn off an SSD system, the VI Technical Guide does allow for the development of termination criteria.

As part of evaluating mitigation, the VI Technical Guide also discusses the use of institutional controls to restrict land use, regardless of whether a mitigation system is in place.


Community Involvement

The VI Technical Guide concludes with a chapter on community involvement, as this is a critical piece of any vapor intrusion investigation.  USEPA recommends the development of a community involvement plan at sites that have vapor intrusion concerns.  USEPA also recommends that all access requests and public communications be in writing in order to document the efforts being made to protect human health.  At sites where an owner may refuse sampling, the VI Technical Guide maintains that reasonable attempts be made to track ownership changes.

Vapor intrusion continues to be an important issue that needs to be addressed at sites with subsurface VOC contamination.  With the publication of the VI Technical Guide, USEPA provides an overall process for evaluating the pathway, although key details and decisions are still left up to the individual regulator.  It is also likely that many state agencies will adopt the VI Technical Guide; especially those states that do not currently have vapor intrusion guidance.


About the Author

Nadine Weinberg is a Partner in the Boston, U.S.A. offices of Environmental Resources Management (ERM).  Ms. Weinberg is currently the Americas Vapor Intrusion Lead for ERM and provides strategic guidance on evaluating vapor intrusion for sites across the country and internationally.  Ms. Weinberg has completed vapor intrusion evaluations including identification of approaches to address vapor intrusion, modeling of the vapor intrusion pathway, development of sampling work plans, assessment of soil gas and indoor air data including the determination of potential human health risks, and negotiations with regulatory agencies. She has represented clients on several litigation cases and has experience writing expert reports, conducting depositions, and participating in trials.

Photograph: Smoke by Ilco, Izmir, Turkey.


Return to the EHS Journal Home Page

EHS Journal - Smoke by Ilco

Tags: , , , , ,

Leave a Comment