Draft Vapor Intrusion Guidance

Graphic adapted from figure by Thomas McHugh, GSI Environmental.
Used by permission.

April 6, 2015

Ecology has updated the table of vapor intrusion screening levels to reflect recent toxicological information.  In addition, the sub-slab attenuation factor was reduced from 0.1 to 0.03 which raises sub-slab screening levels by about a factor of 3.  An Excel spreadsheet with revised groundwater, sub-slab soil gas, and deep soil gas screening levels is now available.

Due to these changes, Table B-1 in Appendix B of the October 2009 draft Guidance for Evaluating Soil Vapor Intrusion in Washington State should no longer be used.  

December 8, 2010

Ecology has suspended for one year work on updating the MTCA cleanup regulation. We will continue work on finalizing the VI guidance.

April 30, 2010

The Washington Department of Ecology is in the process of revising and updating the Models Toxics Control Act (MTCA) cleanup regulation. As part of this effort, Ecology is updating clarifying provisions on addressing threats posed by subsurface vapor phase contaminants.

Ecology has formed an advisory group to provide input on vapor intrusion issues.  In forming this Vapor Workgroup, Ecology has two main goals:
  • Develop a final VI guidance document.  Ecology believes the draft document provides a solid foundation for evaluating this pathway.  However, several issues were identified during the public comment period that Ecology would like to discuss with workgroup members before finalizing the guidance document.

  • Develop draft MTCA rule language. This rule language will support implementation of the phased decision-making process specified in the Washington draft VI guidance document and in recent federal guidance materials.
Workgroup members were chosen to provide a balanced set of knowledge and experience. The group will meet three to four times between May and July, 2010. The first meeting is scheduled for May 13 at the Ecology Headquarters Building in Lacey. 

The first meeting will review the purpose and objectives of the workgroup, discuss the guidance (including public comments received) screening levels, rule revisions, and a strategy for working through remaining issues.
Here is a link to information about the MTCA Cleanup Regulation Update:

Earlier Activity:

The Washington State Department of Ecology’s Toxics Cleanup Program asked for comments on a Draft Guidance for Evaluating Soil Vapor Intrusion in Washington State: Investigation and Remedial Action.

All comments on the document provided to Ecology by November 30, 2009, will be considered when the guidance is finalized.

What is the purpose of this draft Vapor Intrusion Guidance?

This technical guidance is intended to help responsible parties, environmental professionals, and Ecology site managers assess vapor intrusion at cleanup sites in Washington. 

Vapor intrusion (VI) refers to the vapor phase migration of volatile organic and/or inorganic compounds into occupied buildings from underlying contaminated ground water or soil. These substances may contaminate indoor air and impact human health.  This technical guidance document provides techniques for evaluating whether vapor intrusion has the potential to contaminate indoor air. It also recommends ways to protect building occupants.

This draft guidance includes the following:

Chapter 1:       Introduction
Chapter 2:       How to do a preliminary assessment to determine which areas and buildings that vapor intrusion could potentially affect.
Chapter 3:       How to further assess those site areas and buildings identified by the preliminary assessment. 
Chapter 4:       Interacting with the affected public.
Chapter 5:       Vapor intrusion mitigation techniques.
Chapter 6:       Deriving protective subsurface concentrations and evaluating the need for institutional controls.
Chapter 7:       References
Appendix A:     Terminology.
Appendix B:     Shallow groundwater and soil gas screening levels that protect indoor air.
Appendix C:     Soil gas sampling.
Appendix D:     Using the Johnson and Ettinger Model during vapor intrusion assessments.
Appendix E:     Estimating the contribution of vapor intrusion to indoor air using soil gas and indoor air measurement data.

What kinds of comments did Ecology look for?

The draft guidance addresses a wide range of vapor intrusion issues.  With a number of these issues, Ecology recognizes that a national consensus has yet to emerge.  Ecology asked for feedback on:

  • The tiered approach. For example, is using soil gas or groundwater data to make screening decisions reasonably conservative? We have tried to be consistent with the general approaches used by other state and federal agencies. Do you believe this is a reasonable approach for implementing the current MTCA rule provisions? If not, what do you suggest?
  • The list of VI contaminants of potential concern. The draft list of screening levels is designed to provide a reasonably conservative approach for identifying potential VI problems that would be implemented within a tiered decision-making framework. Should additional chemicals be included on the list? Do you believe the benefits of a longer chemical list outweigh the potential costs?  If not, what suggestions do you have for modifying the list?

  • Use of the Johnson and Ettinger model. Do the draft recommendations seem reasonable? If not, what do you suggest?

  • Consideration of background concentrations resulting from indoor and outdoor sources of volatile hazardous substances. Do the draft recommendations seem reasonable?  If not, what do you suggest?

  • Our recommendations concerning petroleum hydrocarbons. Are the draft recommendations reasonable?  If not, what do you suggest?

  • Level of detail and terminology. Ecology received a wide range of comments on earlier drafts related to the use of qualitative terms (like “slightly above”).  Some commenters also felt we should be more consistent, or exact, in how we characterized our recommendations (when, for example, should “expect” be used versus “recommend”). Does the current document provide a reasonable balance between general guidance and specific recommendations?

What is the relationship between this draft guidance and the ongoing MTCA Cleanup Regulation Update?

This draft guidance supports implementation of the Model Toxics Cleanup Act (MTCA) cleanup regulation. Ecology is currently in the process of reviewing and updating this regulation. As part of this process, Ecology is examining the investigation and cleanup requirements applicable to vapor intrusion to determine if changes should be made.

Feedback on this guidance document is helping Ecology determine what rule changes (if any) might be needed to effectively deal with vapor intrusion problems at cleanup sites.

Ecology plans to complete work on the vapor intrusion guidance document in 2010. The rulemaking process is currently scheduled to be completed in 2011. If necessary, Ecology will update the vapor intrusion document to reflect regulatory changes following rule adoption.

What is Ecology doing with the comments received?

All comments received by 5 p.m. on November 30, 2009, are being considered. Although we are unable to respond to each comment individually, Ecology will issue a general responsiveness summary that describes the issues identified by reviewers and explains the changes we made to the draft document in response.

Who should I contact if I have questions?

If you have questions, please contact:

Jim White
(360) 407-7324

Jodi Gearon
(360) 407-7235

This page last revised: April 20, 2015