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COVID-19 Postpones Deadline for Submission of Comments Supplemental Guidance for Vapor Intrusion


As you may know, in February 2020, California Environmental Protection Agency (EPA) released a Supplemental Guidance for Screening and Evaluating Vapor Intrusion - Draft for Public Comments (Draft Supplemental VIG). The public comment period was recently extended to June 1, 2020 and the original public information meetings have been postponed until further notice due to the COVID-19 public health emergency.

The Draft Supplemental VIG document presents information and recommendations on: (i) using attenuation factors (AFs); (ii) a step-wise approach to evaluating vapor intrusion (VI); (iii) evaluating sewers and other subsurface conduits as a potential VI migration route and pathway of exposure; and, (iv) building a California-specific VI database.

Below is a summary of several new items contained in the Draft Supplemental VIG and a discussion of Waterstone’s thoughts on each of them.

1. Vapor Intrusion Attenuation Factors

An attenuation factor is a unitless factor used to convert soil gas concentrations to theoretical indoor air concentrations for the evaluation of vapor intrusion. This is the primary item in the Draft Supplemental VIG for which Waterstone will be submitting comments and it is the item which will affect estimated indoor air concentrations and thus case closures the most.

The Draft Supplemental VIG recommends the use of the USEPA’s empirically-derived AF of 0.03 (from USEPA, 2015[1]) to evaluate vapor intrusion from sub-slab and “near-source” soil gas to indoor air for the initial screening of sites in California. Currently, an AF of 0.001 is being used to estimate indoor air concentrations of volatile chemicals based on soil gas concentrations. The following formula shows how the AF is used:

AF x ConcSoil Vap = ConcAir

Let us compare the two AF values in determining an allowable concentration in soil vapor using the published screening level concentration for perchloroethylene (PCE) indoor air in a commercial/industrial setting of 2 µg/m3.

0.001 vs. 0.03

0.001 x ConcSoil Vap = ConcAir = 2 µg/m3

0.001 x ConcSoil Vap = 2 µg/m3

ConcSoil Vap = (2 µg/m3 / 0.001)

ConcSoil Vap = 2,000 µg/m3

= 2 µg/L



0.03 x ConcSoil Vap = ConcAir = 2 µg/m3

0.03 x ConcSoil Vap = 2 µg/m3

ConcSoil Vap = (2 µg/m3 / 0.03)

ConcSoil Vap = 66.7 µg/m3

= 0.0667 µg/L


So, by increasing the AF from 0.001 to 0.03, California regulators make a giant leap and assume that 30 times more mass of volatile organic compounds (VOCs) travels from the soil vapor through the slab and into a building’s occupied space. This change in the AF will make it infinitely harder to gain regulatory closure for sites with VOC contamination and impossibly hard to get closure for sites with chlorinated VOC contamination. If this becomes the accepted standard for evaluating soil vapor and its effect on indoor air, case closure (if possible at all) will take longer and cost significantly more.

Below is a brief summary of the origins of the 0.03 AF value and our rationale on why it should not be applied to California sites.

The AF value is derived from sub-slab and soil gas data contained in USEPA’s 2012 VI Database (USEPA, 2012[2]) and is a value which USEPA considers to be a reasonably conservative, generic AF which “accounts for the inherent temporal and spatial variability in indoor air and subsurface vapor concentrations”. The sites sampled for USEPA’s 2012 VI Database were primarily residential buildings, but the AF of 0.03 is extended to commercial buildings with the rationale that “in many geographic locations, some commercial enterprises have been established in converted residential buildings”. This is not the case for most California commercial properties. In fact, none of the buildings at the commercial sites studied by Waterstone have been converted from residences.

Based on this logic, it was concluded that commercial buildings are expected to have similar susceptibility to VI and similar interior mixing and dilution as the residential buildings represented in USEPA’s 2012 VI Database. These conclusions were presented in USEPA’s 2015 Oswer Technical Guide for Assessing and Mitigation the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air (USEPA, 2015[3]). This is a one size fits all approach that ends up hurting the vast majority of commercial property owners.

Theoretical considerations noted in USEPA 2015 to support lower AFs for non-residential buildings include higher ventilation rates and thicker concrete slabs with less settling and less cracking, with a note that EPA may consider appropriate building-specific data when evaluating VI for large non-residential buildings. Nonetheless, the Draft Supplemental VIG applies the 0.03 AF to all buildings. We believe it is inappropriate to apply the 0.03 AF to all California sites without accounting for these differences.

Only a handful of the sites studied in USEPA’s 2012 VI Database were located in California, and none of the sites were located in southern California. Many of the sites were located in cold-weather climates and had buildings with basements. This would be expected to yield lower AFs since the buildings are more tightly closed during cold weather and basements are enveloped on all sides by vadose zone soils. We believe this is inconsistent with most California sites.

Waterstone agrees with the points outlined in the Draft Supplemental VIG for why the 0.03 AF is premature and not fully scientifically supported, as summarized below:


  • Few buildings designed for commercial or industrial use are included in the USEPA VI Database.

  • Very few California data are included in the USEPA VI Database.

  • The USEPA VI Database included data where site-specific outdoor air data were rarely collected.

  • The USEPA VI Database studies were performed at a time when building screening techniques and tools were less well-developed.

  • For most buildings in the USEPA VI Database, only one indoor air sample and one subsurface sample were collected per building.

For these reasons we believe implementation of the 0.03 AF is premature and not scientifically supported. This 0.03 AF value has already resulted in increased costs and extended project schedules where regulators have required further study. Although it has not been used to establish cleanup levels, it has made VI studies more extensive, costly, and time-consuming. Further, it has made case closures more difficult for sites that might have previously been quite simple to attain. We believe that AFs should be developed based on California-specific or site-specific studies where both soil gas and indoor air data are available.

2. Transport of Vapor Contamination Through Sewers

The Draft Supplemental VIG document recommends evaluation of sewers as a potential preferential pathway for vapor migration and sampling of sewer air for an additional line of evidence in diagnosing the source of VOCs in indoor air. This may include sampling of the sewer air. The Draft Supplemental VIG notes “Where vapor forming chemicals are likely to have impacted sewer air, and the conduit connects to or has the potential to release vapors below a specific building, then an indoor air investigation for the building should proceed.”

We note that both the sewer pipe itself and utility trench backfill material can act as preferential pathways. These pathways are typically evaluated to help explain why areas not near the chemical use or storage locations but near the sewer pipes have been contaminated. Due to greater void space in the pipe, vapor transport can be at a higher rate in the pipe than the backfill (porous media) and, theoretically, a direct pathway into a building. This assumes that the plumbing systems inside buildings, which have been designed to prevent sewer gases from entering the building, are leaking or have become compromised. While these scenarios are rare, they are certainly possibilities that should be considered. Additionally, plumbing features such as cracked or punctured pipes, loose fittings, degraded toilet gaskets (e.g., wax rings), and dry plumbing traps (e.g., p-traps) can easily be fixed and/or maintained to prevent the flow of vapors into the building.


The Draft Supplemental VIG suggests that sampling the air inside vapor conduits such as sewers should be part of VI evaluations when indoor air results indicate the presence of VOCs, but the VOCs do not appear to be migrating through the subsurface soil.

Typically, a sewer investigation is not part of most VI studies. This would be a costly addition which would benefit only a limited number of cases. However, in those instances when VOCs are present in indoor air, and contaminants are not accounted for in soil vapor, it makes sense to consider the sewer as a potential source.

3. Four-step Process for VI Assessments

The Draft Supplemental VIG outlines a 4-step process to determine whether buildings are potentially affected by VI that may pose a health risk to occupants.

  1. Prioritizing buildings in proximity to source contamination for a VI assessment

  2. Collecting exterior soil gas samples to determine if buildings have potential for VI

  3. Collecting indoor air, subslab soil gas, and outdoor air samples if buildings have potential VI risks

  4. Evaluating the need to manage current and future VI risk based on both indoor air concentrations and soil gas concentrations.

Much of this is already a standard part of a VI evaluation, but the Draft Supplemental VIG outlines additional details that may go beyond what might be part of a typical VI study. We believe these are all considerations that should be part of a VI study and will help to get to the root cause of the problem. However, they are also likely to increase costs and require more time to complete.

Waterstone is not likely to have substantive comments to this portion of the VIG.

4. California VI Database

The Draft Supplemental VIG document proposes the compilation and development of a California-statewide VI database for future review by the CalEPA workgroup to determine if California-specific AFs are justified. The State has added capabilities to the GeoTracker system to accept building-specific data and vapor data.

If the data is properly reviewed and evaluated, it is likely to result in a much more realistic and favorable AF than what currently exists, certainly lower than the 0.03 AF. However, the Draft Supplemental VIG does not outline the standards or methods by which the data will be evaluated and how it would be used to develop California-specific AFs.

However, if the data is evaluated in a conservative manner and sites which are the exceptions or rare cases are given more weight, then it could result in an inappropriate AF.

All of this points to the fact that AFs are used as a one size fits all solution. However, they don’t consider the specifics of any individual site and as a consequence, lose scientific validity for the majority of the sites to which they are applied.

We believe that a far better approach would be to generate site-specific AFs. The data that will likely be collected in the GeoTracker system should allow for a host of factors to be evaluated and correlated to actual vapor migration into indoor spaces - this data could lead to a procedure for development of site or building-specific AFs.

Your Thoughts Are Important

Waterstone is preparing comments and a technical response to submit and is soliciting input from our clients and colleagues. We would like to hear your thoughts, opinions, and details of how this guidance will affect your business operations, property transactions, development projects, ability to finance, site closures, and litigation matters. We are interested in any technical arguments in favor of or in opposition to what is contained in the Draft Supplemental VIG.

To provide comments or discuss any topics further, please contact Mark Shifflett at mshifflett@waterstone-env.com or 714-595-0894.

We also encourage you to submit comments of your own. Click on one of the helpful links below to get your opinions heard.

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