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Preliminary DTSC Study on Attenuation Factors

Prepared by Brianne Archer, P.E. and Jeffrey Dagdigian, PhD

Vapor intrusion occurs when chemical vapors migrate from contaminated groundwater through the soil into the basements or foundations of buildings. These chemical vapors can degrade indoor air, sometimes to the point of posing risks to human health.


In February 2020, the California Environmental Protection Agency (CalEPA) released a Supplemental Guidance for Screening and Evaluating Vapor Intrusion - Draft for Public Comments (Draft Supplemental VIG). The Draft Supplemental VIG was prepared to update the previous guidance, Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air (Final VIG), published jointly by the Department of Toxic Substances Control (DTSC)/CalEPA in October 2011.

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.

The Draft Supplemental VIG recommends the use of the United States Environmental Protection Agency (USEPA) 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. This is a much larger attenuation factor (i.e. more vapors migrate into indoor air from subsurface soil) than the AF of 0.001 that was published for existing commercial buildings in the 2011 Final VIG. The following formula shows how the AF is used:

AF x Conc[Soil Vapor] = Conc[Air]

The 0.03 AF value is derived from sub-slab and soil gas data contained in USEPA’s 2012 VI Database (USEPA, 2012[2]) which USEPA considers to be a reasonably conservative, generic AF that “accounts for the inherent temporal and spatial variability in indoor air and subsurface vapor concentrations”.

However, it has been noted by several commenters that the sites sampled for USEPA’s 2012 VI Database were primarily residential buildings with few locations in California. 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.

Figure 1. Location of Sites Included in the USEPA VI Database Study Area

(From “Empirical Analysis of Vapor Intrusion Attenuation Factors for Sub-Slab and Soil Vapor” presentation, Ettinger and others, December 2018)

Despite these apparent limitations, the 0.03 AF was extended to commercial buildings with the rationale that “in many geographic locations, some commercial enterprises have been established in converted residential buildings”. This is rarely the case for California commercial properties.

Sneak Peek at DTSC’s Preliminary Attenuation Factor Study

With regards to building a California-specific VI database, DTSC is currently preparing their own study using a California-specific database and presented the findings in a recent webinar at the California Land Recycling Conference. The study is currently undergoing peer review and DTSC plans to publish their findings in December 2020.

Why are they completing such a study? Partially because there are fewer homes with full or partial basements in California. California-style buildings typically have different foundations than those in other parts of the country and primarily included in the EPA database.

The DTSC-study analyzed the following AFs for different types of buildings and sampling locations:

-Sub-slab to indoor air AFs (residential, non-residential, and select sites)

-Soil gas to indoor air AFs (residential vs non-residential, selected sites, various sampling depths and lateral locations

-Groundwater to indoor air AFs (residential vs. non-residential, selected sites, various sampling depths and lateral locations).

The main differences were residential versus non-residential buildings and the depths of the soil gas samples collected.

To compile the beginnings of the California-specific VI database, DTSC paired indoor air data with (i) subsurface soil gas (almost 3,000 pairs) and (ii) sub-slab soil gas (600 pairs). Some limitations of this database acknowledged by the DTSC include:

· Including only two sites under the oversight of the California Regional Water Quality Control Board (RWQCB);

· Including only one site with a crawl space;

· Not including the previously identified sites in the USEPA database; and,

· Not including any buildings with basements.

In addition, the empirical statistical evaluation that was completed did not take soil type or HVAC system information into account.

Background Data for Statistical Evaluation

Many volatile chemicals that could present a potential VI issue at contaminated sites can also be found indoors due to source unrelated to subsurface contamination. These sources may include emissions from consumer products, home furnishings, building materials, combustion sources, and other outdoor sources. Therefore, to determine the extent to which VI impacts indoor air concentrations, one must consider the additional contributions of background sources to indoor air concentrations.

When background sources contribute to indoor air, the empirical AF will be biased high relative to the true VI AF. In order to account for this, only sites where the subsurface soil gas concentrations were substantially greater than the background concentrations were considered for statistical evaluation.

For the purposes of culling the DTSC’s VI database for pairs with soil gas data more than 50 times a background concentration, annual outdoor/background data from the California Air Resources Board (CARB) was considered for tetrachloroethylene (PCE) and trichloroethylene (TCE) from 2009 to 2018.

The median of the 90th percentile PCE concentrations over the 10-year period was 0.27 micrograms per cubic meter (ug/m^3) and the median of the annual maximum concentrations was 1.36 ug/m^3. In comparison, the indoor air screening level published by the DTSC in their HERO Note 3 Screening Levels is 0.46 ug/m^3 for a residential scenario and 2.0 ug/m^3 for a commercial/industrial scenario. By comparing the CARB outdoor/background concentrations to the indoor air regulatory limits for PCE, it’s clear that they may have a significant impact on whether the indoor air concentrations exceed the regulatory limits.

Similarly, the median TCE concentrations over the same 10-year period were 0.11 ug/m^3 for the annual 90th percentile and 0.99 ug/m^3 for the annual maximum. In comparison, the indoor air EPA Regional Screening Level (RSL) is 0.48 ug/m^3 for a residential scenario and 3.0 ug/m^3 for a commercial/industrial scenario.

These background concentrations were taken into account to determine if the soil gas/indoor air pairs should be included in the AF calculations. Only those pairs where the soil gas was more than 50 times the background concentration were included to minimize the attenuation from outdoor sources as a proportion of the attenuation from subsurface soil.

DTSC Study – Sub-Slab Attenuation Factors

The DTSC’s study includes the calculation of sub-slab to indoor air attenuation factors for 600 sub-slab and indoor air data pairs. Of the 600 pairs, 558 were non-residential at 30 sites and 42 were from residential buildings at 3 sites. Most of the sub-slab data in California is from non-residential buildings due to homeowner resistance to sub-slab sampling and regulator’s preference to skip straight to indoor air sampling.[3]

The screening level AF for sub-slab to indoor air vapor attenuation is 0.03; however, the 95th percentile combined residential and non-residential AF from the preliminary DTSC study is 0.004, nearly 10 times lower. Excluding residential buildings, the 95th percentile non-residential AF is 0.0031, which is even lower than the combined AF.

DTSC Study – Soil Gas Attenuation Factors

The DTSC’s study also includes the calculation of subsurface soil gas to indoor air attenuation factors for over 2,900 soil gas and indoor air data pairs. Of these pairs, 1,441 were non-residential at 32 sites and 1,485 were from residential buildings at 6 sites.

The screening level AF for subsurface soil gas to indoor air vapor attenuation is 0.03; however, the 95th percentile combined residential and non-residential AF from the preliminary DTSC study is 0.0008, which is 33 times lower than EPA’s AF. Excluding residential buildings, the 95th percentile non-residential AF is 0.0016, which is higher than the combined AF but still much lower than EPA’s AF of 0.03.

As stated above, there are only 6 residential sites in the database and most of those are located in Southern California. During their presentation, DTSC staff noted that the residential AFs do not agree with the previously understood conceptual model for VI from soil gas because they are generally lower than the non-residential AFs. In the October 2011 VIG, the residential attenuation factors were presumed to be double those for commercial buildings.

Compilation of Attenuation Factors from the DTSC Study

The following is a summary of the empirically calculated AFs from the 2020 DTSC study. The AFs are consistently well lower than the 0.03 AF published by EPA and promulgated in the Draft Supplemental VIG. These findings indicate that the 0.03 AF may be overly conservative for use in California. Additional evaluation of California-specific VI data may also be warranted to determine if the conceptual model should be adjusted based on the lower AFs for residential sites and what impact soil type and HVAC operation might play into the empirical AF. Additional work may also be conducted with regards to the soil gas profile by depth and the impact on indoor air concentrations.

Is the DTSC study an improvement over EPA study?

The DTSC study is an improvement over the EPA study for several reasons. First, it is geographically more representative of sites in California with more appropriate climatic conditions. Second, there are a greater number of pairs between indoor air and subsurface/sub-slab soil gas and the pairs were procured from more sites. Lastly, the DTSC’s California-specific VI database also includes more types of structures than the EPA study.

What are DTSC’s Next Steps?

DTSC plans to complete the peer review process of this study and review the comments generated through peer review. The AF Study Report will be revised as appropriate and finalized with a projected completion date of December 2020. We will report back to you then.

[1] [2] [3] Please note that these results have not been finalized and are preliminary findings that should not be cited or referenced, although they were shared by DTSC during the conference webinar.


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