Recently, an article on one of our favorite topics was published in the Journal of Groundwater Monitoring and Remediation, a publication from the National Groundwater Association. The article is titled, Empirically Derived California Vapor Intrusion Attenuation Factors by Rafat Abbasi, William Bosann, and Dan Gallagher. The authors are the Chief of State-Wide Discovery and Enforcement, a Senior Toxicologist, and a Senior Engineering Geologist respectively at the California Department of Toxic Substances Control (DTSC). The article can be found here: NGWA Link
Joe Landeros, GIT from our office went through the article with a fine tooth-ed comb and drafted the following in-depth summary.
Vapor Intrusion (VI) is an important migration pathway for properties contaminated with volatile organic compounds (VOCs). VI attenuation factors (AFs) are a method used for screening properties for exposure risk. The AF is the reduction ratio in chemical concentrations that occur during the migration of soil vapors from the subsurface into indoor air through the various flooring, subflooring, slab(s), and engineered fill materials or lack thereof. Typically, AFs are based on either model estimates or empirical studies of subsurface and indoor air data. Most states use screening AFs based on analysis of large empirical datasets.
The United States Environmental Protection Agency (US-EPA) completed their nationwide study of empirical AFs in 2012 and generated a database containing data from 2,929 paired indoor air and subsurface measurements collected from 913 buildings across 15 states. Chlorinated VOCs make up 97% of the chemicals in the database, and 85% of the data are from residential buildings. Based on their data, the US-EPA recommends an AF of 0.03 when screening buildings for VI with subslab and/or soil vapor data. This VI study categorized subslab data as samples collected approximately six (6) inches below the foundation slab and soil vapor data as samples collected at or below five (5) feet below ground surface.
However, the US-EPA dataset is not representative of conditions in California for a number of reasons. Of the data collected, very few sites were from California, only two (2) small subslab datasets and two (2) small soil vapor datasets, all of which were within the San Francisco Bay general area. Most of the data was collected from New York, Montana, Colorado, and Connecticut, all of which generally have colder climates than California. Over 75% of the indoor air samples were collected in residential homes with basements. However, less than 5% of single-family homes in California have basements. Additionally, due to limited nonresidential data in their database, the US-EPA did not generate attenuation factors for commercial/industrial buildings.
In 2019, the DTSC gathered data from contaminated sites to derive AFs that are representative of climate and building conditions in California. The DTSC database includes measurements from 52 sites across 16 counties, and contains 4,972 paired measurements of both residential and nonresidential properties. Data selected for AF evaluation included 213 buildings, 100 of which were commercial/industrial, and 113 were residential. The DTSC studied database yielded subslab and soil vapor AFs of 0.005 and 0.0009, respectively with a 95th [confidence] percentile. Both the subslab and soil vapor AFs are orders of magnitude below the US-EPA AF of 0.03. Essentially, this means dirty properties can have safe and usable building structures, especially with the assistance of engineering controls in place. Additionally, the results of the DTSC study were compared to other VI AF studies. In 2021, the Department of Defense collected data for 79 buildings from 22 military installations nationwide and calculated an AF of 0.001. In 2018, Derycke et al. collected data for 51 schools from 38 towns across France and calculated an AF of 0.04. In 2021, Lahvis, and Ettinger compiled data from 485 buildings from 36 sites across California. Their study resulted in a subslab AF of 0.002 and a soil vapor AF of 0.001.
When screening a site for potential risk, McAlister GeoScience believes it is important to use the realistic and practical AF that is representative of the site conditions, construction methods, and site use. Nonresidential buildings are typically larger with greater mixing volumes, fewer utility penetrations, and higher indoor air exchange rates. Therefore, nonresidential buildings are more likely to dilute incoming vapors when compared to residential buildings. Additionally, modern commercial buildings in California are constructed with a vapor barrier and/or subslab venting system below a concrete slab-on-grade and above the engineered compacted fill. This type of construction is vastly different than a residential structure with a crawl space or basement, thus it’s not reasonable for the two types of buildings to have comparable AFs.
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