Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 13.2


The collection of soil vapor samples as part of a site characterization and vapor intrusion investigation is discussed in Section 7. The technical background and development of soil vapor and indoor air action levels for assessment of potential vapor intrusion hazards is presented in the HDOH Environmental Hazard Evaluation (EHE) guidance (HDOH, 2016.). The guidance also presents soil and groundwater action levels for screening of potential vapor intrusion hazards. The collection of soil vapor data is recommended for all sites where potentially significant releases of volatile chemicals might have occurred.

A key factor in development of the action levels for potential vapor intrusion risks is the assumed attenuation of subsurface vapors as they intrude a building and mix with indoor air. Assumptions incorporated into the vapor intrusion models are discussed in Sections 2 and 3 of Appendix 1 of the EHE guidance and based on research published by Brewer et al. (Brewer et al., 2014b.). The research represents an important update to indoor air:subslab soil vapor attenuation factors initially proposed by USEPA researchers in the early 2000s and published for public review in 2012 (USEPA 2012b.).

The “empirical database” used to derive the 2012 USEPA attenuation factors was subsequently determined to be unreliable (Brewer et al. 2014b.). The database relied on a single, small-volume (e.g., one-liter), randomly located, subslab vapor sample to represent intruding vapors (attenuation = indoor air sample concentration/subslab vapor sample concentration). Attenuation factors derived from the database were an order of magnitude or more higher (i.e., less attenuation) than attenuation estimated from more rigorous databases of building leakage used by engineers to design heating and cooling and energy systems, and were called into question. Use of the excessively high (“conservative”) attenuation factors caused large numbers of sites with relatively minimal VOC contamination to be unnecessarily flagged for potential vapor intrusion risks.

Reliability of the approach used by USEPA required a high degree of uniformity of VOC concentrations in vapors beneath building slabs. Collection of a vapor sample from another location under the building slab would otherwise generate a different attenuation factor, implying that any single attenuation factor estimated was random and unlikely to be representative of actual vapor intrusion conditions. As discussed by Brewer et al. (Brewer et al., 2014b.), subslab vapor plumes are in fact likely to highly heterogenous. This, in addition to uncertainty regarding actual vapor entry points into a building, negates the validity of the USEPA database for derivation of indoor air:subslab attenuation factors. As an alternative, Brewer et al. (Brewer et al., 2014b), refer back to the original approach proposed by USEPA (USEPA, 2004f.) based on better supported building leakage rates in various climate regions within the United States. Using this approach, attenuation factors estimated for tropical regions where buildings are not heated throughout the year were used to develop soil vapor action levels presented in the HDOH EHE guidance.

Two USEPA vapor intrusion guidance documents were being finalized for publication at the time that the Brewer at al. (Brewer at al., 2014b.) research was published, one for vapor intrusion in general (USEPA 2015c.) and one specific to vapor intrusion associated with petroleum (USEPA 2015b). The research published by Brewer et al. (Brewer et al., 2014b.) is referenced in the USEPA petroleum vapor intrusion guidance. The research is not, however, referenced in the general vapor guidance. This was due to completion of the final review of the guidance document prior to publication of the Brewer et al. (Brewer et al., 2014b.) paper (personal communication, Kapuscinski, 2016.):

(The Brewer et al.) 2014 paper… on subslab attenuation factors was published (Fall 2014) after the intra-agency vetting concluded for the (2015) OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air (OSWER Publication 9200.2-154). By contrast, the (2015) Technical Guide for Addressing Petroleum Vapor Intrusion at Leaking Underground Storage Tank Sites (EPA 510-R-15-001) was prepared and revised somewhat independently.

Although publication of the USEPA vapor intrusion guidance documents postdate publication of the Brewer et al. (Brewer et al., 2014b.) research, presentation of indoor air:subslab vapor attenuation factors in the documents based on the 2012, USEPA “empirical database” should be considered invalid and not referred to for use in site-specific, vapor intrusion investigations.