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Evolving Standards for Due Diligence and Vapor Intrusion: Consider State Guidance and Policy

As regulation of vapor intrusion continues to evolve and escalate costs at contaminated sites, due diligence practices for vapor intrusion must also continue to evolve. Potential purchasers should consider supplementing a standard ASTM E1527-21 Environmental Site Assessment (ESA)[1] to keep pace with the rapidly developing regulation of vapor intrusion consistent with their risk tolerance for the transaction.


ASTM E1527-21 includes potential vapor migration within its scope (i.e., the movement of vapors in subsurface media) but not vapor intrusion (i.e., the intrusion of vapors into buildings). ASTM E1527-21 does not identify any specific tools for evaluating vapor migration beyond the standard methodologies used for identifying releases or potential releases to subsurface media – and references (but does not require) use of ASTM E2600-22,[2] as a supplemental guide for assessing vapor encroachment (i.e., the presence of contaminated vapors at the target property due to an on-site or off-site release).


ASTM E2600-22 provides a two-tiered methodology for screening contaminated sites for the potential presence of vapor encroachment. Under a Tier 1 screening, the environmental professional identifies properties within a prescribed radius from sources of releases of contaminants of concern. Where a potential risk is identified through the Tier 1 screening, additional screening is conducted under a Tier 2 evaluation, either by invasive means (i.e., testing) or by non-invasive means (i.e., review of Phase II reports or similar information).


Both ASTM E2600-22 and ASTM E1527-21 caveat that various issues are not within the scope of the guidance/practice, including health and safety issues. These non-scope issues may significantly limit the utility of the guidance/practice for an assessment of vapor intrusion risks and liabilities.


For example, under neither ASTM E2600-22 or ASTM E1527-21 is the environmental professional responsible for calling out potential health risks and/or liabilities associated with specific chemicals. Nor is such information necessarily appropriate for inclusion in an ESA. Potential purchasers, however, should be aware that the presence of trichloroethylene (TCE) in vapor intrusion presents special liabilities due to regulatory concerns about potential health impacts associated with short term exposure to TCE.


Numerous states have adopted policies to expedite vapor intrusion investigations where TCE is involved.[3] In California, the presence of TCE in indoor air above certain levels can trigger mandatory evacuations of tenants until the vapor intrusion is mitigated – and above certain levels in subsurface media, may trigger an indoor air investigation.[4]


Moreover, ASTM E2600-22 warns that “[t]his guide does not address requirements of any federal, state, or local laws with respect to vapor intrusion. Users are cautioned that federal, state, and local laws, regulations, or policy may impose vapor encroachment screening or vapor intrusion assessment obligations that are beyond the scope of this guide.”[5] This limitation has increased significance as more states begin to adopt such guidance.[6]


In February 2023, after several years of public comment and revisions, Cal-EPA finally published its Final Draft Supplemental Guidance for Screening and Evaluating Vapor Intrusion (“Guidance”). The purpose of the Guidance is to provide an approach for screening buildings for potential health risk to building occupants – and is not intended or appropriate for due diligence investigations. [7] Among other things, the costs, duration, and scope of an evaluation under the Guidance is ill-suited for due diligence practices.[8]


Nonetheless, certain features of the Guidance (and features of other applicable states’ guidance) may inform due diligence at a target property. While a non-exhaustive list, some potentially relevant features of applicable state guidance are discussed below.

For example, California’s Guidance adopts the use of U.S. EPA’s conservative attenuation factor[9] (pending the creation of a state-specific database). Attenuation factors are used to project concentrations of chemicals in indoor air on the basis of subsurface media. Thus, properties where vapor intrusion is not present may nonetheless be tagged by regulatory agencies as a potential vapor intrusion risk based on concentrations of a chemical in the soil, soil gas, or groundwater – triggering a requirement for indoor air monitoring.


Moreover, the Guidance also identifies circumstances where indoor air monitoring is recommended directly (in lieu of further investigation about the potential for vapor intrusion): (i) proximity to a contaminated groundwater plume or release area, (ii) the presence of a groundwater plume present less than 5 feet under the building, or (iii) a conduit (e.g., sewer line) that enters the building and has intersected contaminated media.


As to the inclusion of the investigation of sewer lines, the Guidance cites to a study of dry-cleaning sites in Denmark where sewer lines were responsible for 20% of the buildings with vapor intrusion.[10] According to the Guidance, conventional vapor mitigation systems will not abate vapor intrusion where sewer lines are the point of entry into the building.[11]


Whether or not a potential purchaser elects to conduct indoor air monitoring or investigate sewer lines as part of due diligence, a potential purchaser should be aware that certain scenarios will likely trigger such investigations if the target property were subject to regulatory oversight.


Another significant feature of applicable state guidance for a transaction is the applicable search distance from the target property used for screening purposes to identify potentially affected buildings. The Guidance prioritizes buildings within 100 feet of an area of estimated vadose zone soil contamination and free product extending from a source – with a 30-foot radius for non-UST petroleum releases.[12] By contrast, ASTM E2600-22 uses different terminology and measurements for screening criteria.[13] Thus, if an ASTM E2600-22 report is prepared, the environmental professional should evaluate whether these differences are significant and adapt the minimum screening distance as appropriate and necessary.

In conclusion, potential purchasers of property may want to consider seeking legal and technical advice about the scope of due diligence to adequately address vapor intrusion issues – beyond the preparation of an ASTM-compliant ESA – particularly with an eye toward applicable state guidance or policies.











[1] ASTM E1527-21, Standard Practice for Environmental Site Assessments: Phase I Environmental Site Assessment Process, December 21, 2021 (“ASTM E1527-21”)

[2] ASTM E2600-22, Standard Guide for Vapor Encroachment Screening on Property Involved in Real Estate Transactions, May 18, 2022. [3] A survey of state positions on the short-term risks of TCE is beyond the scope of this article; however, a brief (and non-exhaustive) Google search reveal several states that have adopted some policy on TCE and expedited vapor intrusion investigations, including Wisconsin, Oregon, North Carolina, Minnesota (although Minnesota guidance caveats there is no conclusive evidence of actual short-term risks), Washington, New York, Massachusetts, and Connecticut. [4] See e.g., DTSC’s "Human Health Risk Assessment [HHRA] Note Number 5" (DTSC, 2014) or the SF Bay Regional Water Board’s “Vapor Intrusion Framework” (SF Bay Regional Water Board, 2014). [5] ASTM E2600-22 at 1.1.2. [6] Again, a survey of state vapor intrusion guidance is beyond the scope of this article. A review of state guidance in a database compiled by the environmental consulting firm, Geosyntech, Inc., however, identifies several states that have adopted vapor intrusion guidance documents. https://www.geosyntec.com/vapor-intrusion-guidance [7] Guidance at 2. [8] For example, the Guidance recommends seasonal sampling, which is impractical for most real estate transactions. [9] See OSWER Technical Guidance for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air, Office of Solid Waste Emergency Response, U.S. EPA, June 2015 (“OSWER Guidance”). The OSWER guidance addresses practices for the evaluation of vapor intrusion at CERCLA and RCRA sites. [10] Guidance at 12. [11] Ibid. [12] Ibid. [13] ASTM E2600-22 at 8.1.3.

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