Vapor Intrusion Migration

Vapor intrusion is a relatively new concern among environmental groups. Little is currently established regarding the topic; however, as human health is affected, public awareness of the topic is growing quickly and, as a result, new standards pertaining to vapor intrusion are constantly emerging.

Vapor intrusion is the vapor phase migration of subsurface volatile chemicals into the overlying structures. These chemicals may be a result of wastes buried beneath a property or as a result of local groundwater contamination. Several factors are considered in evaluating the risk of vapor intrusion, including chemical properties and toxicity values, soil parameters, building parameters, and exposure time. Vapors may accumulate to levels that may cause health and safety hazards and risks.

In recent years, several documents have been prepared to address and evaluate the issue. In 1998, the United States Environmental Protection Agency (USEPA) compiled a series of models for estimating the health risks associated with vapor intrusion and indoor air quality. Most noted of these models is the Johnson & Ettinger (J&E) model, which was first developed in 1991 and has since been modified to account for different assumptions. The model takes into account soil characteristics, exposure time, exposure frequency, air exchange, chemical toxicity and properties, building parameters, and physical settings. Under this model, one should be able to determine if the indoor air concentration are at a level that may potentially pose an unacceptable inhalation risk by comparison to cancer risk and non-cancer hazard indices.

In 2002, the USEPA issued the OSWER Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwaters and Soils (Subsurface Vapor Intrusion). The purpose of the 2002 USEPA Subsurface Vapor Intrusion Guide is merely to provide a tool to evaluate the risk of vapor intrusion and is not intended to act as a regulation. The document recommends a tiered approach to evaluating the risk of vapor intrusion in residential settings; however, the guide may be adjusted for commercial/industrial or recreational uses. The guide provides both generic and more site-specific screening levels for soil gas and groundwater. While several states currently utilize this guide as a means to evaluate vapor intrusion risks, many states have opted to implement their own vapor intrusion guide.

For the state of California, two guidance documents have been published: the Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air (DTSC/CalEPA, 2004) and the Use of California Human Health Screening Levels (CHHSLs) in Evaluation of Contaminated Properties (CalEPA, 2005). It should be noted that these documents do not present regulatory cleanup standards. The regulatory agencies have final ruling over a contaminated site.

The latest attempt to provide a standard for assessing the risk of vapor intrusion to indoor air is the American Society for Testing and Materials (ASTM) standard practice document ASTM E 2600-08 (ASTM International, 2008), which was published in 2008. The practice is not required under ASTM E1527 for Phase I Environmental Site Assessments, but may be used as a voluntary supplement to the practice. This practice seeks to determine whether a vapor intrusion condition (VIC) exists on a property the a process consisting of four tiers so that low-risk properties can be screened out quickly (Tiers 1 & 2), and once a potential VIC has been determined for a property, confirmation (Tier 3) and mitigation (Tier 4) is to follow. A VIC is defined as “the presence or likely presence of any chemicals of concern (COC) in the indoor air environment of existing or planned structures on a property caused by the release of vapor from contaminated soil and groundwater either on a property or within close proximity to the property, at a concentration that presents an unacceptable health risk to occupants.”

As this is a relatively new concern, there is much progress to be made. The emergence of these guidelines and standards demonstrate that we are taking a step in the right direction. Public awareness is a great driving force for establishing standards as the regulatory agencies are met with immense political pressure to do so. In time, we can expect vapor intrusion standards to be much more omnipresent.

For more information call me at 310-615-4500 or email me at tmen@partneresi.com.