Vapor Intrusion
Regulatory Problems and Pitfalls

Rod B. Thompson, SESCO Group

Science and Regulation

The regulatory guidance and science understanding of vapor intrusion has been in a rapid state of change for many years now. The frequency and nature of these changes complicate decisions regarding investigation, liability claims and the need for mitigation. Because common environmental constituents like petroleum and chlorinated compounds are so volatile, vapor intrusion has become the driver of many risk-based cleanups. While it is important to assess the health risk from any potential exposure, the difficulties associated with definitive conclusions regarding exposure levels and the need for mitigation is complex and often misleading.

What is Vapor Intrusion?

Vapor intrusion is the process by which compounds that are released to the soil or groundwater volatilize and migrate through the environment in sufficient concentration to enter an occupied structure. When vapor levels inside a structure are significant (e.g. an order of magnitude above regulatory limits) mitigation decisions are straightforward. However, the measured levels inside structures are often much lower and the question of the source of the chemical is difficult to resolve and frequently misleading. There are numerous sources for indoor air concentrations of common contaminants, including:

• Ambient air
• Consumer products
• Lifestyle habits
• Attached garages
• Building construction

Contributions of volatile chemicals from sources other than subsurface vapor intrusion are considered to be “background” and are termed Indoor Air Background (IAB). When interpreting indoor air measurements, it is appropriate to consider the contributions of background sources to the indoor air concentrations. Often what is measured, or a significant portion of what is measured, in the indoor air inside the structure is IAB. It is easy to misinterpret the source of measured concentrations inside the structure. Misinterpretation often results in unnecessary litigation and regulatory mandates to mitigate.

Indoor Air Background

IAB is present in all structures. No single number or source can be used to represent common background values within structures. Personal lifestyle and consumer product choices, construction design, building materials, etc., all contribute to varying levels of indoor air background (ITRC, 2007). For this reason IAB values are most commonly listed in ranges, using the results of large sample sets (studies) to present probabilities or percentiles indicating the frequency with which a certain background value can be expected to occur inside the structure. There have been numerous compilations of IAB ranges and values, and guidance on how to use these values. Most notably are the United States Environmental Protection Agency (USEPA) (2002; 2008), Dawson and McAlary (2009), New York (2006), and New Jersey (2005). All of these entities request that background should be considered whenever indoor air sampling results fall within these ranges.

Determining whether or not the source is from the subsurface vapor intrusion or IAB requires a number of investigation considerations relative to the vapor migration “process.”

Vapor Migration Process

Vapor migration and intrusion is a process. To understand vapor intrusion, it is critical to understand the entire process from the source origin through environmental transport, migration and successive attenuation, all the way to the exposure point inside a structure. Understanding requires both empirical data and thorough science knowledge of vapor migration. This understanding should provide the basis for a written, descriptive Conceptual Site Model (CSM). While most investigators purport to have a CSM, few produce a written document. A written CSM is significantly clearer and more useful than any claim of “conceptual understanding.” An excellent discussion of the need for and guidance on constructing, a CSM is provided by the Interstate Technical Resource Council (ITRC, 2007).

What empirical data are needed? Data relative to the strength and composition of the originating source, dissolved phase groundwater transport, soil structure relative to vapor transport, subsurface soil gas concentrations and the degree of attenuation in each discreet step are all essential. Indoor air data are also needed. There is little value in obtaining indoor air data without subsurface data. Definitive conclusions regarding the source of exposure levels inside the structure are difficult and require numerous sources of data in order to make reliable conclusions regarding vapor intrusion exposure levels.

What Data is Necessary to Conclude Mitigation is Necessary?

Recently the USEPA (2008) has determined that many regulatory health protective levels fall within common indoor air background ranges. For instance, the EPA recently defined indoor air background ranges for tetrachloroethylene (PCE) at the 50th and 95th percentiles as 0.7-7.5 ug/m3 (EPA, 2008a). Most current regulatory levels fall within this range. In Indiana 3.2 ug/m3 is the current acceptable chronic exposure level in residential structures with children (IDEM, 2006). Important information that is useful in understanding the source of indoor air measurements includes, but is not limited to:

• Indoor air data taken under conditions conducive to vapor transport
• Ambient or outside air concentrations
• Subsurface data representative of vapor concentrations and ratios or contaminants
• Source strengths in groundwater or soil
• Comprehensive understanding of migration pathway
• Comprehensive understanding of attenuation
• Comprehensive understanding of exposure levels

The dilemma of deciding if the source is from the subsurface or from IAB is solved by deciding a “weight of evidence” from all of this data. There is significant and often unreliable variability in much of this data (EPA, 2008). It is important to understand the uncertainty and what conclusions can be reliably drawn from each piece of data. For instance, most regulatory agencies are reluctant to accept soil gas measurements (New York, New Jersey). Without question, the only way to assess a completed vapor pathway when indoor air values fall within the IAB is with a thorough and complete weight of the evidence evaluation consistent with the conceptual site model.

How to measure and weigh the evidence is critical and SESCO Group will supply more information the next few months as these complicating issues are more fully explored.