Vapor intrusion into structures on a property can potentially create significant liability and have a material impact on property value. As such, it is a growing concern for property owners, prospective purchasers of property and the environmental professional conducting a due diligence investigation.

To respond to this industry concern, ASTM recently approved ASTM E 2600, Standard Practice for Assessment of Vapor Intrusion into Structures on Property Involved in Real Estate Transactions. The standard describes a process consisting of four tiers to assess if a vapor intrusion condition (VIC) exists, and what the alternatives for mitigation might be. The VIC definition and other key terms are defined in Table 1. The first two tiers are screening tiers designed to quickly and inexpensively identify a potential vapor intrusion condition (pVIC). If the potential for vapor intrusion cannot reasonably be screened out at the Tier 1 and/or Tier 2 levels, and assuming the transaction is going to proceed, the process identifies three options: (1) proceed with a more site-specific and comprehensive investigation (Tier 3) in the hope that this investigation will eliminate vapor intrusion concerns; (2) proceed directly to mitigation (Tier 4) on the assumption that preemptively conducted mitigation may be more cost-effective to address a pVIC; or (3) gain more certainty on the presence of a pVIC through additional investigation.

A generalized flowchart that interprets the screening process included in Tiers 1 and 2 of the standard is provided in Figure 1. Prior to initiating the screening evaluation, it is assumed that (1) the structures on the property being investigated (the target property) were not designed to be intrinsically safe from vapor intrusion; and (2) there are no significant preferential pathways, natural or man-made, which can accelerate the movement of vapors into structures on the target property from contaminated soil and/or groundwater either on the target property or nearby.

Vapor intrusion screening under tier 1

Enlarge this picture Figure 1: A generalized flowchart that interprets the screening process included in Tiers 1 and 2 of the ASTM vapor intrusion standard. ©Copyright 2008 The Buonicore Group, with permission.

The information required in Tier 1 is the same information collected as part of an ASTM E 1527 Phase I investigation. The first test in the screening process is a search distance test to identify any known or suspected contaminated sites in the area of concern (AOC). If there are none, no further action is required.

The standard identifies a primary and secondary AOC in the search distance test, distinguished by whether the radius search is completely around a target property (primary AOC) or limited solely to the up-gradient direction (secondary AOC). Also, the search distances are different for sites contaminated with non-petroleum hydrocarbon chemicals of concern (COC), versus sites contaminated with petroleum hydrocarbon COC. The search distances are shorter for petroleum hydrocarbons because they are known to undergo bio-degradation in the vadose zone in the presence of oxygen.

To identify the AOC for contaminated sites with non-petroleum hydrocarbon concerns, the search distance is 1/3 mile around a target property, and beyond that a source of contamination is only of concern if it is up-gradient of the target property (up to the applicable search distances identified in the ASTM E 1527 Phase I standard). For sites with petroleum hydrocarbon concerns, the search distance is 1/10 mile around a target property, and beyond that a source of petroleum hydrocarbon contamination is only of concern if it is up-gradient of the target property (up to the applicable search distances identified in the ASTM E 1527 Phase I standard). Hence, for example, a gas station site with a release would be in the primary AOC if located anywhere within 1/10 mile of the target property. It would be in the secondary AOC if located up-gradient of the target property between 1/10 mile and 1/2 mile (the applicable ASTM E 1527-specified search distance).

If a known or suspect contaminated site is located in the AOC, then typically the next check would be to determine if there is a hydraulic or physical barrier between the potentially impacted structure on the property and the source of contamination. For example, if there is a river between the source of contamination and the structure, the river will likely act as a hydraulic barrier to any migrating contaminant vapors. Hence, this suspect source of contamination in the AOC may be eliminated from concern.

If there are no hydraulic or physical barriers, the next check would be to determine if the known or suspect contaminated site in the AOC may have released COC. If not, the site may be eliminated from concern.

If COC are likely to be present at a contaminated site in the AOC (e.g., perchloroethylene from a former dry cleaner), then the location of this site, relative to the target property, becomes important. If the source of contamination is located up-gradient of the property, there is greater concern than if it is located down-gradient or cross-gradient.

To understand why, it is necessary to understand the definition of the critical distance in the standard. Effectively, the critical distance is the distance that contaminant vapors can potentially migrate in any direction through the vadose zone along a path of least resistance. For example, it is the distance in the vadose zone that volatile vapors from a contaminated groundwater plume can migrate to reach and intrude into a structure on the target property.

The standard conservatively identifies the critical distance and distinguishes between non-petroleum hydrocarbons and petroleum hydrocarbons. If the distance is less than 100 feet (except for dissolved petroleum hydrocarbon COC in which case the critical distance is shorter: less than 30 feet), then it is presumed that migrating contaminant vapors can potentially impact a structure on the target property. If the critical distance is greater than or equal to 100 feet (or 30 feet for dissolved petroleum hydrocarbon COC), then it is presumed unlikely that migrating contaminant vapors can impact a structure on the target property. The critical distance for petroleum hydrocarbon COC in free product form (above the water table) is the same as for non-petroleum hydrocarbons, i.e., 100 feet.

If a source of contamination is located down-gradient from a target property, it can be eliminated from concern if it is beyond the critical distance. If the source of contamination is cross-gradient, it can be eliminated if it is beyond the critical distance plus a so-called “buffer” to account for the width of the plume at the edge nearest the target property or contamination puddling above the water table (a condition that may, for example, be associated with a gasoline spill). If the source of contamination is up-gradient of the target property or on it, then it can not be eliminated at this step.

If the source of contamination has not been eliminated from concern, then the environmental professional may be able to eliminate the source by using experience with local subsurface geology and soil characteristics. For example, if the soil is clay or otherwise has a very low permeability, this might eliminate the source from concern, depending on its location.

The Tier 1 assessment concludes either that vapor intrusion is not a likely concern at the target property or that a pVIC exists. If a pVIC exists, further investigation may proceed to Tier 2, preemptively proceed to mitigation as described in Tier 4, or take some other course of action (e.g., withdraw from the deal).

Vapor intrusion screening under Tier 2

More refined screening takes place in Tier 2. If data are available in state regulatory files or elsewhere that can provide insight into the extent of contamination associated with the source (e.g., plume delineation, COC concentrations, etc.) and the status of any remediation efforts, then the screening methodology continues by assessing whether the nearest plume edge is beyond the critical distance. If so, the source may be eliminated from concern. If not, then the concentrations are compared to existing generic state risk-based concentrations (RBCs) that have typically been established for soil, soil gas and groundwater. If concentrations are below the state RBCs, then the source may be eliminated from concern. If above, then the environmental professional may determine RBCs on a site-specific basis, e.g., using the ASTM E 1739 standard. If the concentrations are below the site-specific RBCs, the source may be eliminated from concern.

If there is no plume information associated with the source of contamination (or if the site-specific RBCs have been exceeded), then it may be appropriate to evaluate whether or not sampling (e.g., soil, soil gas and/or groundwater at or near the target property) is a viable option. Sampling should not be pursued unless it is believed that useful information can be collected. If sampling is not pursued, then a pVIC is presumed to exist. If sampling is conducted and the results indicate that vapor intrusion is unlikely, then the source of contamination may be eliminated from concern.

In summary, a source of contamination may be eliminated in the Tier 1 and Tier 2 screening methodology if:

• The source is not in the AOC
• There is a hydraulic/physical barrier between the source and the target property;
• The presence of COC is unlikely;
• The contaminant source is down-gradient from the target property and beyond the critical distance;
• The contaminant source is cross-gradient from the target property and beyond the critical distance plus a buffer to account for plume width/puddling;
• Subsurface geology and soil characteristics are such that vapor intrusion is unlikely;
• The nearest plume edge to the structure on the target property is beyond the critical distance;
• COC concentrations are below state generic RBCs;
• COC concentrations are below site-specific RBCs; or
• Sampling results indicate that vapor intrusion is unlikely to be a concern.

If a pVIC remains after the Tier 2 screening, the environmental professional and client may choose to proceed to a Tier 3 investigation, proceed directly to pre-emptive mitigation in Tier 4, or take some other action, such as further investigation to reduce the uncertainty.

Implications for environmental professionals

Environmental professionals today have two choices on how to deal with vapor intrusion in a Phase I: either make sure the client is aware that vapor intrusion, as an indoor air quality issue, is a non-scope consideration in the ASTM E 1527 Phase I; or at the client’s request, include it as a supplement to the Phase I, much the same as asbestos might be added to the scope of services. If it is to be included, the ASTM E 2600 vapor intrusion standard provides a methodology that can be followed to screen for a pVIC and, if one exists, determine the most appropriate course of action. PE

Figure 1: ASTM E 2600 Key Terminology

1. Vapor Intrusion Condition (VIC), defined as “the presence or likely presence of any chemical of concern in the indoor air environment of existing or planned structures on a property caused by the release of vapor from contaminated soil or groundwater either on the property or within close proximity to the property, at a concentration that presents or may present an unacceptable health risk to occupants.” The standard only deals with indoor air emissions emanating from contaminated soil or groundwater.
2. Potential Vapor Intrusion Condition (pVIC), defined by the standard when screening indicates the possibility of a VIC, but where there is insufficient data to ascertain the presence or likely presence of any chemical of concern in the indoor air environment.
3. Chemical of Concern (COC), defined as a chemical in the subsurface environment that is known or reasonably expected to be present, that can potentially migrate as a vapor into an existing or planned structure on a property, and that is generally recognized as having the potential for an adverse impact on human health. COC meet specific criteria for volatility and toxicity, and include volatile organic compounds, semi-volatile organic compounds, and volatile inorganic analytes such as mercury.
4. Critical Distance (CD), defined as the lineal distance in any direction from the nearest edge of a contaminated plume to the nearest structure on the target property, or to the nearest property boundary if there are no structures on the target property.