A former industrial site located near a residential area which caused a vapor intrusion issue.

Based upon feedback from the marketplace since the ASTM E 2600-08 vapor intrusion standard was published in March 2008,^[1] the standard is currently undergoing revision. There are two proposed technical revisions that will enable the area of concern (AOC) for vapor intrusion screening in real estate transactions to be significantly reduced and save considerable time.

The first technical revision is directed at differentiating in Tier 1 between what steps to take when groundwater flow direction cannot be estimated (in the Phase I investigation) and when it can. The second revision would eliminate the secondary area of concern (AOC) in Tier 1 as field experience to-date has shown it not to be necessary. Moreover, it can waste valuable time and money investigating sites a considerable distance from the target property (TP) that are highly unlikely to result in a vapor intrusion problem.

Starting with the basics

An old waste dump area located near a residential area that created a vapor intrusion issue.

These proposed revisions in the standard have the potential to significantly reduce the AOC, and thus the time and cost of investigations. The existing standard identifies a primary and secondary AOC in the Tier 1 search distance test, distinguished by whether the record search is done within the area completely around a target property (primary AOC), or whether the up-gradient direction beyond the primary AOC (secondary AOC) is included as well.

Also, the search distances are different for sites contaminated with volatile and semi-volatile chemicals of concern (COC) – such as many chlorinated solvents – versus sites contaminated with petroleum hydrocarbon COC – such as BTEX. The search distances are shorter for petroleum hydrocarbons because they are known to undergo significant bio-degradation in the presence of oxygen. The primary AOC search distances were determined based upon conservative consideration of both plume length and the distance vapors might travel along a path of least resistance from a source (such as contaminated groundwater) through the vadose zone directly to a structure on a TP. The secondary AOC was included to keep the search distances in the E 2600-08 standard consistent with those in the E 1527-05 Phase I standard.^[2]

There is sound technical basis for establishing the primary AOC. Plume length research was conducted for both volatile chemical plumes (such as chlorinated solvent plumes from dry cleaners) and volatile petroleum hydrocarbon plumes from leaking underground storage tank sites. In order to be conservative, the plume length selected to determine the primary AOC was based upon the 90th percentile distance.^[3-6] For (non-petroleum hydrocarbon) COC plumes – such as may be associated with dry cleaners – 90 percent of the time the plume length was less than approximately 1,590 feet. For volatile petroleum hydrocarbon COC plumes, 90 percent of the time it was less than approximately 390 feet.

Using experience from vapor intrusion sites in the U.S. on the distances vapors may migrate through the vadose zone, and adding them to the 90th percentile plume length, the primary AOC search radii were determined, i.e., 1/3 mile (1,760 feet) for non-petroleum hydrocarbon COC, and 1/10 mile (520 feet) for petroleum hydrocarbon COC.

Under the proposed technical revisions, if it is not possible to estimate groundwater flow direction in the Phase I investigation, then the Tier 1 screen must consider all known or suspect COC-contaminated property surrounding the TP within the search radii of the primary AOC, as defined in the existing standard. However, if it is possible to estimate groundwater flow direction (and assuming four quadrants can be distinguished around a TP: an up-gradient, two cross-gradient, and a down-gradient), then the AOC can be further reduced. This is important because each known or suspect contaminated property within the AOC may need to be investigated further, and may even require a file review at state regulatory offices (in accordance with Tier 2 of the standard). The key consideration is where the known or suspected contaminated property is located with respect to the TP.

Contaminated sources located up-gradient of the TP

For contaminated sources – such as a drycleaner with a PERC release or a gas station with a release from a leaking underground storage tank – located up-gradient of the TP, the focus would continue to be those contaminated properties within the primary AOC distances (i.e., 1,760 feet for COC sources and 520 feet for petroleum hydrocarbon COC sources).

For contaminated sources located cross-gradient of the TP

A former gas station site.

When a contaminated property (such as a drycleaner with a PERC release or a gas station with a release from a leaking underground storage tank) is located cross gradient from the TP, the length of the plume associated with the cross-gradient source is not really relevant. However, its width is relevant. According to the E 2600-08 standard, what matters for cross-gradient sources is whether the nearest edge of the contaminated plume is within the critical distance from the nearest structure on the TP. The critical distance as defined in E 2600-08 effectively is the maximum distance a vapor can reasonably be expected to migrate in relatively permeable soil, assuming the path of least resistance is directly from the nearest edge of the contaminated media (such as groundwater) to the nearest structure on the TP. The distance of concern from the structure on the TP to the property that created the contamination (e.g., a dry cleaner), would be the critical distance plus a distance to account for the plume width at that point. While the critical distance numbers are specifically identified in E 2600-08 for petroleum hydrocarbon and non-petroleum hydrocarbon contamination, nothing prescriptive is mentioned in the standard about how to deal with plume width if such information is not available. A suggested approach is described later.

For contaminant sources located down-gradient of the TP

For contaminated sites – such as a dry cleaner with a PERC release or a gas station with a release from a leaking underground storage tank – located down-gradient of the TP, plume length and width matter little and the focus would only be on the critical distance. Hence, the AOC can be reduced from 1,760 feet to 100 feet for COC contamination, except for petroleum hydrocarbon COC contamination where the AOC can be reduced from 520 feet to either 100 feet (when LNAPL or free product is presumed to be present), or 30 feet (when only dissolved petroleum hydrocarbons are presumed to be present in the groundwater).

Suggested methodology for dealing with plume width at cross-gradient sources

A former drycleaners. Such sites present high potential risk in any vapor intrusion screen.

For contaminated properties located cross-gradient from the TP, plume width must be taken into consideration in selecting an appropriate distance of concern (D_concern). The question is, what would be a reasonably conservative estimate for plume width (a default value) to use in the screening process assuming no actual plume information is available?

One approach is to base the default plume width on 1/3rd of the plume length^[7,8] and, to be conservative, use the 90th percentile plume length (PL90) discussed previously, regardless of where the contaminated property is located in the cross-gradient quadrant. Plume width, for example, would matter little if the contaminated property is located close to the boundary separating the cross-gradient quadrant from the down-gradient quadrant. On the other hand, it would be of much greater concern if the contaminated property is located close to the boundary separating the cross-gradient quadrant from the up-gradient quadrant.

Assuming symmetry of the plume on both sides of the source, one-half of the plume width (PW) would be added to the critical distance (D_critical) to establish the distance of concern. This can be expressed as:

D_concern = D_critical + 1/2 PW = D_critical + 1/2 (PL90/3)

Comparing this approach for default plume width with actual plume data^[3-6] suggests the approach is reasonable. For non-petroleum hydrocarbon COC contamination sources (such as a dry cleaner) located cross-gradient from the TP:

D_critical = 100 ft.
PL90 = 1,590 ft.

Therefore:

D_concern = 100 + 1/2 (1,590/3) = 365 feet

The default E 2600-08 search radius in the cross-gradient quadrant can then be reduced from 1,760 feet to 365 feet for non-petroleum hydrocarbon COC.

For LNAPL or free product petroleum hydrocarbon sources (such as a gas station with a release from a leaking underground storage tank) located cross gradient from the TP:

D_critical = 100 ft.
PL90 = 390 ft.

Therefore:

D_concern = 100 + 1/2 (390/3) = 165 ft.

Using this approach, the default E 2600-08 search radius in the cross-gradient quadrant could then be reduced from 520 feet to 165 feet for LNAPL petroleum hydrocarbon sources. If only dissolved petroleum hydrocarbons (in groundwater) are presumed to be present:

D_critical = 30 ft.
PL90 = 390 ft.

Therefore:

D_concern = 30 + 1/2 (390/3) = 95 ft.

Using this approach, the default E 2600-08 search radius in the cross-gradient quadrant could then be reduced from 520 feet to 95 feet for dissolved petroleum hydrocarbon sources.

Status of E 2600-08

Several changes are planned for the ASTM E 2600-08 standard that should improve its practicality, clarity and consistency. These changes were balloted in April 2009. After discussion of these changes and any further revisions, there will be another ballot in summer 2009. If all goes well, it is anticipated that the revised standard could be approved at the October 2009 ASTM meeting in Atlanta and published by the end of the year.

Furthermore, using reasonable assumptions, it may be possible to reduce significantly the default distances of concern, which could significantly reduce the investigation effort in many cases with minimal reduction to the protectiveness of the screening process. PE