US EPA Office of Solid Waste and Emergency Response Final Guidance for Assessing and Mitigating the Vapor Intrusion Pathway from Substance Sources to Indoor Air (External Review Draft)

EPA Office of Solid Waste and Emergency Response Final Guidance for Assessing and Mitigating the Vapor Intrusion Pathway from Substance Sources to Indoor Air (External Review Draft) by the U.S. Environmental Protection Agency Office of Solid Waste and Emergency Response, April 2013

Comments on the guidance will be accepted through May 24

Vapor intrusion is a potential human exposure pathway ─ a way that people may come into contact with environmental contaminants while performing their day-to-day indoor activities. Figure 2-1 summarizes the vapor intrusion pathway. The exposure route of general interest for vapor intrusion is inhalation of toxic vapors present in indoor air. ….methane and certain other volatile chemicals can also pose explosion hazards when they accumulate in confined spaces. 

The primary contamination source need not, however, be on the property of interest to pose a vapor intrusion problem. Depending on the geology and amount and form of contamination in the source zone(s), contaminated groundwater plumes can be long and narrow and can flow beneath a property located a mile or more away from the primary source. Soil gas plumes tend to extend in both lateral directions and can be larger in lateral extent relative to groundwater plumes. The primary source(s) of vapor intrusion (e.g., contaminated soil, or buried drums) may be present on a neighboring property or on a property some distance away. Even “greenspace” properties that have not previously been occupied or developed may contain contamination by vapor-forming chemicals due to migrating plumes of contaminated groundwater or migrating soil gas. Therefore, EPA recommends that the potential for vapor intrusion be considered at all properties being considered for redevelopment or proximate to industrial and commercial use areas (EPA 2008a).

At many sites, the vapor source in soil or groundwater is not in contact with the bottom of the subject building. Under these circumstances, a volatile chemical that is present in a source zone or groundwater must volatilize from the source medium and enter the pore space around and between the subsurface soil particles in the soil column above the groundwater table, which is called the unsaturated soil zone or vadose zone. If the vapor source is in the vadose zone, the vapors have the potential to migrate radially in all directions from the source via diffusion (i.e., upward toward the atmosphere, laterally outward, and downward toward the water table, which may eventually lead to groundwater contamination). Diffusion, which is caused by the random motion of molecules, affects the distribution of soil vapors when there are spatial differences in chemical concentrations in the soil gas. The net direction of diffusive transport is toward the direction of lower concentrations.

If the vapor-forming chemicals are dissolved in groundwater at the groundwater table (i.e., volatile chemicals are in the uppermost reaches of an unconfined – “water table” – aquifer), fluctuations in the water table will tend to transport the volatile chemicals upward (during periods of rising water table) or expose impacted water above the water table to soil gas (during periods of falling water table). The latter will facilitate the episodic formation of vapors in the vadose zone. Rising water tables also will bring the vapor source closer to the building(s).

Even small pressure differentials may cause advective flow of gas into or out of the building through pores, cracks, or openings in the building floor or basement walls. As a result of the construction of foundation walls and floor slabs, a perimeter crack (i.e., space between the floor slab and walls) may be created and serve as an entry location for soil vapors. This perimeter crack is often obscured by wall coverings, and may not be accessible for inspection or direct testing. Vapors have been observed to migrate through what appears to be intact concrete floors and walls, which may, in fact, have small unobserved fractures or porous areas from improper curing. In addition, conduits may be present to facilitate soil gas entry into buildings. These conduits may include utility (e.g., sewer, water, or electrical) penetrations and floor drains, which can be considered preferential (structural) pathways. Although floor drains are designed to allow water to drain away from the building, they are usually not designed or constructed to eliminate soil gas entry.

United States: EPA Draft Vapor Intrusion Guidance Is Now Subject To Public Comment by William J. Walsh, April 22, 2013, mondaq
The Environmental Protection Agency (EPA) has released and is seeking comments on draft general guidance on methods of addressing indoor vapor intrusion (VI) from contaminated soil and groundwater plumes (Draft VI Guidance) and a companion draft guidance focused particularly on petroleum hydrocarbon contaminants, such as gasoline, diesel and jet fuel The Draft VI Guidance updates the EPA draft vapor intrusion guidance issued in 2002, which never was finalized.

The risk management goal for determining that no remedial action is warranted remains a finding that the lifetime cancer risk (based on a reasonable maximum exposure condition) is 1 in 10,000 or less, and the non-cancer risk presents a hazard index of less than 1, although Draft VI Guidance states that a risk manager may decide that a risk level of less than 1 in 10,000 is unacceptable and that response is warranted without further explanation concerning how the traditional remedy selection criteria apply in this situation. Screening levels (i.e., levels that allow for early determination of a remedy) are set at a 1 in 1 million risk level. EPA reserves the right to consider taking action based on short-term and acute exposures.

Similarly, a preemptive approach may be taken because it is more cost-effective to design, install, operate and monitor engineered exposure controls to mitigate vapor intrusion in newly constructed buildings, or in buildings to be constructed in the future, that are in areas of vapor-forming subsurface contamination, rather than potentially allow vapor intrusion to occur later and assess it after the fact. … The Draft VI Guidance contains extensive direction concerning obtaining public comment and implementing institutional controls at individual sites. The Draft recommends that regulators be notified of significant changes in building ownership, uses, access by the general public, or building construction (e.g., renovations), which may affect its risk management decisions pertaining to potential vapor intrusion assessment and mitigation, subsurface remediation, or institutional controls. …
Comments on the guidance will be accepted through May 24.

[Refer also to:

The Assessment Review Board Reduces Methane Contaminated Property’s Taxation Value to Zero by John M. Bulman, Weirfoulds LLP, March 2, 2012
An insurance broker the property owner consulted could not obtain insurance on the house and property because of the methane. Unable to bear the costs of bringing the methane control system up to standard, the owners consulted a broker about selling the property, only to find that their real estate broker would not list the house for sale. In refusing the listing, the broker said that no one would be interested in buying the property because they would not be able to either insure or mortgage it because of the methane levels….The methane problem, it ruled, is more than a mere nuisance, posing a real hazard. ]

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