December 7, 2017
by Wesley Robb
Vapor intrusion occurs when there is a migration of chemicals containing vapor from a subsurface source such as soil, groundwater, or soil vapor into an overlying building (Figure 1). As a result of past activities, some areas are more prone to the risk of vapor intrusion for its surrounding structures. For example, erstwhile sites of dry cleaners or filling stations could possibly place an area at risk as toxic chemicals like petroleum hydrocarbons and chlorinated solvents were used in these locations. (For more information on vapor intrusion, visit www.constructionspecifier.com/the-growing-concern-with-vapor-intrusion.)
If poisonous vapor chemicals are present in the soil, groundwater, and/or soil vapor, they may migrate into a nearby structure. This may adversely affect a building’s indoor air quality (IAQ), leading to potential health problems for building occupants. (Chemicals typically encountered in a vapor intrusion scenario can have varied health effects—from lung cancer, decreased kidney function, damage, and cancer, heart failure, and ischemia [decreased oxygen supply] to increased sensitivity to allergens, immune system impairment or failure, auto-immune effects, skin irritation, rash, discoloration, dermatitis, liver damage or tumors, steatosis, and death of liver cells.) Addressing vapor intrusion sites is an outgrowth of years of legal precedence, which flows from the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), or ‘Superfund,’ and the Resource Conservation and Recovery Act (RCRA). These laws developed the framework for managing and remediating hazardous waste sites, and form the framework for legal precedence in vapor intrusion litigation.
There have been documented cases of lawsuits brought upon building owners, where health problems caused by vapor intrusion were cited as the reason. In 2010, a summary judgment was granted against property owners who, as the federal district court in Nevada determined, knowingly developed housing over a groundwater plume contaminated by dry-cleaning chemicals. (In the Voggenthaller v. Maryland Square case, the federal district court in Nevada cited homeowners were endangered by the contaminated groundwater and chemical vapors emanating from the groundwater plume.)
As more studies are conducted by U.S. Environmental Protection Agency (EPA) and similar agencies, awareness is growing about the threat of vapor intrusion. EPA has paid particularly close attention to trichloroethylene (TCE) because findings have revealed this industrial solvent poses potential human health hazards, including cancer. (Of notable concern is the possibility fetal heart defects may result if pregnant women are exposed to TCE in their earliest stages of gestation.)
More architects are specifying chemical vapor barriers when designing a new structure in locations where vapor intrusion may be a concern. On receiving these specifications, contractors are able to hire certified installers to put in a chemical vapor barrier system during construction even before the foundation is laid. This practice helps mitigate the risk vapor intrusion poses.
Chemical vapor barriers are not to be confused with standard moisture barriers (often simply referred to as ‘vapor barriers’). The latter is designed to keep moisture out of a building in an attempt to reduce the potential for mold, which can also be a health risk, and water damage. Chemical vapor barriers, on the other hand, are designed to not degrade or allow penetration of vapor-phase chemicals. The fact moisture barriers are often described as ‘vapor barriers’ in the industry can lead to confusion.
Generally, there are two approaches to the construction of a vapor mitigation system. One option consists of the installation of a polyethylene or polyolefin tarp product. Another, more advanced construction product, uses a spray-applied composite membrane in addition to specified tarp materials to provide the spray material a substrate on which to adhere. The polyethylene/polyolefin barrier materials are often recycled from post-consumer products, making them less expensive. However, in the absence of a spray-applied composite membrane, these barriers are often difficult to seal around penetrating uprights and slab perimeters.
Such mitigation systems work well for new structures, but what if an unprotected building is already standing in an area where vapor intrusion is a potential risk? When a foundation is already laid, is there any way to retrofit a building so a chemical vapor barrier can be applied after it has been constructed? The short answer is ‘yes.’
According to EPA, recognition of soil vapor intrusion to buildings and other enclosed spaces only occurred in the 1980s, sparked by concerns over radon intrusion. (Visit www.epa.gov/vaporintrusion/what-vapor-intrusion for more information on vapor intrusion.) Hence, buildings constructed before 1980 and for some time after had no specifications to reduce risk of vapor intrusion.
With the passage of time, awareness of vapor intrusion from manmade chemicals contaminating soil, groundwater, and soil vapor has grown. Products specifically designed to retrofit an existing structure in order to mitigate vapor intrusion risk are available. These sealant materials are applied on top of the slab as opposed to a barrier installed below. They are distinguished from the aforementioned spray-applied coatings because they are rolled onto the sub-slab similarly to concrete paint or sealer.
Depending on the layout, these materials can be applied to both the basement floor and foundation walls. Many industrial building owners find this method attractive as it can act as a chemical vapor barrier as well as a finished floor surface.
So what steps should a building owner or facility manager take if their particular structure is, in fact, at risk for vapor intrusion?
Factors to consider
To develop an effective and efficient vapor mitigation plan, a few factors need to be taken into account. The first is location—where is the structure in relation to the contaminated vapor? The vapor plume must be characterized to take effective action—are there pressure differentials drawing contaminants from
the soil gas into the structure? Are there noticeable pathways in the form of cracks or utility penetrations allowing more access? Understanding the answers to these questions is imperative to assembling a good vapor mitigation strategy.
Before establishing a vapor mitigation strategy, a qualified professional should be contracted to assess the potential for vapor intrusion and sample as needed. It should be noted assessment of IAQ can be complicated by products (e.g. nail polish, paint, lubricants, inks, and dyes) routinely used inside the building. Understanding the types of volatile chemicals present will better assist the implementation of a vapor mitigation system.
HVAC systems on many buildings can potentially result in negative pressure on a building’s interior. When these negative pressures are present inside of a building, it is more likely at risk for vapor intrusion. This negative air pressure can draw soil vapor through the semi-permeable building slab, as well as through cracks and unsealed uprights into the breathable space.
Temperature differentials should also be factored into the equation. When there is a substantial temperature gradient between the insides of a structure and the subsurface below, the resulting negative air pressure can bring vapors into a building’s interior from below via the stack effect. Finally, wind blowing over a structure, along with stack effect, can create the negative pressure gradient necessary to permit vapor intrusion.
Another factor is the number of openings present in the slab concrete of a given structure. Openings such as cracks, open seams, and utility penetrations can be transmission conduits through which chemical vapors in the subsurface soil gas can pass. These vapors are much more likely to find their way into a building when their conduits are left open after installation of a vapor barrier mitigation system.
Inspection and plan
An experienced installer and personnel familiar with the building must be tasked with identifying and detailing aspects of the structure’s characteristics affecting its risk for vapor intrusion. Is there something unique about the building’s sub-slab? In some situations, elements may only get discovered when reviewing the building’s blueprints. It is imperative to know these characteristics to properly design an effective mitigation system.
Various parts of the building could have been built at different times, and therefore, have different physical characteristics as a result of this time lapse. For these reasons, finding an individual with first-hand knowledge of the structure and perhaps insight into its construction can be very valuable. Building additions often have very different soil than the rest of the structure. Over time, soil is compacted, and as building additions are made at a later time, the soil’s integrity can be drastically different and must be accounted for when designing a system. To draw up an effective plan, layouts of the building’s suction points, pipes and blowers, electrical work, and mechanical details must be included. This also allows for a more accurate cost estimate and scope of work.
Installing the system
The first step should be to seal all openings in the foundation floor and walls through which vapor can easily make its way up. By sealing off these potential pathways, the bulk of the vapor mitigation system does a more effective job. This practice cannot be thought of as sufficient, but it is very helpful in improving the system’s overall performance.
The next step is to install the mitigation system itself. A couple of options are available, but the most commonly employed one is sub-slab depressurization (SSD) system. It is most frequently used as a result of a few competitive factors. SSDs are generally regarded as the most robust solution to mitigate the risk of vapor intrusion. They are designed to control the upward migration of chemical vapors from the soil by creating a negative pressure beneath the sub-slab, as well as venting and piping away the chemical vapors from the interior breathing zone.
SSD systems have been widely employed to mitigate vapor intrusion for existing buildings. Some of the drawbacks to this system are they can be difficult to design and consume electricity. The systems do have to be switched on, and because of their mechanical nature, require a level of maintenance. Although the negative pressure created below the slab can greatly reduce the vapor intrusion risk, it does not limit it fully as adsorption and penetration of these chemicals can still occur through the concrete slab and openings.
SSDs are often specified because they are well-known and similar to the radon mitigation assemblies that have been installed for many years. They also have the advantage of being active systems which, through the use of fans, pumps vapors accumulating below a building’s slab out from under the building and away into the open air. This means they may be particularly suitable for heavily contaminated sites.
An alternative system is a retrofit coating available for the purpose of blocking vapor intrusion in existing structures. This material can be applied directly to the concrete slab, basement walls, and around penetrating uprights. This is frequently very cost-effective when compared to SSD systems as it does not require the deconstruction and replacement of the basement slab. However, some retro-coatings are non-loadbearing so it is important to select the proper type for the specific traffic load.
Non-loadbearing coatings may provide a perfect solution for areas such as crawlspaces or when a protective concrete application is placed on top. Both loadbearing and non-loadbearing types have the advantage of low maintenance over SSD systems. Unlike the SSD system, the product is always ‘on,’ requires no electricity, and has no ventilation fan needing upkeep or replacement.
In order to ensure maximum protection from vapor intrusion, some projects may require both an SSD system and a retrofit coating.
As more scientific data regarding vapor intrusion comes out, and building owners become aware of the health risk to their occupants and the potential long-term liability associated with vapor intrusion in their buildings, interest in mitigations systems for existing buildings will only increase. This is especially true in high-risk locations, such as urban areas, densely developed for a century or more.
Wesley Robb is the director of technical strategies and applications at Vapor Mitigation Strategies, a Wellington Environmental Company. He has 23 years of environmental field and laboratory experience, including several years of soil vapor sampling and analyses. Since joining Wellington in 2004, Robb has managed onsite activities of various kinds, including underground storage tank removals, soil remediation, Phase I investigations, and vapor/soil/groundwater sampling. He can be reached at firstname.lastname@example.org.
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