Understand the Emerging Issue of Vapor Intrusion

Chemical Engineering Progress, Oct 2007 by Traister, Matthew

The migration of vapors from contaminated groundwater and soil into aboveground buildings can adversely affect indoor air quality by creating an exposure pathway that may not have been addressed previously.

Vapor intrusion (VI) is an emerging concern that may impact thousands of hazardous, brownfields and commercial/industrial waste sites across the United States. While the VI pathway itself is not new, several factors - including some site-specific discoveries, revised regulatory guidance, and updated toxicity information about certain chlorinated solvents (e.g., trichloroethylene, or TCE) - have converged into a "perfect storm" of events, the sum of which threatens to challenge a well-established environmental paradigm.

This article focuses on the complex technical, legal and public relations issues that confront individuals and corporations faced with undertaking a VI investigation. It discusses the scope of the VI problem in the context of historical regulatory requirements and practices, and reviews recent developments that have influenced this emerging technical issue - and promise to continue to shape it.

The vapor intrusion pathway

The U.S. Environmental Protection Agency (EPA) has defined vapor intrusion as the migration of volatile chemicals from the subsurface into overlying buildings (1). Soil vapor, or soil gas, is the air found in the pore spaces between soil particles. Soil vapor can enter a building or structure through convective and/or diffusive means, and commonly enters via cracks or perforations in building floors and walls (Figure 1 ). These vapors migrate primarily as a result of pressure differences between the building's interior and exterior. For instance, heating, ventilation and air-conditioning (HVAC) systems can create a negative pressure within the structure, effectively drawing the vapors into the building. Thus, a waste site that has groundwater and/or soils contaminated with one or more volatile organic compounds (VOCs) may pose a potential VI threat to inhabitants of adjacent buildings and/or structures.

As part of the EPA'S strategic plan, one of the goals of the Office of Solid Waste and Emergency Response (OSWER) is to reduce or control the risk to human health and the environment at more than 374,000 contaminated sites. In order to effectively reduce or control that risk, it is necessary to determine whether specific exposure pathways exist. If the pathway does indeed exist, the site is evaluated to ascertain whether the levels of contamination pose a significant risk to human health or the environment.

Clearly, ascertaining whether a site represents a true VI concern will require extreme diligence and care on the part of the investigator. To put VI in the proper perspective, a review of the history that has brought us to this point is in order.

A look back at vapor intrusion

As the environmental movement took hold in the 1970s and stories of toxic waste sites like Love Canal made headlines, the EPA was given the task of coordinating, managing and enforcing the multitude of environmental programs that Congress would develop, including the Clean Air Act, Clean Water Act, Resource Conservation and Recovery Act (RCRA), and Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), also known as the Superfund law.

The 1980s saw a rapid rise in the identification of hazardous waste sites. As the investigation of these sites proceeded, EPA's mission of protecting human health took center stage. Toxicological studies and human health risk assessments were conducted to prioritize sites for evaluation and assess the degree to which remedial action was required.

In their simplest form, these investigations considered the various pathways by which humans could be exposed to the chemicals of concern and calculated a quantitative assessment of the incremental risk . If that risk exceeded certain thresholds (e.g., an incremental risk of one in a million), remedial action was initiated.

Consider the case of an active electronics manufacturing plant in an industrial park adjacent to a residential neighborhood. If, as a result of past industrial activity, the facility experienced a leak in an underground tank that contained TCE, it is possible that the local groundwater (which may serve as a potable water source for the town or the neighboring residential development) may have been contaminated. In such circumstances, based on the scientific community's knowledge of the fate and transport of chlorinated solvents in the environment at that time, it was frequently concluded that the greatest human health risk associated with TCE exposure was due to ingestion of contaminated groundwater. The alltoo-common solution was to place the affected residential neighborhood on public water supplies so mat this exposure pathway was broken and, therefore, posed no further risk.

Unfortunately, for several reasons, such exposure pathway evaluations have traditionally focused on ingestion or dermal contact with soil or groundwater, rather than inhalation exposures. First, health effects and lexicological data are considerably more abundant for the ingestion and dermal-contact pathways than for inhalation exposures. second, evaluating the VI pathway is invariably more complex, usually involving the use of computer models to predict the indoor air concentration associated with a particular groundwater concentration, taking into account the reduction (or attenuation) that occurs as the vapor travels through the soil. Finally, background concentrations of these VOCs, which often originate from the use of common household chemicals, can mask the extent to which VI is (or is not) occurring.