Issue Date: April 18, 2011
EPA Moves On Vapor Intrusion
The Environmental Protection Agency is going to add vapor intrusion to its list of criteria for deciding if a contaminated area should be put on Superfund’s National Priorities List. This will be the first major change to the Superfund hazard ranking system since the law was passed in 1980. Vapor intrusion is the process by which volatile chemicals move from a subsurface source, such as contaminated soil or groundwater, into the indoor air of overlying homes and buildings. Its inclusion as a hazard criterion may not have a big impact on the testing and remediation of existing buildings, but it may cause old, closed Superfund sites to be reevaluated. It also has the potential to change the standards for new construction.
In addition, the agency is finalizing its nearly 10-year-old proposed guidance document on evaluating vapor intrusion in buildings. EPA will use the guidance to determine if and when human exposure from this pathway poses an unacceptable health risk. Issued as a draft document in November 2002, the final guidance document is not a part of the Superfund hazard ranking system, but the work on the two pieces would have to be coordinated, according to the agency.
Vapor intrusion occurs when heating, ventilation, and air-conditioning create negative pressure within a home or building that draws volatile compounds inside. If the compounds are toxic, as might be the case for contamination from a nearby Superfund site, there is the threat of a health risk.
The compounds of greatest health concern from groundwater are polychlorinated hydrocarbons. Trichloroethylene and tetrachloroethylene are the most common chemicals found in vapor intrusion assessments. “The highly chlorinated compounds are found because they are more resistant to breakdown by biological processes,” says Henry J. Schuver, an environmental scientist studying vapor intrusion at EPA’s Office of Resource Conservation & Recovery. Other compounds viewed as potential health problems are dichloroethane, benzene, toluene, ethylbenzene, and xylenes.
Observers compare chemical vapor intrusion with the problem of radon gas leaking into buildings. But even though the mechanisms are similar, radon is easier to measure and remove because its underground source is consistent and predictable.
State and federal regulators have been aware of the existence of vapor intrusion for many years, but only since the early 1990s has it been recognized as a potential health problem. Greatly increased public awareness of vapor intrusion over the past few years has driven many states to establish their own standards for remediation, a situation complicated by the fact that researchers are still trying to figure out the best way to measure vapor intrusion.
“Prior to the 1990s, we knew that landfill gas, gasoline, and things like that could cause problems, but we didn’t realize that relatively low concentrations of volatile compounds would cause an indoor problem,” says David J. Folkes, president of EnviroGroup Ltd., a Colorado engineering firm that has been working on this issue since that time. A well-publicized 1993 case of vapor intrusion at the Colorado Department of Transportation Materials Testing Laboratory site in Denver was one of the first to bring the problem to national attention.
EPA published its first vapor intrusion guidelines under the Resource Conservation & Recovery Act hazardous waste regulations in 2001 and followed that with proposed guidelines under the Superfund law in 2002. It wasn’t until last year, after a Government Accountability Office report criticized the agency’s lack of action on vapor intrusion, that EPA picked up the pace in making vapor intrusion part of the Superfund evaluation process.
Currently, four pathways to chemical exposure are considered under the hazard ranking system that is used to put a site on the National Priorities List: groundwater migration, soil exposure, surface water migration, and air migration. Vapor intrusion would be a fifth pathway to be factored into the ranking system. When screening a site, EPA assigns a score for each pathway and combines the results using a root-mean-square equation to determine an overall site score. A site becomes eligible for the priorities list when the total score hits 28.5.
Folkes notes that although the science of predicting and evaluating vapor intrusion will be better, the scope of the problem will not change. “Incorporating vapor intrusion into the hazard ranking system, however, will very likely increase the number of Superfund sites on the National Priorities List,” he says.
People involved in the actions at the state level agree that the change is overdue. “There are Superfund cases out there that are being driven primarily by vapor intrusion,” says John E. Boyer, an environmental scientist with the New Jersey Department of Environmental Protection. “Including it in the National Priorities List ranking system is the next logical step.”
Many states have already surpassed EPA in regulating vapor intrusion at contaminated sites. More than half of the states, including California, Colorado, Massachusetts, New Jersey, and New York, for example, have their own regulations and guidance for this problem. The rest use EPA’s draft guidelines or modifications of them.
Technical guidance is also available from the Interstate Technology & Regulatory Council, a coalition of state environmental regulators, various industry representatives, and other stakeholders. The ITRC guidance on vapor intrusion, published in 2007, provides information on assessment and mitigation. A voluntary standard released in 2008 by ASTM International addresses assessing the risks from vapor intrusion at contaminated sites.
But these early guidelines are becoming out of date as more is learned about the way underground vapors behave. For example, as state regulators do more assessments of vapor intrusion situations, they are finding that chemical vapors move in ways that are not as predictable as the early science seemed to indicate. The vapors seem to shift in ways that are not related to contaminated soil locations and groundwater movement, observers point out.
The dilemma is over just how much regulation is needed when measuring and assessing the contamination from vapor intrusion is still so difficult. “It’s not simply a question of going into a building and measuring the indoor air, because what you measure could be from anything,” says Paul C. Johnson, a chemical engineer who pioneered this field. “Somehow you have to figure out a way to confidently link what you see in the indoor air with something in the subsurface.”
Johnson, dean of the Ira A. Fulton Schools of Engineering at Arizona State University, devised with his colleague Robert A. Ettinger in 1991 the first model to define the relationship between chemicals contaminating groundwater and soil and the concentration of those chemicals that would migrate inside a home or building. The Johnson-Ettinger model is still used as a screening tool for vapor intrusion. “Many states have it as part of their guidance for how to assess exposure pathways,” Johnson says.
Accurately assessing the risks from vapor intrusion is complicated. Low levels of contaminants from soil and groundwater contamination do not behave in an easy-to-predict manner. And the concentration of volatile chemicals already in buildings may be a bigger source of contamination than the volatile chemicals coming from the contaminated soil and groundwater.
Johnson is still researching the behavior of chemical vapor contaminants in buildings. “We are discovering in our latest studies that you get very temporal behavior from these compounds,” Johnson says. “Your home might not have any vapor intrusion today, but it might have some tomorrow.”
EPA’s Schuver agrees: “Vapors are less easily controlled, compared to, say, water contamination. They are subject to movement changes by very small pressures.” He adds that the permeability of houses and buildings to chemical vapors remains largely unexplored.
“Background chemicals are another issue,” Schuver adds. Chemicals are always going to be found indoors, and these can be the same as the contaminants from underground vapors, complicating the assessment of vapor intrusion. Many household products, building materials, or furnishings can confound the effort to measure vapor intrusion.
“You may live in an area where the air is pretty clean, but you’re still going to have indoor air contamination,” Johnson adds. “People may have some kind of glue, or cleaning product, or something in the garage that will impact air quality at or above the same level as a subsurface source. It makes the assessment very difficult.”
Because of the obstacle of measuring vapor intrusion after the fact and the complicated guidance required to assess the problem, Folkes and others think many states will just require subsurface ventilation systems to be installed in buildings being constructed near contaminated plumes, without waiting for evidence of possible risk.
Schuver concurs that there is wisdom in that. It’s impossible to predict what the vapor intrusion might be for a building that isn’t there, he says. “The building itself, or the absence of it, changes the distribution of gases below the surface,” he says. Also, it is much cheaper to install a ventilation system as part of construction. Estimates for retrofitting ventilation systems in existing buildings vary widely.
Despite the many complex scientific questions and ongoing research efforts to address them, EPA has announced that it will issue its final guidance on assessing vapor intrusion by Nov. 30, 2012. The agency is in the midst of a series of public information gathering sessions that conclude this month on adding vapor intrusion to the Superfund hazard ranking system, but no date has been set for a final decision.
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