Vapor Intrusion Guidance

The following text was derived from the testimony of Carl Johnson, Deputy Commissioner for Air and Waste Management, before the New York State Assembly Standing Committee on Environmental Conservation regarding vaporization of contamination from soil and groundwater into indoor air (April 21, 2005).

Vapor Intrusion Pathways

"Vapor intrusion" refers to the process by which volatile chemicals move from a subsurface source into the indoor air of overlying or adjacent buildings. The subsurface source can either be contaminated groundwater or contaminated soil which releases vapors into the pore spaces in the soil.

Vapors can enter buildings in two different ways. In rare cases, vapor intrusion is the result of groundwater contamination which enters basements and releases volatile chemicals into the indoor air. In most cases, vapor intrusion is caused by contaminated vapors migrating through the soil directly into basements or foundation slabs. Although the Department historically has evaluated soil gas pathways, improvements in analytical techniques and the knowledge gained from remedial sites in New York and other states has increased our understanding of how vapor intrusion occurs.

Historically, we thought that vapor intrusion was only an issue where the source of the contaminants was very shallow and the magnitude of the contamination was very great. We now know that our previous assumptions about the mechanisms that could lead to exposure to vapor intrusion were not complete. The result is that additional work may be required to investigate or remediate sites that are in the operational or monitoring phase, or that have already been closed. Separate ranking systems have been developed to account for the two different sources of contaminated vapors. Because we now recognize the need to take a different sampling approach, when the Department evaluates a site for vapor intrusion, both sources can now be effectively considered.

Contaminated soil vapor is not the only possible source of volatile chemicals in indoor air. Chemicals are part of our everyday life. Volatile chemicals are found in many household products, such as paints, glues, aerosol sprays, new carpeting or furniture, refrigerants and recently dry-cleaned clothing. Volatile chemicals are also emitted by common commercial and industrial activities. Indoor air may also become affected through the infiltration of outdoor air containing volatile chemicals.

Overall Strategy

Acting together, the Department and NYSDOH are leading the State's proactive approach to vapor intrusion. We have developed a joint strategy to evaluate the vapor intrusion pathway at all of the remedial sites in the State. This strategy is discussed in two complementary documents which have been released to the public. Using this approach, New York will become a national leader in the development and implementation of a comprehensive approach to address the issue of vapor intrusion.

With its focus and expertise on identifying, understanding, preventing and mitigating risks to human health from exposure to environmental contamination, NYSDOH has drafted a document entitled "Guidance for Evaluating Soil Vapor Intrusion in the State of New York." In turn, the Department has developed a strategy, "Evaluating the Potential for Vapor Intrusion at Past, Present and Future Sites." (Note: the title of the strategy was changed to "Strategy for Prioritizing Vapor Intrusion Evaluations at Remedial Sites in New York State"). This document delineates the process by which the agencies will prioritize remedial sites for vapor intrusion evaluations. With regulatory authority over remedial sites, the Department will continue to oversee investigation and cleanup of remedial sites.

The joint agency strategy reflects the following concepts:

Vapor intrusion evaluations are among the top priorities of our agencies;
The likelihood of vapor intrusion-related exposures varies from site to site;
The number of sites at which vapor intrusion evaluations are warranted is quite large; and
Revisiting this issue concurrently at all volatile chemical sites where remedial or corrective actions have been implemented is not feasible, resulting in the need to prioritize these sites.

The Department's portion of this strategy describes the conditions under which the State will conduct the vapor intrusion evaluations, and the order in which sites will be addressed. For example, at remedial sites where there are ongoing environmental investigations, we will evaluate the vapor intrusion pathway as part of the remedial investigation. At sites where remedial decisions already have been made which do not address vapor intrusion, the Department and NYSDOH will seek to have the responsible parties conduct vapor intrusion evaluations. If the responsible parties are unwilling or unable to conduct an evaluation, or there is no responsible party to do so, we will use the criteria in the Department's strategy to rank the site for the perceived likelihood of current or potential exposures.

Because the number of sites at which evaluations for vapor intrusion are expected to be made is quite large, the Department will prioritize its efforts. Simply put, sites where the perceived likelihood of exposure is great will be scheduled for vapor intrusion evaluations sooner than sites where the perceived likelihood of exposure is small.

The Department's evaluation of a vapor intrusion pathway at a specific site will initially involve a review of existing environmental data to see if sufficient information is already available to assess possible vapor impacts. If a vapor intrusion problem is suspected, we may recommend additional sampling, monitoring or mitigation actions. Additional sampling would be used to determine the extent of soil vapor contamination and to verify our initial findings. Monitoring, or sampling on a recurring basis, is typically conducted if there is a significant potential for vapor intrusion to occur if building conditions change. Mitigation steps are intended to prevent exposures associated with soil vapor intrusion. Mitigation may include sealing cracks in the building's foundation, adjusting the building's heating, ventilation or air-conditioning system to maintain a positive pressure to prevent infiltration of subsurface vapors, or installing a sub-slab depressurization system beneath the building. In most instances, mitigation of residential structures will require a sub-slab depressurization system.

Subsurface vapor samples consist of both soil vapor samples collected from the ground away from buildings (soil gas samples) and sub-slab soil vapor samples collected from immediately beneath the foundation or slab of a building (sub-slab samples). Indoor air samples are collected from the basement or crawlspace and from the lowest level of living space. We collect these samples primarily during the heating season, because we believe that soil vapor intrusion is most likely to occur when a building's heating system is in operation and air is being drawn into the building. Outdoor air samples are collected outside of the buildings, and are used to characterize site-specific outdoor air background conditions. The length of a soil vapor investigation will depend on the data that is collected during the investigation, and is only considered to be complete when all of the exposures and potential exposures have been addressed.

As with groundwater contamination, the Department will ask the party responsible for contaminating the site to pay for and perform the vapor intrusion evaluation, as well as any site investigations and the installation and long-term operation and monitoring of any mitigation system which would be required. If the responsible party refuses to perform these activities, or if no viable entity remains, the State will take responsibility for them, and will pursue cost recovery as in any Superfund site.

Time Line for State Action

Assuming that the Department will be responsible for performing the work at approximately 150 legacy sites - sites for which a remedial decision was issued prior to January 1, 2003 and where the Department's information indicated that disposal of any quantity of chlorinated hydrocarbons occurred - we should complete our initial site characterization work to prioritize all the legacy sites by December, 2006. We expect to commence field work before the beginning of the next heating season. Although some field work, such as soil gas sampling, will begin in the summer, indoor sampling cannot begin until October, when the heating season starts. As the site investigations progress, mitigation systems will be installed at buildings where vapor intrusion sampling demonstrates a need, and will continue until all legacy sites have been fully characterized and the need is completely defined.

Generally, the sites scoring highest according to the criteria described in the Department's vapor strategy will be addressed first. As additional information becomes available to us, through the results of vapor sampling performed by private entities, evaluations of particular sites may be deferred or expedited.

Because the State's decisions on mitigation measures will vary from site to site, it may appear that we are applying our vapor intrusion policy and guidance inconsistently. In reality, however, decisions on how to address exposure to vapor intrusion will be made on a site-by-site basis, after a comprehensive review of individual subsurface vapor, indoor air and outdoor air results, and after the consideration of additional site-specific parameters, such as sampling results; sources of volatile chemicals; background levels; and applicable guidelines for volatile chemicals in the air. This is the most appropriate approach to ensure the protection of public health.

The Department views the use of vapor mitigation systems as a short-term solution to the vapor intrusion problem. By addressing the source of the contamination, and ensuring that steps are taken to remediate and monitor the soil and groundwater which provides a pathway for the migration of these chemicals, the Department can provide effective long-term protection of the public health from vapor migration.

Conclusion

The process for evaluating vapor intrusion exposure is evolving, as we learn more about this highly complex phenomenon. We will refine and improve our approach to addressing vapor intrusion as we work through the investigation and mitigation of specific sites. Nonetheless, New York's efforts are leading the Nation in the development and implementation of strategies to address vapor intrusion. Through our efforts, our citizens will be better protected from the chronic health problems which can result from exposure to volatile chemicals in indoor air.