GWMR Focus Issue on Vapor Intrusion

The timing of this GWMR special focus issue roughly coincides with the 20th anniversary of the emergence of soil vapor intrusion to indoor air as an exposure pathway of concern for human health risks at sites with contaminated soil and ground water. In 1988, the Indoor Radon Abatement Act was promulgated and chlorinated solvent vapor intrusion was identified in the Hillside School in Needham, Massachusetts. At about the same time, the Johnson and Ettinger screening-level model was being developed to assess vapor intrusion risks at the Rocky Mountain Arsenal. In general, however, this pathway was seldom assessed until vapor intrusion cases in Colorado in the late 1990s gained attention in the press and raised the issue to national prominence. There are now about two dozen state-level guidance documents, a few federal guidance documents, and multiple documents from private companies or nongovernment organizations. Internationally, there is interest in vapor intrusion in several other countries (Canada, UK, EU, and Australia, to name a few). There are significant differences between jurisdictions in approaches, methods, policies, and screening levels (by up to several orders of magnitude). This indicates the limits of our collective understanding of the topic because the laws of physics, building ventilation practices, and human toxicology do not vary much between states or countries. The development of a simple recipe for assessing vapor intrusion has been elusive; regulatory agencies generally would like an assessment approach that minimizes false negatives (falsely concluding that vapor intrusion poses no significant risk), whereas responsible parties generally would like an approach that is practicable and minimizes both false negatives and false positives (falsely concluding that vapor intrusion does pose an unacceptable risk). Assessments can be challenging because typical indoor air concentrations in residences not affected by vapor intrusion approach risk-based target concentrations for several common chemicals. Considering all the attention paid to the vapor intrusion pathway, there has been very little funding for relevant pure or applied research in the past decade. For example, the Indoor Radon Abatement Act of 1988 allocated about $15 million/year of research and technical assistance funding. Recently, federal funding has been extended to a few studies of VOC vapor intrusion through the Environmental Security Technology Certification program (ESTCP) and the Strategic Environmental Research and Development Program (SERDP), although in much smaller amounts. Industry-funded programs have been few and small and largely led by the American Petroleum Institute and its member companies. As a result, much of what has been learned in the past decade has been based on studies conducted on an ad hoc basis with limited resources. Also, much of what has been learned has been shared in conference proceedings, or other non-peer-reviewed literature. This was a stimulus for this focus issue, which started about a year ago with an invitation sent to a large number of individuals active in this field, but with few or no publications in archival journals. The articles in this issue intentionally span a range of topics, including regulatory and public perspectives as well as traditional archival peer-review papers on sampling methods, empirical data variability, mathematical modeling, background concentrations, biodegradation of petroleum compounds, and more. We are grateful to the dozens of authors and coauthors as well as the dozens of peer reviewers for devoting their time to share the information contained herein. Schedule and space constraints prevented some papers from being published in this issue, and they will appear in coming issues. It will be interesting to see how the assessment and management of this exposure pathway progress. We need to develop and demonstrate the use of new data collection methods leading to confident characterization of long-term average exposures, because it is difficult to practicably reduce the uncertainty inherent in current point-in-time and point-in-space sampling methods given the spatial and temporal variations seen at research sites. In addition, we need improved techniques to discriminate between subsurface and indoor sources of indoor air contaminants, and improved predictive capabilities to address future-use scenarios at sites without buildings currently present or buildings where access for monitoring is denied. There are also several policy-related issues to consider, like those leading to screening levels that vary by several orders of magnitude, data sufficiency for reliable assessment, and relations with owners and occupants of potentially affected properties. There are significant challenges to overcome before we can screen and assess VOC vapor intrusion confidently and cost-effectively at all sites; in the meanwhile, there is an amazing opportunity for creative solutions to the current challenges. The readership of GWMR will no doubt be significant contributors to these solutions. We hope you enjoy this issue and look forward to future articles on this challenging and important topic.

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