Tetra Tech, Inc. (Tetra Tech) has been contracted by the United States Air Force (USAF), HSW/PKAH to conduct an investigation of multiple lines of evidence (MLE) used in assessing risk associated with the vapor intrusion (VI) pathway. To that end, research is being conducted at four Air Force installations where VI is suspected. The purpose of the investigations is to evaluate a variety of parameters related to VI in order to develop a better understanding of the processes that lead to a complete VI pathway, and ultimately, to develop tools for use by the Air Force in assessing VI at other installations.

Detailed site investigations were conducted at three of the sites (Cape Canaveral Air Force Station, Kelly Air Force Base, and Travis Air Force Base). The investigations were designed to generate data to evaluate MLE for vapor intrusion at each of the sites. Site-specific air exchange rate measurements were made at each site using a methodology developed specifically for this project. The method, which uses helium as the tracer, has been easy to apply and cost effective. At the fourth site the method is being compared against a standard method of calculating air exchange rate that is more expensive and more difficult to implement. A project report is forthcoming that will compare the two methods.

Vapor intrusion was simulated at each of the first three sites using two models: the widely used Johnson and Ettinger model and the recently completed ViM model developed by Tetra Tech. The ViM model was found to provide more information to explain vapor intrusion than did the Johnson and Ettinger model. Because the Johnson and Ettinger model assumes steady-state conditions, while the ViM model does not, it was found at such sites the predictions of the two models could be substantially different.