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For over twenty years, the evaluation of vapor intrusion has been a topic of the U.S. These findings indicate that tree sampling may be an appropriate method to detect contamination at shallow depths at sites with VI. Results indicate that a 30-cm diameter tree characterizes soil concentrations at depths less than 20 m over an area of 700 to 1,600 m2, the volume of a typical basement. Results indicate that in-planta concentrations are significantly and positively related to PCE concentrations in groundwater samples collected at depths less than 20 m (adjusted R2 values greater than 0.80) and in soil samples (adjusted R2 values greater than 0.90). Regression models were developed to assess the relation between PCE concentrations in over 400 tree-core samples with PCE concentrations in over 50 groundwater and 1,000 soil samples collected from a tetrachloroethylene- (PCE-) contaminated Superfund site and analyzed using gas chromatography. Trees could provide a similar subsurface sample where roots act as the "sampler' and are already onsite. Traditional VI assessments are often time-, cost-, and labor-intensive whereas traditional subsurface methods sample a relatively small volume in the subsurface and are difficult to collect within and near structures. Vapor intrusion (VI) by volatile organic compounds (VOCs) in the built environment presents a threat to human health. Accurate characterization of subsurface contamination near and under the foundation of buildings is paramount in determining VI risk however, the subsurface volume sampled by traditional soil-gas samples is relatively small and limited by soil porosity, tortuosity, and the pumped soil-gas volume. These direct and indirect methods for measurement of VI risk are invasive, time, and resource intensive (on the order of thousands to hundreds of thousands of dollar), 9, 10 or may not be done at all due to the inability to safely collect samples. 8 Vapor intrusion risk is typically measured with direct methods (e.g., subslab sampling of soil gas) or indirect methods (e.g., groundwater, soil, or soil-gas sampling). 3 Because indoor-air quality is highly spatially and temporally variable, and because many of the chemicals that pose threats can be derived from sources inside buildings, such as cleaning products or building materials, the presence of contaminant concentrations in the shallow subsurface below structures is measured and used to screen for VI risk, which is based on a target excess lifetime cancer risk of 1 × 10 −9 and Hazard Quotient of 1.
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This study was supported by the Navy Environmental Sustainability Development to Integration Program (NESDI) Program, as part of the study on Improved Strategies for Assessment of Vapor Intrusion, under direction by the Space and Naval Warfare (SPAWAR) Systems Center Pacific. In response to the need for future research and development on reducing high costs and uncertainties of VI assessment strategies (Department of the Navy, DON, 2008), the objective of this report is to identify existing best practices, knowledge and data gaps, and future research into new strategies and techniques. Nevertheless, experience indicates that this guidance also leads to higher costs in mitigation, public relations, and potential litigation. Virtually all of the current regulatory documents generated are guidance and have not been promulgated as law, allowing for negotiation on the approach to follow in accordance to site-specific conditions. This exposure pathway has attracted significant attention from regulatory agencies over the past decade in response to several well-publicized cases. Vapor intrusion (VI) refers to the movement of chemical vapors from contaminated soils or groundwater through the subsurface and into buildings (indoor air).