Article pubs.acs.org/est
Long-Term Evaluation of the Controlled Pressure Method for Assessment of the Vapor Intrusion Pathway Chase Holton,† Yuanming Guo,† Hong Luo,†,‡ Paul Dahlen,† Kyle Gorder,§ Erik Dettenmaier,§ and Paul C. Johnson*,† †
School of Sustainable Engineering and the Built Environment, Ira A. Fulton Schools of Engineering, Arizona State University, Tempe, Arizona 85287, United States ‡ Chevron Energy Technology Company, 3901 Briarpark Dr., Houston, Texas 77042, United States, § Restoration Installation Support Team, Hill Air Force Base, 7290 Weiner St., Building 383, Hill AFB, Utah 84056, United States S Supporting Information *
ABSTRACT: Vapor intrusion (VI) investigations often require sampling of indoor air for evaluating occupant risks, but can be confounded by temporal variability and the presence of indoor sources. Controlled pressure methods (CPM) have been proposed as an alternative, but temporal variability of CPM results and whether they are indicative of impacts under natural conditions have not been rigorously investigated. This study is the first involving a long-term CPM test at a house having a multiyear high temporal resolution indoor air data set under natural conditions. Key observations include (a) CPM results exhibited low temporal variability, (b) false-negative results were not obtained, (c) the indoor air concentrations were similar to the maximum concentrations under natural conditions, and (d) results exceeded long-term average concentrations and emission rates under natural conditions by 1−2 orders of magnitude. Thus, the CPM results were a reliable indicator of VI occurrence and worst-case exposure regardless of day or time of year of the CPM test.
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INTRODUCTION At sites where volatile organic compounds (VOCs) are found in the subsurface, VOCs might migrate to the indoor air of overlying buildings. This vapor intrusion (VI) can pose health risks to building occupants. Over the past decade, guidance documents for VI pathway assessment have been promulgated by various groups.1−7 While these vary in their recommendations, they favor a multiple-lines-of-evidence approach (MLE), similar to that in the United States Environmental Protection Agency (USEPA)1,2 and Interstate Technology and Regulatory Council (ITRC)7 guidance. The MLE generally involves the collection of groundwater, soil gas, and indoor air for chemical analyses and screening-level or more complex transport modeling. Indoor air sampling results are often the most heavily weighted in decision-making as they are a measure of indoor exposure and are of the greatest interest to occupants and the public. One issue with relying on indoor air sampling is that it can be confounded by temporal variability and indoor sources.8−10 Temporal variability ranging from less than order-of-magnitude to several orders of magnitude has been reported.11−13 Folkes et al.11 report variations of about 2× in a decades-long monthly sampling data set, while USEPA12 report indoor PCE concentrations varying by over 2 orders of magnitude in weekly monitoring conducted over a one-year period at a duplex. Holton et al.13 performed a study that included monitoring in a house for 2.5 years at 2−4 h intervals; the indoor air TCE concentrations varied by 3 orders of magnitude © 2015 American Chemical Society
within periods of days and weeks. Their data were used to project outcomes of common sampling strategies and they concluded that mischaracterization of occurrence and magnitude of VI exposure was likely at homes with VI behavior like the one studied. Other MLE components are also subject to variability with time and location. For example, spatial and temporal variability in subslab soil gas concentrations can also be an order-ofmagnitude or more.11,14,15 Variability associated with MLE components often hinders decision-making and in some cases drives numerous sampling efforts that fail to result in definitive exposure characterization. As more is learned about MLE data variability and difficulties with decision-making due to this issue, it is becoming clear that additional VI pathway assessment methods are needed. New approaches need to be robust and lead to consistent results that are not sensitive to the timing of the assessment or sampling location. Temporal variability in indoor air and subslab soil gas concentrations is often attributed to temporal variability in indoor-outdoor and indoor-subsurface pressure differentials. These can vary by about 1−10 Pa for residential buildings16,17 and can be influenced by wind, indoor-outdoor temperature differences, and ambient pressure changes. Therefore, some Received: Revised: Accepted: Published: 2091
October 26, 2014 January 9, 2015 January 20, 2015 January 20, 2015 DOI: 10.1021/es5052342 Environ. Sci. Technol. 2015, 49, 2091−2098
Article
Environmental Science & Technology have proposed what we refer to as “controlled pressure methods” (CPM) that manipulate indoor-outdoor pressure conditions over short time periods to either induce or suppress VI; examples include the pressure mapping and tracer testing method applied by Nazaroff et al.,17 and the procedure applied by McHugh et al.9 and Beckley et al.10 McHugh et al.9 used box fans to direct air flow into and out of buildings to create overpressurized and under-pressurized conditions, and data were collected to calculate VOC mass discharge rates and discriminate between indoor source release, outdoor vapor intrusion, and subsurface vapor intrusion. The CPM is of interest because it can be completed within a day or two and the soil gas sample collection below the foundation of a building that is often disruptive to building occupants is not required. Furthermore, it is likely less expensive than VI assessment approaches that require multiple rounds of sampling.18 There are, however, questions that still need to be answered before practitioners have confidence that CPM testing consistently leads to accurate VI pathway assessment decisions. For example, (a) are CPM results significantly different from day to day or season to season?; (b) can CPM results be used to accurately assess chronic (multiyear) and acute (multiweek) exposures?; and (c) are CPM results dependent on the specific operating conditions, such as pressure differential, so that different practitioners might obtain different results? To answer the key questions above, long-term CPM monitoring data sets are needed from sites where long-term monitoring data are also available under natural conditions. The goal of this study was to collect a comprehensive data set of that type, with a focus on answering questions (a) and (b) above. To gain insight into the behavior of different sources under CPM conditions and because radon monitoring is often discussed as a surrogate for chemical VI monitoring, both chlorinated hydrocarbons (CHCs) and radon were monitored in this study.
control building under-pressurization, which is measured relative to outdoor air and subslab soil gas. The underpressurization is large enough that the building is constantly under-pressurized at a fairly stable level during the test, even with temporal fluctuations induced by wind and temperature, ideally with variations
