Creation of a Sub-slab Soil Gas Cloud by an Indoor Air Source and Its

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Creation of a Sub-Slab Soil Gas Cloud by an Indoor Air Source and Its Dissipation Following Source Removal Chase Holton, Yuanming Guo, Hong Luo, Paul Dahlen, Kyle Gorder, Erik M. Dettenmaier, and Paul C. Johnson Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01188 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 20, 2018

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Environmental Science & Technology

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Creation of a Sub-Slab Soil Gas Cloud by an Indoor Air Source and Its Dissipation

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Following Source Removal

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CHASE HOLTON†, YUANMING GUO†, HONG LUO †⊥, PAUL DAHLEN†, KYLE

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GORDER‡, ERIK DETTENMAIER‡, AND PAUL C. JOHNSON†║*

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†School of Sustainable Engineering and the Built Environment, Ira A Fulton Schools of

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Engineering, Arizona State University, Tempe, AZ 85287, ⊥Chevron Energy Technology

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Company, 1200 Smith St., Houston, TX 77002, ‡Hill Air Force Base, 7290 Weiner St.,

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Building 383, Hill AFB, UT 84056, and ║Department of Civil and Environmental

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Engineering, Colorado School of Mines, Golden, CO 80401

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ACS Paragon Plus Environment


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ABSTRACT It is accepted that indoor sources of volatile organic compounds (VOCs) can

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confound vapor intrusion (VI) pathway assessment. When discovered during pre-

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sampling inspection, indoor sources are removed, and air sampling is delayed, with the

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assumption that a few hours to a few days are sufficient for indoor source impacts to

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dissipate. This assumption was tested through the controlled release of SF6 and its

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monitoring in indoor air and soil gas at a study house over two years. Results show that

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indoor sources generate subsurface soil gas clouds due to fluctuating direction in the

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exchange between soil gas and indoor air, and that it may take days to weeks under

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natural conditions for a soil gas cloud beneath a building to dissipate following indoor

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source removal. The data also reveal temporal variability in indoor air and soil gas

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concentrations, long-term seasonal patterns, and dissipation of soil gas clouds over days

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to weeks following source removal. Preliminary modeling results for similar conditions

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are consistent field observations. If representative of other sites, these results suggest that

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a typical 1 – 3 day waiting period following indoor source removal may not be sufficient

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to avoid confounding data and erroneous conclusions regarding VI occurrence.

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INTRODUCTION Subsurface to indoor air vapor intrusion (VI) pathway assessment often involves

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indoor air and sub-slab or exterior soil gas sampling, with indoor air results being

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weighted most heavily for estimating exposure and soil gas results being used to

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corroborate that indoor air impacts are the result of subsurface sources (1). The latter is

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done by comparing soil gas and indoor air concentrations; if the former is greater than the

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latter and the ratio of the two is within published ranges for attenuation factors (2), then it

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is concluded that there is an active connection between contaminants in soil and/or

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groundwater and indoor air.

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This approach can be confounded by indoor air sources and bi-directional

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exchange of soil gas and indoor air. Indoor sources of volatile organic chemicals (VOCs)

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can produce indoor air concentrations above health-based screening levels (3, 4).

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Attempts to identify them often rely on inventory surveys and occupant interviews (5),

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although those do not always lead to identification of all indoor sources (6). Thus,

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methods to identify VOC sources are being evaluated. These include portable detector

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screening (6), controlled pressure method testing (CPM) proposed by McHugh et al. (7),

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tested by Beckley et al. (8), and studied long-term by Holton et al. (9), compound

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concentration ratio analysis, and use of compound-specific isotope analysis (CSIA) (10).

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Some indoor sources are portable and can be removed (e.g. cleaning solvent containers),

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while others are fixed parts of building construction or function (e.g., insulation). This

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paper focuses on the former.

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When portable indoor sources are identified, they are removed and indoor

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sampling is usually delayed; some rules of thumb used in practice are to wait a period

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sufficient for three building volumes to be exchanged (530 h to reach 1000

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ppbv. With time, the concentration of SF6 in indoor air and soil gas at sub-slab and 0.9 m

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BS depths leveled off to within about a factor of two.

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Following the stop of SF6 release on t = 680 d, indoor air and sub-slab soil gas

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responded quickly with concentrations dropping to below 10 ppbv in less than 72 h and

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96 h, respectively. The similar response between indoor air and sub-slab soil gas is likely

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related to the high permeability of the gravel layer beneath the foundation and the

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likelihood of advection being the dominant transport mechanism in this layer. As with the

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start of SF6 release, soil gas at 0.9 m BS responded slowly, with concentrations remaining

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above 150 ppbv 360 h (15 d) after halting the release of SF6. It is important to note that

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the vertical trend of decreasing concentrations with increasing proximity to indoor air

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matches the expected soil gas trends for subsurface VOC sources, even though the soil

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gas profile is the result of a removed indoor air source and not the result of a subsurface

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source. During the test period (655 < t < 695 d), daily 24-h average differential pressure

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measured between sub-slab soil gas and indoor air at location 5 ranged from -0.7 to 0.3

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Pa (SI Figure S2) and air exchange rate ranged from approximately 0.2 – 0.4 h-1. 11


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Figure 6 results were obtained while the land drain lateral connection to the local

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land drain system was open. Figure 7 presents results of an indoor source removal test

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with the lateral connection closed. Prior to source removal (t = 1467 d) the concentrations

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of SF6 in indoor air and soil gas at both depths were within a factor of two and similar to

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those in Figure 6. Upon stopping the SF6 release at = 1467 d, indoor air concentrations

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dropped to near or below 10 ppbv within 72 h, similar to the previous case with the open

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land drain connection. In contrast, sub-slab soil gas concentrations did not decrease

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below 10 ppbv until about 560 h after stopping the indoor SF6 release. Concentrations of

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SF6 in soil gas at 0.9 m BS decreased the slowest and even more slowly than Figure 6

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results, with concentrations falling below 100 ppbv only after about 970 h. Later in the

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test, sub-slab soil gas concentrations reversed their declining trend and increased to above

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10 ppbv, which is likely the result of upward diffusion from deeper soil gas. Differential

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pressure values are not available between sub-slab soil gas and indoor air during this

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period (1460 < t < 1515 d) due to a computer failure; however, differential pressure

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values are typically indicative of downward flow during the later summer and early fall

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(e.g., ranging between -0.7 and 0.3 Pa in 2011).

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Indoor source release and removal modeling studies. Preliminary modeling

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was performed using the three-dimensional, multicomponent, numerical model developed

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by Abreu and Johnson (16), with modification needed to simulate indoor source release

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to indoor air at a constant emission rate. The intent was to examine if preliminary

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modeling results resembled qualitative trends in the field so simple geometry and release

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scenarios were employed, rather than field-site specific model conditions. 12



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The release scenarios included: (a) creation of a subsurface soil gas cloud at a

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constant building over-pressurization for 720 h and (b) removal of the indoor source and

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continued monitoring at a constant over- and under-pressurization conditions for 1440 h.

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Table 1 provides details of the two scenarios simulated, while SI Table S1 summarizes

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the model inputs. A plan view schematic of the model domain, foundation cracks, and

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sampling locations is shown in SI Figure S3. The 10 Pa pressure was used in the second

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simulation to evaluate vapor cloud dissipation under CPM testing (13)

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Figure 8 shows the predicted subsurface soil gas cloud after 720 h of indoor

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source release at contour depths of sub-slab (0.15 m BS), 1 m BS, and 1.8 m BS. Soil gas

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concentrations are greatest below the building foundation and along the perimeter crack.

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For the second model period, the effect of source removal was simulated for

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building under- and over-pressurization conditions, as shown in Figures 9 and 10,

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respectively. These show SF6 concentrations in indoor air and soil gas at locations A

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(along a crack) and B (building center) highlighted in SI Figure S3. For location A, soil

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gas concentrations are presented for sub-slab, 1 m BS, and 1.8 m BS while only sub-slab

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concentrations are presented for location B because concentrations at 1 m BS and 1.8 m

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BS remained within 10% of sub-slab values.

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Indoor air concentrations decrease by over two orders-of-magnitude within 12 h of

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indoor source removal in both over- and under-pressurization simulations, although the

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response is faster for the over-pressurization scenario. In both cases, sub-slab

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concentrations take over 120 h (5 d) to decrease by an order-of-magnitude and 1 m BS

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concentrations take over 240 h (10 d) to drop an order-of-magnitude at location A. The

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soil gas results at location B are similar for both simulations with nearly uniform 13


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concentrations at each depth. The results at location B increase slowly for over 120 h

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before steadily decreasing.

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The simulation results for location A are more similar qualitatively to the removal

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test results from the field studies than those for location B. For example, prior to removal

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of the indoor source, SF6 concentrations in indoor air and soil gas at location A are within

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a factor of two in the simulations, and after removal of the indoor source, sub-slab soil

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gas concentrations decrease to levels below those at 1 m BS. The temporal trends and

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differences in concentrations with depth at location B are different from the field studies,

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but still indicate that indoor source-created soil gas clouds can remain for extensive

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periods after indoor source removal. Location A and B differ in their proximity to the

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crack; thus, location B experiences significantly less advective flow than location A.

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Implications for pathway assessment. The study results presented above

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demonstrate a few key points relevant to VI pathway monitoring and indoor source

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removal. First, it is clear that indoor sources, in combination with bi-directional soil gas

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flow, can create subsurface vapor clouds. Second, the extent and mass storage in these

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vapor clouds can vary daily and seasonally, depending on the indoor source release rate,

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indoor air exchange rate, chemical properties, and predominant direction of soil gas flow.

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For example, the soil gas cloud at the study site was larger and contained more mass in

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the summer than in the winter.

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With respect to the removal of known indoor sources and the timing of VI

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pathway sampling, it is likely that typical waiting periods (24 – 72 h) might be sufficient

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to assess the significance of current indoor air impacts following source removal, for

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chemicals with low sorption potential like SF6. More strongly sorbing compounds are 14



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anticipated to dissipate slower than a tracer like SF6 due to increased partitioning-related

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storage in soil moisture and on soil surfaces. Soil gas sampling after typical waiting

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periods, however, may lead to erroneous conclusions regarding the presence of a

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subsurface source and completeness of the VI pathway. The first assertion is supported

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by the rapid declines in indoor air concentrations within one day following source

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removal at the field site and in the modeling results. The second assertion is built from

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the temporal and spatial trends in the soil gas profiles at the field site and typical

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conventions for data interpretation. As shown in Figures 6 and 7, soil gas concentrations

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1-m below the foundation declined very slowly and persisted for weeks, and this resulted

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in vertical soil gas profiles that resembled what is expected for long-term subsurface

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VOC sources (e.g., decreasing concentrations in moving from a source to indoor air). A

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practitioner seeing a vertical profile like those represented by the post-source removal

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data in Figures 6 and 7 could erroneously conclude that a long-term subsurface source

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was present and that VI was possible, even though no long-term source was present.

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It is not possible now, given the single study site and modeling results, to

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recommend post source removal waiting periods before soil gas sampling; the data and

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observations from this study site may not be representative of other VI-sites, so caution

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should be exercised in extending lessons-learned at this site to other VI sites. Having

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noted that, it appears that interpretation of a snapshot data set could be confounded by the

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indoor source history for weeks following its removal, and that the temporal analysis of

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several sampling events under natural conditions would be needed to determine if VOCs

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in soil gas were the result of an indoor or long-term subsurface VOC source. Such

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waiting periods and data collection requirements might be impracticable at sites where 15


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there is uncertainty regarding the impacts of indoor sources, so in the future, practitioners

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might consider alternate investigation strategies, such as use of CPM testing (7, 9, 13) or

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application of CSIA (10) for VI pathway assessment. Finally, in the future, this data set

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might also be useful for another purpose: to evaluate the effects of dynamic

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environmental variables on concentration changes with time, as in Johnston and

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MacDonald Gibson (17).

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ASSOCIATED CONTENT

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Supporting Information

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Additional information as noted in the text. This information is available free of charge

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via the ACS Publications website.

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AUTHOR INFORMATION

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Corresponding Author

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*E-mail: [email protected]

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ACKNOWLEDGEMENTS

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The research was funded by the U.S. Department of Defense through the Strategic

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Environmental Research and Development Program (SERDP). The findings and

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conclusions in this article are those of the authors and do not necessarily represent the

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view of the U.S. Air Force or the Department of Defense.

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REFERENCES 1. OSWER technical guide for assessing and mitigating the vapor intrusion pathway from subsurface vapor sources to indoor air; United States Environmental Protection Agency: Washington, DC, 2015; https://www.epa.gov/sites/production/files/2015-09/documents/oswer-vaporintrusion-technical-guide-final.pdf 2.

EPA’s vapor intrusion database: Evaluation and characterization of attenuation factors for chlorinated volatile organic compounds and residential buildings; United States Environmental Protection Agency: Washington, DC, 2012; https://www.epa.gov/sites/production/files/201509/documents/oswer_2010_database_report_03-162012_final_witherratum_508.pdf

3. Dawson, H. E., McAlary, T. A compilation of statistics for VOCs from post-1990 indoor air concentration studies in North American residences unaffected by subsurface vapor intrusion. Ground Water Monitoring and Remediation. 2009, 29, 60-69. 4. Doucette, W. J., Hall, A. J., Gorder, K. A. Emissions of 1,2-dichloroethane from holiday decorations as a source of indoor air contamination. Ground Water Monitoring and Remediation. 2010, 30, 65-71. 5. Vapor intrusion pathway: A practical guideline; Interstate Technology & Regulatory Council: Washington, DC, 2007; https://www.itrcweb.org/documents/vi-1.pdf

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6. Gorder, K.A. and E.M. Dettenmaier. Portable GC/MS methods to evaluate sources of cVOC contamination in indoor air. Groundwater Monitoring and Remediation. 2011, 31 (4), 116-119. 7. McHugh, T. E., Beckley, L., Bailey, D., Gorder, K., Dettenmaier, E., RiveraDuarte, I., Brock, S., MacGregor, I. C. Evaluation of vapor intrusion using controlled building pressure. Environmental Science & Technology. 2012, 46, 4792-4799. 8. Beckley, L., Gorder, K., Dettenmaier, E., Rivera-Duarte, I., McHugh, T. On-site gas chromatography/mass spectrometry (GC/MS) analysis to streamline vapor intrusion investigations. Environmental Forensics. 2014, 15, 234-243. 9. Holton, C., Guo, Y., Luo, H., Dahlen, P., Gorder, K., Dettenmaier, E., Johnson, P. C. Long-term evaluation of the controlled pressure method for assessment of the vapor intrusion pathway. Environmental Science and Technology. 2015, 49, 20912098. 10. McHugh, T. E., Kuder, T., Fiorenza, S., Gorder, K., Dettenmaier, E., Philp, P. Application of CSIA to distinguish between vapor intrusion and indoor sources of VOCs. Environmental Science and Technology. 2011, 45, 5952-5958. 11. McHugh, T. E., De Blanc, P. C., Pokluda, R. J. Indoor air as a source of VOC contamination in shallow soils below buildings. Soil and Sediment Contamination. 2006, 15, 103-122.

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12. Holton, C., Luo, H., Dahlen, P., Gorder, K., Dettenmaier, E., Johnson, P. C. Temporal variability of indoor air concentrations under natural conditions in a house overlying a dilute chlorinated solvent groundwater plume. Environmental Science and Technology. 2013, 47, 13347-13354. 13. Guo, Y., Holton, C., Luo, H., Dahlen, P., Gorder, K., Dettenmaier, E., Johnson, P.C. Identification of alternative vapor intrusion pathways using controlled pressure testing, soil gas monitoring, and screening model calculations. Environmental Science and Technology. 2015, 49, 13472-13482. 14. Johnson, P. C., Holton, C., Guo, Y., Dahlen, P., Luo, H., Gorder, K., Dettenmaier, E., Hinchee, R. Integrated field-scale, lab-scale, and modeling studies for improving our ability to assess the groundwater to indoor air pathway at chlorinated solvent-impacted groundwater sites. Strategic Environmental Research and Development Program (SERDP) Project ER-1686, 2016; https://www.serdp-estcp.org/Program-Areas/EnvironmentalRestoration/Contaminated-Groundwater/Emerging-Issues/ER-1686/ER-16862 15. Holton, C. Evaluation of vapor intrusion pathway assessment through long-term

monitoring studies. Ph.D. Dissertation, Arizona State University, Tempe, AZ, 2015. 16. Abreu, L., Johnson, P. C. Effect of vapor source-building separation and building construction on soil vapor intrusion as studied with a three-dimensional numerical model. Environmental Science and Technology, 2005, 39 (12), 4550-4561. 3



17. Johnston, J., MacDonald Gibson, J. Quantifying spatiotemporal variability of tetrachloroethylene in indoor air due to vapor intrusion: a longitudinal, community-based approach. Journal of Exposure Analysis and Environmental Epidemiology. 2013, 24, 564-571.

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TABLES Table 1. Summary of indoor source modeling scenarios.

Subsurface Soil Gas Cloud Creation

Indoor Source Removal and Soil Gas Cloud Dissipation

Pressure Pressure Indoor Sourcea Differentialb Simulation Differentialb Simulation Simulation Emission Rate Duration outdoor indoor outdoor indoor # (P -P ) (P -P ) Duration [h] [g/s] [h] [Pa] [Pa] 1

4.00E-04

-2

720

-2

1440

2

4.00E-04

-2

720

10

1440

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Chemical-specific properties of sulfur hexafluoride (SF6) were used in simulations.

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Positive pressure differential values indicative of building under-pressurization and negative values indicative of building over-pressurization.

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FIGURES

Figure 1. Conceptual schematic of the indoor source release field experiment.

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Figure 2. Daily 24-h average SF6 concentrations in indoor air and sub-slab soil gas at location 3 during constant source release with error bars spanning the daily maximum and minimum values. Dashed lines indicate periods of source removal.

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Figure 3. Mass of SF6 in soil gas below the study house determined from synoptic soil gas survey data with error bars spanning the uncertainty in the calculations.

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Figure 4. SF6 concentration contour plots from t = 329 d (summer event) for soil gas for sub-slab, 0.9 m BS, and 1.8 m BS depths, following 201 days of steady SF6 release.

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Figure 5. SF6 concentration contour plots from t = 515 d (winter event) for soil gas for sub-slab, 0.9 m below slab (BS), and 1.8 m BS depths, following 387 days of SF6 release.

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Figure 6. SF6 concentrations in indoor air and soil gas at sub-slab (SS) and 0.9 m BS depths at location 3 for 655 d < t

Creation of a Sub-slab Soil Gas Cloud by an Indoor Air Source and Its

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