Article pubs.acs.org/est
Evaluation of Vapor Intrusion Using Controlled Building Pressure Thomas E. McHugh,†,* Lila Beckley,† Danielle Bailey,† Kyle Gorder,‡ Erik Dettenmaier,‡ Ignacio Rivera-Duarte,§ Samuel Brock,∥ and Ian C. MacGregor⊥ †
GSI Environmental Inc., Houston, Texas, United States Hill Air Force Base, Utah, United States § Space and Naval Warfare Systems Center Pacific, San Diego, California, United States ∥ Air Force Center for Engineering and the Environment, San Antonio, Texas, United States ⊥ Battelle Memorial Institute, Columbus, Ohio, United States ‡
S Supporting Information *
ABSTRACT: The use of measured volatile organic chemical (VOC) concentrations in indoor air to evaluate vapor intrusion is complicated by (i) indoor sources of the same VOCs and (ii) temporal variability in vapor intrusion. This study evaluated the efficacy of utilizing induced negative and positive building pressure conditions during a vapor intrusion investigation program to provide an improved understanding of the potential for vapor intrusion. Pressure control was achieved in five of six buildings where the investigation program was tested. For these five buildings, the induced pressure differences were sufficient to control the flow of soil gas through the building foundation. A comparison of VOC concentrations in indoor air measured during the negative and positive pressure test conditions was sufficient to determine whether vapor intrusion was the primary source of VOCs in indoor air at these buildings. The study results indicate that sampling under controlled building pressure can help minimize ambiguity caused by both indoor sources of VOCs and temporal variability in vapor intrusion.
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significant variation in indoor radon concentrations.6 Significant temporal variability has also been observed in VOC vapor intrusion. As a result of indoor VOC sources and temporal variability, neither the presence nor the absence of elevated VOC concentrations in indoor air during a single sampling event provides definitive information about vapor intrusion. Collection of indoor air samples under controlled building pressure conditions may address concerns regarding both indoor sources and temporal variability. As an alternative to conducting multiple sample events, building operating conditions can be manipulated to induce a negative building pressure during a scheduled sampling event. Induced negative building pressure serves to reduce temporal variability by ensuring that building pressure conditions support the flow of soil gas into the building. Conversely, positive building pressure can be used to “turn off” or minimize vapor intrusion (see Abstract graphic). Indoor air samples collected under these positive pressure conditions serve to characterize the impact of indoor and outdoor (ambient) sources on vapor intrusion. The goal of this study was to evaluate the utility of a vapor intrusion investigation program that utilizes indoor air samples collected under induced negative and positive building pressure
INTRODUCTION At sites where VOCs have been released into the subsurface, the migration of VOCs into buildings, a process known as vapor intrusion, is an increasing concern. In the last 15 years, vapor intrusion impacts have been identified at a number of sites across the country. As a result, the evaluation of vapor intrusion as a potential mechanism for exposure to VOCs originating from the subsurface is now routinely required as part of the investigation and cleanup of contaminated sites. The understanding of transport mechanisms, however, is rapidly evolving. Evaluation methods also continue to evolve. The USEPA issued draft guidance on vapor intrusion in 20021 and is currently working to finalize that guidance by the fall of 2012.2 The revised guidance is expected to place an increased emphasis on the measurement of VOC concentrations in indoor air for identification of vapor intrusion impacts.2 The evaluation of vapor intrusion is complicated by a variety of problems including indoor sources of VOCs and temporal variability. VOCs are found in a wide range of consumer products including household cleaners, glues, fuels, and furnishings, and commonly result in VOC concentrations in indoor air above risk-based screening levels.3 For most buildings, the movement of soil gas through the building foundation is primarily through advection.4 Temporal changes in building pressure can result in temporal changes in the magnitude and direction of gas flow through the foundation.5 For radon, these temporal changes in pressure result in © 2012 American Chemical Society
Received: Revised: Accepted: Published: 4792
December 13, 2011 March 19, 2012 April 9, 2012 April 9, 2012 dx.doi.org/10.1021/es204483g | Environ. Sci. Technol. 2012, 46, 4792−4799
Environmental Science & Technology
Article
while the data logger records the minimum and maximum pressure differences observed during each measurement period, 5 min for this study. For this study, one port was open to the indoor atmosphere and the other was isolated to the exterior of the building. Indoor/outdoor pressure differences were measured on one instrument by extending tubing from the “external” port through a crack in a window or door and using tape to seal the remainder of the opening. Cross-foundation pressure differences were measured with a second instrument by extending tubing from the “external” port through a hole drilled through the floor slab and sealing the hole with modeling clay or Teflon tape. Building Pressure Control. Building pressure was controlled using a small, 20 in. box fan (rated at 2500 CFM) placed in a window (single-family residences and small commercial buildings) or a larger, 36 in. floor fan (rated at approximately 10 000 CFM) in a doorway for larger commercial buildings. Negative building pressure was induced by setting the fan to blow out of the building and positive pressure was induced by setting the fan to blow into the building. Fan speeds were adjusted to achieve a target pressure difference across the building envelope of 1−5 Pa. For each induced pressure condition, the fan was operated for at least 12 h prior to the start of VOC and radon sample collection and continued throughout the sample collection period. Subslab Sample Point Installation. Sample points for the collection of subslab gas samples were installed by drilling a 3/8 in. hole through the building slab and into the underlying soil or fill material to a depth of 3−4 in. below the base of the foundation. A length of 1/4 in. or 1/8 in. nylon tubing was placed in the hole within a sand pack of 2−4 in. such that the end of the tubing was located at or just below the base of the foundation. The hole through the foundation was filled with hydrated bentonite clay to 1 in. below grade and covered with modeling clay at the surface. The end of the tube was fitted with a compression fitting to provide an airtight attachment to the sample train. The end of the tube was plugged when samples were not being collected. Three subslab sample points were installed in each test building distributed across the building foundation, see SI Figures S.1−S.6. Collection of Air and Soil Gas Samples. The program included three indoor, three subslab, and one outdoor sampling locations at each building (Table 1). Indoor air and outdoor air samples for VOC and SF6 analysis were collected in 6 L evacuated stainless steel Summa canisters using an 8 h flow controller. Subslab gas samples for VOC and SF6 analysis were collected as grab samples over a period of approximately 2 min in 1 L or 6 L Summa canisters. A 60 mL gastight polyethylene syringe incorporated into the sample train was used to purge the sample point of three line volumes of gas prior to sample collection. A vacuum gauge incorporated in the Summa canister sample train was used to monitor the vacuum and ensure sample collection. Samples for radon analysis were collected as grab samples using a 60 mL gastight syringe to fill a 0.5 L Tedlar bag with 200 mL of gas. One set of samples was collected from each sample point during each pressure condition. In order to evaluate sample accuracy, field duplicate samples were collected at least one for every 10 samples. Sample Analysis Methods. Summa canister samples were analyzed for VOCs by EPA Method TO-15,7 with indoor and outdoor samples analyzed in selective ion monitoring (SIM) mode to achieve lower detection limits. Although the laboratory reported results for a standard list of VOCs, the primary VOCs
conditions to provide an improved understanding of the potential for vapor intrusion.
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MATERIALS AND METHODS Overview of Testing Program. In order to evaluate the impact of controlled building pressure on vapor intrusion investigation results, the pressure control method has been applied eight times at six buildings across the U.S. The buildings were either slab-on-grade or had finished basements. The layout and volume for each building are provided in Supporting Information (SI) Figures S.1−S.6. At two buildings (Arizona State University (ASU) Research House in Utah and an office building at Moffett Field in California), sampling was conducted under baseline (i.e., uncontrolled), induced negative pressure, and induced positive pressure conditions. At these two buildings, the sampling program was implemented twice in order to evaluate the reproducibility of the results. At four additional commercial/industrial buildings at Travis Air Force Base (AFB), California, Jacksonville Naval Air Station (NAS), Florida, Marine Corps Recruit Depot (MCRD) Parris Island, South Carolina, and Tinker AFB, Oklahoma, sampling was conducted under induced negative pressure and induced positive pressure conditions, but not baseline conditions. During the pressure control study, each pressure condition was maintained for at least 12 h prior to initiation of sample collection and then samples were collected over an 8 h testing period. Throughout each pressure control period, indoor/ outdoor and cross-foundation pressure differences were measured to verify successful pressure control. Radon was used as a naturally occurring soil gas tracer and radon concentrations in indoor air were used to evaluate the effect of the induced building pressure conditions on the movement of soil gas into the test buildings. For five of the six test buildings, sulfur hexafluoride (SF6) was released inside the building and used to measure building air flow rates. The sample collection program implemented for each pressure condition is summarized in Table 1. Implementation of the Table 1. Types of Air Samples Collected in Each Building for Pressure Conditiona matrix
number of locations
indoor air
3
outdoor air subslab soil gas
1 3
analyte
location
VOCs, SF6, radon VOCs, SF6, radon VOCs, SF6, radon
distributed across building based on building layout upwind location three locations distributed across the building foundation
a
Note: Indoor and outdoor samples for analysis of VOCs and SF6 collected using a flow controller over 8 h period. Subslab soil gas samples for analysis of VOCs and SF6 and all samples for radon analysis collects as grab samples.
pressure control method requires building access on three consecutive days (for induced positive and induced negative pressure evaluations only) or four consecutive days (if the baseline evaluation is included). Measurement of Pressure Differences. In order to understand the potential for advective air flow into and out of the building, pressure differences were measured using Omniguard 4 differential pressure transducers with data loggers. A pressure transducer contains two pressure ports and displays pressure differences across the ports in increments of 0.25 Pa, 4793
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sources was calculated using the simplifying approximation that 100% of the air flow into the building is attributable to outdoor air:
of interest were benzene and toluene (two VOCs not known to have subsurface sources at any of the sites), and one or two chlorinated VOCs known to have subsurface sources at the site. The detection limits for the target VOCs ranged from 0.03 to 0.05 ug/m3. Analysis of SF6 was performed using a modified NIOSH Method 6602 with a detection limit of 10 ug/m3.8 Analysis of VOCs and SF6 was conducted by Columbia Analytical Services (CAS), Simi Valley, California. Radon was used as a naturally occurring tracer for the movement of soil gas into the buildings.9 Samples collected for radon analysis were analyzed at the University of Southern California (USC) Earth Sciences department using the extraction method of Berelson, 198710 and the analysis method of Mathieu, 1998.11 This analysis does not have a defined detection limit; however, measurement accuracy decreases with decreasing radon concentration. The measurement accuracy for a sample containing 0.2 pCi/L radon is estimated to be ±30%.9 Outdoor radon concentrations typically range from 0.2 to 0.7 pCi/L,9 indicating that outdoor radon concentrations can typically be measured with an accuracy of ±30% or better. For ASU House Round 1 and Moffett Field Round 1, the radon analysis of subslab samples was done on-site using a RAD7 portable radon meter (Durridge Company, Billercia, MA). As operated, the RAD7 provides an accuracy of ±30% or better for samples with ≥50 pCi/L radon (typical for subslab samples). This accuracy was verified through the collection of split samples also analyzed at USC. Air Flow through Buildings. In order to determine the air flow rate through the test buildings and movement of indoor air across the slab, a tracer gas (SF6) was released inside the buildings during each investigation. At the initiation of the investigation, a cylinder of SF6 was set up in a central room of the lowest building level at each building and set to release SF6 at a constant flow rate for the duration of the sampling program. The tracer gas was released for more than 12 h prior to collection of any samples. The building occupants were allowed to conduct all normal activities but were asked to keep windows closed and minimize the opening of doors to the outside. The air flow through the building was calculated as follows: AF =
RRTG × C TG − source C TG − air
Md,outdoor = CAA × AF
Where Md, outdoor is the mass discharge through the building originating from outdoor sources (ug/h or pCi/h) and CAA is the analyte concentration in outdoor air (ug/m3 or pCi/m3). The difference between the two mass discharge values represents the mass discharge originating from indoor and/or subsurface sources. Md,indoor/subsurface = Md,total − Md,outdoor
(4)
The change in mass discharge (specifically Md, indoor/subsurface) between the negative and positive pressure conditions can be used to distinguish between indoor and subsurface sources because the positive pressure condition suppresses the influx of soil gas into the building but does not affect the discharge from indoor sources.
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RESULTS AND DISCUSSION Effect of Variability on Interpretation of Results. For each pressure condition, the chemical concentrations in indoor air were measured at three locations within each of the test buildings to allow for a statistical characterization of the variability within each building. In many cases, high spatial variability was observed within the buildings such that the standard deviation for the three measurements was similar in magnitude to the mean concentration. However, this spatial variability appeared to reflect true spatial differences in VOC distribution rather than random variability. For example, for the ASU House, chlorinated VOC and radon concentrations were consistently higher at Sample Point 2 compared to the other locations for baseline and negative pressure sampling events. Based on other work conducted by the authors, this sample location is more strongly influenced by soil gas entry than the other two indoor air sample locations. The high spatial variability limits the ability to utilize statistical analyses to determine whether the observed changes in concentrations between pressure conditions were greater than expected from random variability. Instead, a more qualitative evaluation is required. For example, in our experience, infiltration of soil gas into buildings commonly results in indoor radon concentrations >2× outdoor concentrations and the USC laboratory can reliably measure a difference of this magnitude. As a result, when the pressure transducer measurements show that pressure control has been attained, the observation of indoor radon concentrations >2× outdoors during the negative pressure condition and within 2× of outdoors during the positive pressure condition at all three measurement locations provides evidence that the pressure control has been successful in controlling the movement of soil gas across the building foundation. Although not as rigorous as formal statistical testing, this type of qualitative analysis is useful for evaluation of highly variable environmental data. When the various types of measurements (i.e., differential pressure, radon, and VOCs) provide consistent results, the data can be reliably interpreted. For ASU House and Moffett Field, the pressure control method was implemented twice consecutively in order to evaluate the consistency of the results. For both buildings, the changes in radon and VOC concentrations in response to changes in controlled building pressure were similar for both
(1)
Where AF is the air flow through the building (m3/h), RRTG is the rate of tracer gas release (m3/h), CTG‑source is the concentration of SF6 tracer gas in the source cylinder (fraction), and CTG‑air is the average concentration of SF6 tracer gas measured in indoor air at the three indoor sample locations (fraction). Mass Discharge. For each target analyte, the mass discharge is the mass per unit time of the analyte moving through the building. The total mass discharge through the building was calculated as follows: Md,total = C IA × AF
(3)
(2)
Where Md, total is the total mass discharge through the building (ug/h or pCi/h), CIA is the analyte concentration in indoor air (ug/m3 or pCi/m3), AF is the air flow through the building (m3/h). The flow of outdoor air into a building is typically much greater than the flow of soil gas into a building (i.e., outdoor air accounts for >90% of the flow). As a result, the mass discharge through the building originating from outdoor 4794
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Table 2. Average Radon Concentration (pCi/L) in Study Buildingsa pressure condition baseline
negative
building
indoor
outdoor
ASU House (round 1) ASU House (round 2) Moffett Field (round 1) Moffett Field (round 2) Travis AFB Jacksonville NAS Parris Island Tinker AFB
0.39 ± 0.06 0.38 ± 0.14 0.65 ± 0.09 1.00 ± 0.05 NM NM NM NM
0.48 0.10 0.12 0.18 NM NM NM NM
positive
indoor
outdoor
± ± ± ± ± ± ± ±
0.18 0.03 0.03 0.26 0.4 0.1 0.11 0.21
2.44 1.87 0.46 0.62 0.67 0.23 0.26 0.25
1.49 1.99 0.20 0.16 0.06 0.15 0.05 0.07
indoor
outdoor
± ± ± ± ± ± ± ±
0.09 0.07 0.05 0.33 0.3 0.1 0.22 0.27
0.03 0.09 0.06 0.30 0.27 0.1 0.38 0.18
0.06 0.07 0.11 0.10 0.06 0.0 0.12 0.03
Note: Radon concentrations in indoor air are average ± standard deviation of single measurements at each of three indoor locations. Outdoor air concentrations are for single measurements.
a
concentrations in indoor air were similar to outdoor concentrations (i.e., 0.4 Pa in the expected direction. For two buildings (Travis AFB and Jacksonville NAS), the average cross-foundation pressure differences were close to zero for both the positive and negative pressure conditions. The magnitude of the crossfoundation pressure difference is most likely a function of the permeability of both the foundation and the underlying soil. For example, when the foundation permeability is high and the soil permeability is low, the cross-foundation pressure difference should be small relative to the pressure difference across the building envelope. Despite the small pressure differences measured across some of the building foundations, the radon results (Table 2) indicate that the differences were sufficient to control the flow of soil gas. Under induced positive pressure conditions, radon concentrations in indoor air were similar to radon concentrations in outdoor air in all five buildings where pressure control was achieved indicating that the positive pressure was sufficient to inhibit significant soil gas flow through the building foundation. Under negative pressure conditions, radon concentrations in indoor air were greater than outdoor concentrations in four of the five buildings and more than twice outdoor concentrations in three of the five buildings. At Tinker AFB, cross-foundation pressure differences were high compared to the other buildings. For the induced negative pressure condition, the average radon concentration in subslab samples was 86.5 pCi/L (see SI Table S.1) and yet radon 4795
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Figure 1. (a) Mass discharge through ASU house normalized to mass discharge during baseline conditions. For each pressure condition, the measured mass discharge (SI Table S2) was divided by the mass discharge under baseline conditions. Solid blue bars show mass discharge from outdoor (A) sources. Hatched red bars show mass discharge from indoor or subsurface (S/I) sources. Data shown are from second round of testing. (b) Mass discharge through moffett building 107 normalized to mass discharge during baseline conditions. For each pressure condition, the measured mass discharge (SI Table S2) was divided by the mass discharge under baseline conditions. Solid blue bars show mass discharge from outdoor sources. Hatched red bars show mass discharge from indoor or subsurface sources. Data shown are from second round of testing. Negative mass discharge values are an artifact of analytical variability.
indoor air were generally similar under negative and positive pressure conditions. When changes were observed, they were often correlated with changes in outdoor concentrations. For the chlorinated VOCs (i.e., VOCs with known subsurface
sources), the concentration response to the controlled pressure conditions was generally similar to radon (i.e., the concentrations in indoor air were generally higher under negative pressure conditions compared to positive pressure conditions). 4796
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Table 3. Average VOC Concentrations (ug/m3) in Study Buildingsa pressure condition baseline building
negative
positive
indoor
outdoor
indoor
outdoor
indoor
outdoor
ASU House (round 1)
Benz: Tol: DCE: TCE:
0.48 2.20 0.13 6.80
± ± ± ±
0.07 0.40 0.02 0.44
0.5 1.5
