Application of CSIA to Distinguish Between Vapor Intrusion and

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Application of CSIA to Distinguish Between Vapor Intrusion and Indoor Sources of VOCs Thomas McHugh,†,* Tomasz Kuder,‡ Stephanie Fiorenza,§ Kyle Gorder,|| Erik Dettenmaier,|| and Paul Philp‡ †

GSI Environmental, Houston, Texas; School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma, United States § BP America, Houston, Texas, United States Hill Air Force Base, Utah, Unites States

)

‡

bS Supporting Information ABSTRACT: At buildings with potential for vapor intrusion of volatile organic chemicals (VOCs) from the subsurface, the ability to accurately distinguish between vapor intrusion and indoor sources of VOCs is needed to support accurate and efficient vapor intrusion investigations. We have developed a method for application of compound-specific stable isotope analysis (CSIA) for this purpose that uses an adsorbent sampler to obtain sufficient sample mass from the air for analysis. Application of this method to five residences near Hill Air Force Base in Utah indicates that subsurface and indoor sources of tricholorethene and tetrachloroethene often exhibit distinct carbon and chlorine isotope ratios. The differences in isotope ratios between indoor and subsurface sources can be used to identify the source of these chemicals when they are present in indoor air.

’ INTRODUCTION At sites where volatile organic chemicals (VOCs) have been released into the environment, such as dry cleaners, gas stations, and industrial sites with historic use of solvents, the migration of these chemicals from the subsurface into buildings, a process known as vapor intrusion, is a significant concern.1 Although the use of chlorinated VOCs has decreased, a variety of consumer products such as hobby craft glues, cleaners, and lubricant sprays still contain trichloroethene (TCE), tetrachloroethene (PCE), and/or other chlorinated VOCs.2 In addition, gasoline, cigarette smoke, and consumer products containing petroleum distillates are sources of petroleum VOCs.3 In residences not affected by vapor intrusion, concentrations of PCE commonly range from 0.9 to 7.4 ug/m34 compared to a U.S. Environmental Protection Agency (USEPA) screening value for residential indoor air of 0.4 ug/m3.5 Concentrations of benzene in indoor air commonly range from 2.5 to 10 ug/m3 compared to a USEPA screening value of 0.3 ug/m3. Due to the ubiquity of indoor and ambient sources, VOCs detected in the air of buildings located in close proximity to the subsurface VOC contamination may have originated from either the subsurface source or from an ambient or indoor source. Compound-specific stable isotope analysis (CSIA) is an analytical method used to measure the ratio of stable isotopes in r 2011 American Chemical Society

specific chemical compounds. CSIA has been used in a wide variety of forensic applications to distinguish between different sources of the same chemical, for example, natural vs synthetic drugs or chemicals manufactured using different synthetic pathways6 or to distinguish different sources of contaminants in groundwater.7 CSIA has been used to characterize PAHs in aerosols emitted from indoor sources;8 however, to our knowledge, CSIA has not previously been used to evaluate the sources of VOCs in indoor air. For VOCs (e.g., TCE or PCE), differences in carbon, chlorine and/or hydrogen isotope ratios have been observed among batches of the same chemical species obtained from various suppliers. These differences in isotope ratios have been attributed to differences in source materials used in synthesis of the VOCs and differences in synthesis process.9
11 When VOCs are released into the environment, their isotope ratios can change over time due to isotope effects associated with in situ degradation, and to a lesser extent, with physical attenuation.7 Biological degradation of VOCs, such as TCE, PCE, and Received: March 24, 2011 Accepted: June 8, 2011 Revised: June 3, 2011 Published: June 08, 2011 5952

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Environmental Science & Technology benzene, typically results in an enrichment of the heavy isotope species for the remaining undegraded substrate,12,13 while various physical remediation processes (e.g., volatilization) may result with enrichment or depletion of the heavy isotopes in the remaining substrate.14
17 As a result, VOCs at historically contaminated sites commonly differ in their isotope ratios from those values reported for the original manufactured products. At sites where the isotope ratios for the subsurface source of the target analyte is distinct from the isotope ratios of potential indoor sources owing to past in situ degradation or physical transformation, CSIA is expected to be particularly useful for distinguishing between vapor intrusion and indoor sources of VOCs. CSIA techniques applied to VOCs rely on conventional gas chromatography for separation of the target analytes prior to their isotope ratio determination. To date, the best published CSIA quantitation limits for VOCs in environmental matrices required a minimum of several tens up to several hundreds ng of the target analyte to be introduced into the gas chromatographic column.7 For indoor air samples, attainment of this mass range can require a sample volume of more than 100 L. As a result, adsorbent tubes offer the most practical means to collect the sample for transport to the analytical laboratory. Use of adsorbent tubes for collection of vapor-phase samples, as with any phase transfer, has the potential to result in isotopic fractionation if the transfer efficiency is less than 100%. For this project, we conducted a laboratory study to evaluate the potential for isotope effects associated with the use of the adsorbent tube samplers. This was followed by application of CSIA to distinguish between vapor intrusion and indoor sources of VOCs at five residences located near Hill Air Force Base in Utah.

’ MATERIALS AND METHODS Collection of Field Samples. Samples for CSIA were collected from five residences located near Hill AFB in Utah (Supporting Information (SI) Figure S1). Four of the five residences were located above groundwater contaminated by TCE and other chlorinated VOCs. The fifth residence was connected to a sanitary sewer line that received discharge water containing chlorinated VOCs. Each of the residences was selected for testing based on the prior detection of TCE (three residences) or PCE (two residences) in indoor air during sampling conducted by Hill AFB. The previously detected compound (i.e., either TCE or PCE) was the target analyte for the sampling program. At each residence, the sample collection program included collection of at least one indoor air sample and at least one of the following subsurface samples: soil gas, sewer headspace gas, and/or groundwater sample. Prior to initiation of indoor air sample collection, the target analyte concentration in indoor air was measured onsite using a field portable GC/MS (HAPSITE ER by Inficon, Syracuse, NY). Based on the measured concentration, the sample collection volume was established to achieve a target sample mass of 100 ng to 300 ng (however, for a few samples, where target analytes were present at low concentrations, the final mass collected in adsorbent tubes was below 100 ng). The target VOC was collected by pumping indoor air through an adsorbent tube using calibrated pumps (AirCheck XR5000 by SKF, Inc.) at a flow rate of 90 mL/min as described in USEPA Method TO-17.18 The sample collection time generally ranged from 6 to 28 h depending on the concentration of the target analyte in indoor air.

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Several replicates were collected simultaneously to provide material for duplicate runs and for determination of both carbon and chlorine isotope ratios. The glass adsorbent tubes were 6 mm OD, 4 mm i.d., and 11.25 cm long. The tubes were fritted at one end and packed with a short bed of quartz wool, a 2.5 cm bed of Tenax GR, a separating bed of quartz wool, a 2.5 cm bed of Carboxen 569, then a retaining bed of quartz wool, held in place by stainless steel gauze and a retaining spring. For a subset of samples, a back-up tube was placed in series behind the primary tube to check for breakthrough of sample during collection. Adsorbent tubes for CSIA were shipped at ambient temperature (transit times varied between 2 and 6 days) to the University of Oklahoma and then refrigerated prior to CSIA. The effect of sample handling will be further discussed in the section on the sorbent validation study. Back-up tubes for evaluation of breakthrough were shipped to Beacon Environmental Services in Bel Air, Maryland. The back-up tubes were analyzed for chlorinated VOCs by USEPA Method TO-17. No VOCs were detected on any backup tubes (detection limit = 5 ng) indicating an absence of VOC break-through during sample collection. Soil gas samples were collected into Tedlar bags and later transferred onto adsorbent tubes. Each soil vapor point was purged of three or more line volumes of gas using a 60 mL plastic gastight syringe connected to the sample point using a three-way valve. Soil vapor was then pumped into one or more 5 L Tedlar bags using a 1 L glass syringe. The VOCs concentrations in each 5 L Tedlar were measured on-site using the portable GC/MS as described above for indoor air samples. Using the concentration results, the soil vapor samples were transferred to adsorbent tubes using the procedures described above for indoor air sample to achieve a mass of the target analyte of between 100 ng and 300 ng. Groundwater samples were collected in 40 mL VOA vials and preserved with hydrochloric acid for carbon CSIA or with sulfuric acid for chlorine CSIA. The samples were stored on ice and shipped to the University of Oklahoma for CSIA. Stable Isotope Analysis of TCE and PCE in the Environmental Samples. The carbon isotope ratios of TCE and PCE were determined by a PT-GC-IRMS (purge-and-trap-gas chromatography
isotope ratio mass spectrometry) protocol, adopted from the methods used in the University of Oklahoma in CSIA of VOCs.16,19 VOCs in aqueous samples were preconcentrated following the purge and trap (PT) approach described elsewhere.16 VOCs in sorbent tubes were analyzed after thermal desorption onto a standard PT unit (Eclipse 4660, OI Analytical) equipped with a thermal desorption accessory (OI Analytical). The accessory replaces the PT sparger and permits samples collected onto adsorbent tubes to be analyzed using the PT protocol. The tube desorption temperature was set to 350 °C (maximum recommended for Tenax GR). After thermal desorption, VOCs collected on the Eclipse’s trap were transferred onto a GC-IRMS (Varian 3400 gas chromatograph
Thermo Finnigan 252 isotope ratio mass spectrometer) and the analysis proceeded as for VOCs recovered from a water sample. The chlorine isotope ratios of TCE and PCE were determined by a PT-GC-MS (purge-andtrap-gas chromatography
mass spectrometry) protocol, utilizing a standard quadrupole MS (Agilent 7890, Agilent 5790) in SIM (selective ion monitoring) mode, following the principles described by Sakaguchi et al.20,21 Chromatographic peaks of TCE or PCE were introduced into the GC from the PT transfer line interfaced into the split
splitless injector (split  10 to 20 was used). To calibrate the 37Cl/35Cl of the target analytes, the PT 5953

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Table 1. Isotope Ratios for TCE and PCE in Consumer Products δ13C% (VPDB)a

product

a c

δ37Cl% (SMOC)b

Ace Premium Lub-E lubricant (11 oz. spray can)

PCE
32.2 ( 0.3 (n = 3)

PCE
0.3 ( 0.1

Brakleen Brake Parts Cleaner (19 oz. spray can)

PCE
25.0 ( 0.2 (n = 3)

PCE
1.4 ( 0.3 TCE: 4.5

Birchwood Casey Gun Scrubber (16 oz. spray can)

TCE:
31.1 ( 0.3 (n = 3)

E6000 glue (2 fl. oz. squeeze tube)

PCE:
34.2 ( 0.2 (n = 3)

PCE:
4.4 ( 0.1

E6000 glue (3.7 fl. oz. squeeze tube)

PCE:
37.4 ( 0.2 (n = 3)

PCE:
2.6 ( 0.3

Alleenes 7800 glue (2 fl. oz. squeeze tube)

PCE:
31.9 ( 0.2 (n = 3)

PCE: 0.4 ( 0.2

Sprayway 706 Brake Parts Cleaner (20 oz. spray can)c

TCE:
31.8 ( 0.3 (n = 5)

TCE:
0.1

Sprayway 706 Brake Parts Cleaner (20 oz. spray can)c Smith & Wesson Heavy Duty Bore Cleaner (10 oz. spray can)

PCE:
36.5 ( 0.2 (n = 5) PCE:
36.7 ( 0.2 (n = 3)

PCE:
3.3 ( 0.4 PCE:
2.4 ( 0.1

Eezox Premium Gun Care (3 oz. spray can)

TCE:
24.0 ( 0.1 (n = 3)

TCE: 3.2 ( 0.3

Eezox Premium Gun Care (4 fl. oz. can)

TCE:
24.1 ( 0.2 (n = 5)

TCE: 3.1

Eezox Premium Gun Care (1.5 oz. drip bottle)

TCE:
23.2 ( 0.1 (n = 3)

TCE: 4.7 ( 0.9

Sprayway C-60 (16 oz. spray can)

TCE:
29.8 ( 0.3 (n = 8)

TCE:
3.2 ( 0.4

Carbon isotope ratios reported with standard deviations of n replicates. b Chlorine isotope ratios reported with standard deviations of n = 2, if applicable. The same product contained both TCE and PCE.

peaks were bracketed by several coinjected peaks of TCE or PCE standard, as applicable. The 37Cl/35Cl of the standard compounds was previously determined offline by the MeCl method. The indoor air samples, and also some of the soil vapor samples, contained interfering nontarget VOCs that could not be chromatographically resolved from TCE. Carbon CSIA of those indoor air samples required 2-D GC separation, utilizing a polar phase (DB-Wax) and a nonpolar phase (DB-MTBE) in a sequence, similar to a previous CSIA study of ethylene dibromide occurring in the presence of gasoline hydrocarbons.7,22 Chlorine CSIA did not require 2-D GC because coeluting compounds did not produce ions interfering with those of TCE. δheavy E ¼ ðR sample =R standard
1Þ  1000

ð1Þ

Isotope ratios for element E are presented using delta (δ) notation (eq 1). The heavy/light isotope ratio (R = heavyE/lightE, for example, 13C/12C or 37Cl/35Cl) is normalized against the appropriate isotope ratio international standards (VPDB and SMOC, respectively) and reported in parts per thousand (%). The analytical error (precision defined by sample duplicates, accuracy defined by TCE/PCE standards of known isotope composition processed and analyzed as described above for the groundwater or thermal desorption tube samples) was (0.7 % or better for δ13C and (1 % or better for δ37Cl. Commercial Product Survey. The commercial products (miscellaneous solvents/degreasers and glues) evaluated in this study were purchased in 2010 from retailers in the U.S. Carbon isotope ratios of the liquid products were determined by split injection of the liquid product diluted in pentane onto a GCIRMS instrument (Agilent 6890, Thermo Finnigan XL). To ensure that the measured ratios were accurate, a TCE+PCE standard was run before and after each product sample. The glue samples were dissolved in methylene chloride and then diluted in pentane. Pentane addition precipitated resin present in those samples. The resin was allowed to settle and the samples for injection were taken from the clear supernatant solution. Chlorine isotope ratios of the liquid products were determined by purge and trap-GC/MS. Small aliquots of the products dissolved in methanol were mixed, spiked into clean dilution water, and analyzed following the protocol described above. In addition, carbon isotope ratios of vapor escaping from the commercial

product containers were determined by placing each product in a sealed glass jar connected to the PT sparger inlet. A product container in normal storage position was sealed in the jar, and aliquots of headspace were introduced in succession after progressively increasing storage time. Vapor release rates for the same containers were screened in a separate experiment prior to isotope analysis, to allow accurate calculation of the volumes of headspace required to deliver a fixed mass of analyte. Three consecutive headspace injections followed by CSIA were performed from each storage jar. The injected volumes were reduced as the analyte concentrations in the jar increased over time. This setup would permit detecting transient isotope effects associated with the initial stage of vapor release, if any. Once the VOCs were introduced into the PT device, the analytical protocol was identical to that used for standard PT-GC-IRMS analysis of the same analytes. Adsorbent Validation. A laboratory study was conducted to evaluate the potential for isotope fractionation associated with the use of adsorbent tubes for sample collection. Full mass of the analyte in concentrated aqueous solution was injected into the adsorbent tube, followed by drawing a volume of air (i.e., a “challenge volume”) at 40 and 100 mL/min. The air was humidified to 60% at 23
24 °C. The mass of the water injected with the target VOC was negligible compared to that introduced in humidified air. The key variable tested was the challenge volume, to identify the potential problems caused by sampling a large volume of air. Additionally, data was collected to determine the effects of the time elapsed between sampling and analysis, the temperature of storage prior to analysis, the air humidity (varied from 30 to 90% at 23
24 °C) and the presence of nontarget VOCs. The mass of the nontarget VOCs (methylene chloride, ethanol, 2-propyl alcohol, toluene and p-xylene, at weight ratios of 27:29:29:7:7) exceeded the mass of the target VOCs by a factor of 250. Tubes loaded with the target compounds were analyzed following the CSIA protocols described above.

’ RESULTS AND DISCUSSION Commercial Product Survey. The carbon isotope ratios of TCE and PCE present in the commercial products (Table 1) approximated the normal δ13C range observed in the petrochemical precursors of those compounds.7 The chlorine isotope 5954

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Figure 1. Carbon isotope ratios of TCE (normalized to initial δ13C = 0%) injected on Tenax GR/Carboxen 569, flushed by variable air volume (challenge volume), analyzed within 24 h: TCE at relative humidity H = 60% ((); H = 30% (+); H = 90% (0); TCE at H = 60% and with other VOCs in sample ()). Additional replicate tubes loaded at H = 60% were analyzed after 18 days hold time at room temperature (b) and after 18 days at 4 °C (O).The solid line is an empirical regression line for the H = 60% data over the natural log of the challenge volume, with a slope of 0.41 ( 0.08 (R2 = 0.7, n = 52) The Y-axis error bar represents the range of normal analytical uncertainty of carbon CSIA.

ratios in the same products (Table 1) were similar to the range of chlorine isotope ratios in TCE and PCE reported to date.7 It is noteworthy that differences of several % units were observed for different containers of the same commercial products (e.g., as apparent for E6000 glue). Such differences demonstrate that specific isotope fingerprints cannot be attributed to a given category of products (e.g., E6000 glue) but only to a specific batch of containers manufactured at a specific time at a specific facility. As a result, the full range of isotope ratio values from multiple products provides the most appropriate bracket for evaluation of unknown indoor sources. Considering that both carbon and chlorine isotope ratios of TCE and PCE span several % units, the strongest discrimination between vapor intrusion (subsurface sources) and indoor VOCs sources will require that the isotope ratios of the target VOCs in the subsurface source fall outside the range of ratios defined for commercially available consumer products containing the target analyte. This is most likely to occur at sites where biodegradation resulted in enrichment of the heavier isotopes (for example, the case of Residence 1 in this study). The carbon isotope ratios of TCE or PCE vapor emitted from the closed product containers were consistently depleted in 13C relative to the liquid products, but the average δ13C difference between the liquid product and the vapors was only 0.3 ( 0.1% (SI Table S1). These results were in good agreement with the prior observed isotope effect associated with a combination of phase equilibrium and vapor diffusion.16 Cl isotope ratios of vapor were not determined, but the difference in δ37Cl between liquid product and vapor can be estimated at
1 to
2% following the same mechanism of fractionation, where the net isotope effect is augmented by both diffusive elements and phase equilibrium favoring volatilization of the lighter 12C molecules.16 The carbon isotope effect was within the stated uncertainty of (0.5%, and the chlorine isotope effect was within or near to the stated uncertainty of (1%. Sorbent Tube Validation for CSIA. Use of Tenax GR/ Carboxen 569 adsorbent tubes to collect vapor-phase samples

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Figure 2. Carbon isotope ratios for TCE in consumer products (from Table 1 and ref 7), Residence 1 indoor air, and groundwater samples from two nearby monitoring wells. Error bars for indoor air sample represent uncertainty associated with analytical precision ((0.7%), potential fractionation associated with large sample volume (
1.5%, i.e., the maximum observed in the sorbent validation study for 100 L sample, cf. Figure 1), and storage of sample at room temperature for three days (
1.0%, extrapolated from the maximum observed in the sorbent validation study for 18 days of storage, under assumption that the effect is correlated linearly with the time of storage, cf., Figure 1). The analytical precision for the groundwater samples is (0.5% and no other sources of bias or uncertainty were identified for these samples.

for CSIA did not result in a measurable change in carbon or chlorine isotope ratios for PCE or TCE under a baseline sampling condition of 2.5 L sample volume at 60% humidity for samples analyzed within 24 h. Figure 1 summarizes carbon isotope fractionation for TCE samples collected under a wider range of conditions. The measured carbon isotope ratios for TCE were affected by challenge volume, in that fractionation increased logarithmically with the challenge volume, and holding time at room temperature (but not at 4 °C). In both cases, the measured isotope ratios were enriched in 13C relative to the original sample. There was no difference between sampling at 40 mL/min vs 100 mL/min (data not shown). Based on limited testing, variations in humidity and the presence of nontarget VOCs in the sample matrix do not appear to cause a large fractionation effect. Increased concentrations of water vapor and nontarget VOCs appeared to reduce the fractionation associated with large challenge volume. Unlike TCE, PCE fractionation caused by the sorption
desorption process was negligible, independent of the sample volume and holding time (SI Figure S2). In the case of chlorine isotopes in TCE, there was no discernible fractionation connected to sampling volume (SI Figure S3). The observed fractionation of TCE might be explained by the adsorption properties of Tenax GR and Carboxen 569, specifically, the fractionation of TCE could be attributed to strong retention on the Carboxen 569 bed23 resulting in less than 100% recovery of TCE from this adsorbent. A standard practice of refrigeration of the sorbent tubes immediately after sampling and shipping the samples while refrigerated is proposed to reduce the potential of isotope ratio artifacts similar to those described above. As shown in Figures 2 and 3, no attempt was made to calculate accurate data correction based on the sorbent validation data. Instead, the maximum error was estimated (maximum sampling volume combined with the effect of sample storage) and applied as a default error bar for the data. While it might be tempting to use 5955

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Figure 3. Carbon and chlorine isotope ratios for PCE or TCE in samples collected in and around four residences: Indoor air (b), groundwater (0), soil gas (), sewer headspace gas (4), and PCE-containing glue found in Residence 3 (+). Dashed box shows range of isotope ratios measured for commercially available consumer products, see Table 1 and ref 7. For carbon isotope ratios: the error bars for gas-phase samples represent uncertainty associated with analytical precision ((0.7% for TCE and PCE), potential fractionation associated with large sample volume (up to
1.5% for TCE), and storage of sample at room temperature for three days (
1.0% for TCE). See Figure 2 for additional details on the uncertainty determination. For chlorine isotope ratios: the error bars for gas-phase samples represent uncertainty associated with analytical precision ((1.0% for TCE and PCE). The analytical precision for the groundwater samples is (0.5% for carbon isotope ratios and (1% for chlorine isotope ratios and no other sources of bias or uncertainty were identified for these samples.

the apparent logarithmic relationship between the sampling volume and the isotope ratio bias, such correction would be problematic due to the complexity of the process. Discrimination of Vapor Intrusion from Indoor VOC Sources. To evaluate the utility of CSIA to distinguish between vapor intrusion and indoor sources of VOCs, we investigated five residences near Hill AFB in Utah. The sampling program included a total of six water samples, six soil gas, two sewer gas headspace, six indoor air samples, and one indoor source sample that were all collected and analyzed for carbon and/or chlorine stable isotope ratios. The five residences were selected from the routine Hill AFB vapor intrusion monitoring program area based on prior detections of PCE or TCE in indoor air. For each residence, one or more subsurface samples were collected in the vicinity of the residence. The goal of the analyses was to use observed differences in carbon and chlorine isotope ratios between indoor and subsurface samples and in one case, of a consumer product identified on site as the suspected PCE source, to identify the likely source of the target VOC in indoor air. Figures 2 and 3 summarize the results from the residences tested. The results are shown without correction for the fractionation observed during the laboratory validation study for TCE associated with high sample collection volumes. It is possible that the TCE values reported are biased by up to +2% for the largest volume indoor air samples (i.e., the true δ13C values would be more negative), due to isotope

fractionation increasing with the sample volume and the unrefrigerated state of the tubes during transit. The error bars shown in Figures 1 and 2 represent the worst case scenario: the maximum expression of the fractionation related to increased sampling volume and unrefrigerated storage of the samples (during transit), and do not include any mitigating effects from humidity and nontarget VOCs that were also observed during the laboratory study. While the conclusions regarding the source of TCE in indoor air samples are not compromised by this modest uncertainty, the accuracy of future CSIA evaluations for TCE would be improved by identifying and using adsorbent media that do not show this fractionation effect. As demonstrated by good performance of the selected sorbent combination for PCE (SI Figure S2), the problem of fractionation can likely be eliminated by selection of an adsorbent with appropriate affinity for a given target analyte. The measured carbon and chlorine isotope ratios for TCE and PCE in indoor air samples were interpreted by comparing these values to (i) the range the isotope ratios observed in the unaltered commercial products and (ii) the ratios measured in subsurface samples matched to the individual residences. As compared to isotope ratios in unaltered product, isotope ratios of VOCs in subsurface samples will often show enrichment of heavy isotope species due to biological degradation in the saturated or vadose zone. If the isotope ratio for the target VOC in the indoor air sample is similar to the isotope ratio in the subsurface sample 5956

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Environmental Science & Technology and is enriched in the heavy isotope species relative to the range observed for manufactured products, then this provides evidence for the influx of degraded VOCs from subsurface, that is, vapor intrusion. When the isotope ratio of the target VOC in the indoor air sample is within the range observed for commercial products and depleted in the heavy isotope relative to the target VOC in matched subsurface samples, this provides evidence that the target VOC in indoor air originates from an above ground source. The strength of either interpretation depends on the extent of characterization of the range of variability in isotope ratios in manufactured products and in the site-specific subsurface source. While volatilization of previously degraded VOCs into the vadose zone and/or degradation of VOCs in the vadose zone create pools of vapor enriched in the heavy isotopes, several physical processes are known to create an opposite effect. For example, in volatilization scenarios dominated by vapor diffusion, the lighter isotope species are preferentially removed, that is, vapor phase will be enriched in the lighter isotopes.16 The magnitudes of such isotope depletion tend to be relatively insignificant in most volatilization scenarios; however, there is a potential for transient effects in gas diffusion, where the front of expanding vapor plume is significantly depleted in the heavier isotope species.14,15 Based on calculated vapor diffusion coefficients for carbon and chlorine isotope species (ref 16 and references therein), one diagnostic feature of such expanding vapor plume would be a depletion of the 37Cl species, exceeding corresponding depletion of the 13C species. No such phenomena were observed in any of the investigated sites. The most likely explanation of the indoor presence of VOCs depleted in heavy isotopes relative to the subsurface source is an alternate indoor VOC source rather than a fractionation effect associated with transport from the subsurface into the building. Prior to CSIA sample collection, Residence 1 and Residence 5 were suspected of having soil vapor intrusion as the primary source of TCE in the indoor air because previous characterization efforts did not identify household products containing TCE. For Residence 4, the primary source of TCE was suspected to be migration of TCE vapors from the sanitary sewer line. Groundwater in the vicinity of Residence 4 is not known to contain TCE; however, the sanitary sewer line serving Residence 4 receives discharge water that contains TCE and other VOCs. Residence 2 and Residence 3 were suspected of having indoor sources of PCE prior to the CSIA sampling program because PCE was not known to be present in the subsurface in close proximity to these residences and no cases of PCE vapor intrusion have been documented for other residences in this area. At Residence 1 (Figure 2) the value of δ13C for TCE in the indoor air was similar to the values for TCE in the two matched groundwater samples. The δ13C of TCE in the indoor air sample was also much heavier than the range measured for TCEcontaining consumer products. For Residence 1, δ37Cl was not measured; however, the carbon isotope results alone strongly support the conclusion that vapor intrusion is the primary source of TCE in indoor air at Residence 1, confirming the preliminary conceptual model for the residence. PCE is detected only sporadically in groundwater near to Residence 2 and Residence 3. The closest monitoring well with consistent detections of PCE is at a distance of between one and two kilometers (U5
141; SI Figure S1). For Residence 2 (Figure 3A), there was little difference in δ37Cl for PCE in the indoor air and groundwater samples. However, the carbon isotope ratio for the indoor air sample was depleted by 6% relative

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to that of the matched groundwater sample and was within the range measured for consumer products containing PCE, a result that is consistent with an indoor source of PCE in Residence 2. Although the carbon isotope ratio for the groundwater sample was also within the range for consumer products, the observation that the value of δ13C for the indoor air was significantly more negative than that of the subsurface sample provides evidence for an indoor source. At Residence 3 (Figure 3B), a search of the residence identified a tube of E6000, a PCE-containing glue commonly used for hobby crafts. The tube of glue was placed in a sealed glass jar and a headspace sample was collected on an adsorbent tube. Carbon and chlorine isotope ratios of PCE in the indoor air closely matched the isotope ratios of the E6000 but were significantly lighter than the isotope ratios for the groundwater sample collected from the matching groundwater monitoring well. These results indicated that the source of PCE in Residence 3 was the E6000 glue. Even without locating the indoor source, the isotope results would have been supportive of an indoor source because the isotope ratios for the indoor air sample were more negative than the isotope ratios for the subsurface sample. For Residence 4 (suspected sewer source), the carbon and chlorine isotope ratios for TCE in indoor air, sewer headspace, and three groundwater samples representative of the discharge into the sewer line were all similar (Figure 3C). Although the carbon and chlorine isotope ratios for all of these samples were within the range measured for consumer products, they are at the heavy end of the range. The close match in isotope ratios for all indoor air and the subsurface samples is consistent with the existing conceptual model that the sewer line is the primary source of TCE in the residence. However, these results cannot eliminate the possibility of an indoor source. For Residence 5 (Figure 3D), a relatively wide range in carbon and chlorine isotope ratios was measured for three groundwater samples collected in the vicinity of the residence (the groundwater sample collected closest to the residence was showing the least enrichment in the heavy isotopes). The carbon isotope ratios for the soil gas were somewhat heavier than the closest groundwater sample but the carbon isotope ratios for the two indoor air samples were somewhat more negative. The chlorine isotope ratios for Residence 5 showed no clear pattern. As a whole, the isotope results for Residence 5 were inconclusive with respect to the source of TCE in indoor air but were suggestive of a contribution from an indoor source. The results obtained confirm that CSIA can be useful for distinguishing between vapor intrusion and indoor sources of VOCs. For two of the five residences included in the study (i.e., Residences 1 and 3), the CSIA results alone were sufficient to provide a definitive identification of the VOC source (i.e., vapor intrusion at Residence 1 and indoor source at Residence 3). At two additional residences (i.e., Residences 2 and 4), the results provided a strong indication of the likely source. The CSIA results were inconclusive at only one of the five residences. At other sites, CSIA is most likely to be useful for distinguishing between vapor intrusion and indoor sources of VOCs when the isotope ratios for subsurface samples of the target VOC fall outside the range observed for consumer products. However, as seen from this study, useful results may also be obtained for sites where the isotope ratios for the subsurface samples fall inside the range for consumer product, but near the upper end of this range. Prior to implementing a large-scale study, the site-specific utility of CSIA for the vapor intrusion pathway can be evaluated by 5957

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’ ASSOCIATED CONTENT

bS

Supporting Information. A map of sampling locations is provided as Figure S1. Figures S2 and S3 show additional results on sorbent validation. Table 1S shows carbon isotope ratios of TCE and PCE vapor emitted from the product containers. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone 713-522-6300; e-mail: [email protected].

’ ACKNOWLEDGMENT This work was funded by the U.S. Air Force Center for Engineering and the Environment Broad Agency Announcement (Contract 09-C-8016) and the U.S. Department of Defense Environmental Security Technology Certification Program (Project ER-201025). The authors thank Dr. Paul Johnson and his research team at Arizona State University for providing access to the ASU Vapor Intrusion Research House (identified as Residence 5 in this paper) and associated property for sample collection. We thank Roger Lee of Sims and Associates for assistance with the soil gas sample collection. We also thank Linnea Heraty and Neil Sturchio at the University of Chicago for help in calibration of chlorine isotope ratios of the standard TCE and PCE. Finally, we thank two reviewers for helpful comments that improved the clarity of the manuscript. ’ REFERENCES (1) Fitzgerald, J. One regulatory perspective on the vapor intrusion pathway. Ground Water Monit. Rem. 2009, 29, 51–52. (2) Hers, I.; Zapf-Gilje, R.; Li, L.; Atwater, J. The use of indoor air measurements to evaluate intrusion of subsurface VOC vapors into buildings. J. Air Waste Manage. Assoc. 2001, 51, 1318–1331. (3) Williams, P. R. D.; Panko, J. M.; Unice, K.; Brown, J. L.; Paustenbach, D. J. Occupational exposures associated with petroleumderived products containing trace levels of benzene. J. Occup. Environ. Health 2008, 5, 565–574. (4) Dawson, H. E.; McAlary, T. A compilation of statistics for VOCs from post-1990 indoor air concentration studies in North America residences unaffected by subsurface vapor intrusion. Ground Water Monit. Rem. 2009, 29, 60–69. (5) USEPA. Regional screening levels for chemical contaminants at superfund sites. http://www.epa.gov/reg3hwmd/risk/human/rb-concentration_table/index.htm (accessed January 11, 2011). (6) Meier-Augenstein, W. An Introduction to the Forensic Application of Stable Isotope Analysis; John Wiley & Sons: West Sussex, UK, 2010. (7) A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants using Compound Specific Isotope Analysis (CSIA), EPA 600/R-08/148; U.S. Environmental Protection Agency: Washington, DC, 2008.

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