Kyle Gorder

Evaluation of Vapor Intrusion Using Controlled Building Pressure

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.

Application of CSIA to distinguish between vapor intrusion and indoor sources of VOCs.

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.

Microfabricated Gas Chromatograph for On-Site Determinations of TCE in Indoor Air Arising from Vapor Intrusion. 2. Spatial/Temporal Monitoring

We demonstrate the use of two prototype Si-microfabricated gas chromatographs (μGC) for continuous, short-term measurements of indoor trichloroethylene (TCE) vapor concentrations related to the investigation of TCE vapor intrusion (VI) in two houses. In the first house, with documented TCE VI, temporal variations in TCE air concentrations were monitored continuously for up to 48 h near the primary VI entry location under different levels of induced differential pressure (relative to the subslab). Concentrations ranged from 0.23 to 27 ppb by volume (1.2-150 μg/m(3)), and concentration trends agreed closely with those determined from concurrent reference samples. The sensitivity and temporal resolution of the measurements were sufficiently high to detect transient fluctuations in concentration resulting from short-term changes in variables affecting the extent of VI. Spatial monitoring showed a decreasing TCE concentration gradient with increasing distance from the primary VI entry location. In the second house, with no TCE VI, spatial profiles derived from the μGC prototype data revealed an intentionally hidden source of TCE within a closet, demonstrating the capability for locating non-VI sources. Concentrations measured in this house ranged from 0.51 to 56 ppb (2.7-300 μg/m(3)), in good agreement with reference method values. This first field demonstration of μGC technology for automated, near-real-time, selective VOC monitoring at low- or subppb levels augurs well for its use in short- and long-term on-site analysis of indoor air in support of VI assessments.

Identification of Alternative Vapor Intrusion Pathways Using Controlled Pressure Testing, Soil Gas Monitoring, and Screening Model Calculations

Vapor intrusion (VI) pathway assessment and data interpretation have been guided by an historical conceptual model in which vapors originating from contaminated soil or groundwater diffuse upward through soil and are swept into a building by soil gas flow induced by building underpressurization. Recent studies reveal that alternative VI pathways involving neighborhood sewers, land drains, and other major underground piping can also be significant VI contributors, even to buildings beyond the delineated footprint of soil and groundwater contamination. This work illustrates how controlled-pressure-method testing (CPM), soil gas sampling, and screening-level emissions calculations can be used to identify significant alternative VI pathways that might go undetected by conventional sampling under natural conditions at some sites. The combined utility of these tools is shown through data collected at a long-term study house, where a significant alternative VI pathway was discovered and altered so that it could be manipulated to be on or off. Data collected during periods of natural and CPM conditions show that the alternative pathway was significant, but its presence was not identifiable under natural conditions; it was identified under CPM conditions when measured emission rates were 2 orders of magnitude greater than screening-model estimates and subfoundation vertical soil gas profiles changed and were no longer consistent with the conventional VI conceptual model.

On-Site Gas Chromatography/Mass Spectrometry (GC/MS) Analysis to Streamline Vapor Intrusion Investigations

Distinguishing between vapor intrusion and indoor sources of volatile organic compounds (VOCs) is a significant challenge in conventional vapor intrusion assessments. For this research project, the authors developed a step-by-step protocol to streamline building-specific investigations by using on-site gas chromatography/mass spectrometry (GC/MS) analysis and building pressure manipulation to determine the source of VOCs in indoor air during a 1-day field investigation. Protocol validation included implementation in industrial buildings and testing alongside conventional methods. The new protocol compares favorably to conventional approaches, yielding more definitive results in less time. This article presents three case studies which illustrate application of the protocol.

Long-Term Evaluation of the Controlled Pressure Method for Assessment of the Vapor Intrusion Pathway

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.

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

It is accepted that indoor sources of volatile organic compounds can confound vapor intrusion (VI) pathway assessment. When they are discovered during pre-sampling inspection, indoor sources are removed and air sampling is delayed, with the assumption that a few hours to a few days are sufficient for indoor source impacts to dissipate. This assumption was tested through the controlled release of SF6 and its monitoring in indoor air and soil gas at a study house over 2 years. Results show that indoor sources generate subsurface soil gas clouds as a result of fluctuating direction in the exchange between soil gas and indoor air and that it may take days to weeks under natural conditions for a soil gas cloud beneath a building to dissipate following indoor source removal. The data also reveal temporal variability in indoor air and soil gas concentrations, long-term seasonal patterns, and dissipation of soil gas clouds over days to weeks following source removal. Preliminary modeling results for similar conditions are consistent field observations. If representative of other sites, these results suggest that a typical 1-3 day waiting period following indoor source removal may not be sufficient to avoid confounding data and erroneous conclusions regarding VI occurrence.

Emission rates of chlorinated volatile organics from new and used consumer products found during vapor intrusion investigations: Impact on indoor air concentrations

Abstract Consumer products can emit chlorinated volatile organic compounds (CVOCs) that complicate vapor intrusion (VI) assessments. Assessment protocols acknowledge the need to remove these products during VI investigations, but they can be problematic to identify and locate. Predicting if the products cause detectable air concentrations is also difficult since emission rate information is limited and can vary with product use and age. In this study, the emission rates of 1,2-dichloroethane, trichloroethene, tetrachloroethene, and carbon tetrachloride from four consumer products identified as indoor sources during VI field investigations were measured under laboratory conditions using a flow through system. Emissions of PCE from an adhesive container tube ranged from 1.33 ± 1.13 μg/min (unopened) to 23.9 ± 2.93 μg/min (previously opened). The laboratory-measured emission rates were used to estimate indoor air concentrations, which were then compared to concentrations measured after the products placed were into an actual residence. The estimated and measured indoor air concentrations were generally comparable, showing that emission rate information can be used to determine the relative impact of internal CVOC sources.