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.

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.

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.

Temporal variability of indoor air concentrations under natural conditions in a house overlying a dilute chlorinated solvent groundwater plume.

Current vapor intrusion (VI) pathway assessment heavily weights concentrations from infrequent (monthly-seasonal) 24 h indoor air samples. This study collected a long-term and high-frequency data set that can be used to assess indoor air sampling strategies for answering key pathway assessment questions like: "Is VI occurring?", and "Will VI impacts exceed thresholds of concern?". Indoor air sampling was conducted for 2.5 years at 2-4 h intervals in a house overlying a dilute chlorinated solvent plume (10-50 µg/L TCE). Indoor air concentrations varied by 3 orders of magnitude (<0.01-10 ppbv TCE) with two recurring behaviors. The VI-active behavior, which was prevalent in fall, winter, and spring involved time-varying impacts intermixed with sporadic periods of inactivity; the VI-dormant behavior, which was prevalent in the summer, involved long periods of inactivity with sporadic VI impacts. These data were used to study outcomes of three simple sparse data sampling plans; the probabilities of false-negative and false-positive decisions were dependent on the ratio of the (action level/true mean of the data), the number of exceedances needed, and the sampling strategy. The analysis also suggested a significant potential for poor characterization of long-term mean concentrations with sparse sampling plans. The results point to a need for additional dense data sets and further investigation into the robustness of possible VI assessment paradigms. As this is the first data set of its kind, it is unknown if the results are representative of other VI-sites.

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.

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.

Trichloroethylene uptake into fruits and vegetables: three-year field monitoring study.

Trichloroethylene (TCE) contaminated groundwater migrating into communities surrounding Hill Air Force Base (HAFB) in northern Utah prompted a multiyear monitoring program (2001-2003) to examine the extent of TCE uptake and transfer into edible fruits. During the initial sampling in fall 2001, TCE was detected in a small fraction of the 167 fruit and tree core samples collected from 17 private residences. Samples were analyzed using headspace gas chromatography (GC) with electron capture detection (ECD) with limited confirmation by mass spectrometry (MS) in selected ion monitoring mode. In fall 2002, over 300 samples were collected from the same general locations sampled in 2001. No TCE was found in any of the fruit or vegetable samples above the method detection limit (MDL) for the headspace GC/MS method (approximately 0.1 microg/ kg fresh weight, depending on sample size), but TCE was again detected in several fruit tree trunk core samples. The detection of TCE in fruit in 2001, but not in 2002, may have been due to improvements in the analytical procedure or changes in the environmental conditions impacting transfer to fruit. The 2003 monitoring focused on repeated sampling over several months at five locations that were selected to represent the range of exposure scenarios evaluated during the previous years. No TCE was identified in any of the fruit above the MDL during 2003, however TCE was again found in tree core samples as observed in 2001 and 2002.

Trichloroethylene uptake by apple and peach trees and transfer to fruit.

A greenhouse study was conducted to quantify 14C-trichloroethylene (TCE) uptake and transfer into the edible fruit of apple and peach trees. Trees were subsurface irrigated with solutions of 14C [TCE] that bracketed groundwater concentrations (5 and 500 microg/L) found in residential areas surrounding Hill Air Force Base, UT, where trace amounts of TCE had been found in several fruits during a preliminary field survey. Nondosed control trees were grown within the canopy of the dosed trees and in a separate greenhouse. Tissue samples were analyzed for 14C and TCE using combustion/liquid scintillation counting (LSC) and headspace/gas chromatography/mass spectrometry (HS/GC/MS). Tissue was also extracted and analyzed by GC/MS for dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), and trichloroethanol (TCEt), three specific TCE metabolites that have been previously identified in laboratory and field studies. No 14C was detected in the nonexposed control trees. Exposed trees contained levels of 14C that were proportional to the exposure concentration. 14C concentrations were greatest in leaves followed by branches and fruits. At the end of the study, TCE was detected only in roots implying that the 14C in the leaves, branches, and fruit was associated with unidentified nonvolatile TCE transformation products and/or is nonextractable. However, TCAA and DCAA were positively identified only in leaves collected during the first year from an apple tree exposed to the high dose treatment. Additional data for other chemicals and fruittrees are needed to better understand the potential transfer of organic compounds to edible fruit.