Utility of Compound‐Specific Isotope Analysis for Vapor Intrusion Investigations

Different types of data can be collected to evaluate whether or not vapor intrusion is a concern at sites impacted with volatile organic compound (VOC) contamination in the subsurface. Typically, groundwater, soil gas, or indoor air samples are collected to determine VOC concentrations in the different media. Sample results are evaluated using a “multiple lines of evidence” approach to interpret whether vapor intrusion is occurring. Data interpretation is often not straightforward because of many complicating factors, particularly in the evaluation of indoor air. More often than not, indoor air sample results are affected by indoor or other background sources making interpretation of concentration‐based data difficult using conventional sampling approaches. In this study, we explored the practicality of compound‐specific isotope analysis (CSIA) as an additional type of evidence to distinguish between indoor sources and subsurface sources (i.e., vapor intrusion). We developed a guide for decision‐making to facilitate data interpretation and applied the guidelines at four different test buildings. To evaluate the effectiveness of the CSIA method for vapor intrusion applications, we compared the interpretation from CSIA to interpretations based on data from two different investigation approaches: conventional sampling and on‐site GC/MS analysis. Interpretations using CSIA were found to be generally consistent with the other approaches. In one case, CSIA provided the strongest line of evidence that vapor intrusion was not occurring and that a VOC source located inside the building was the source of VOCs in indoor air.     

Observation of Conditions Preceding Peak Indoor Air Volatile Organic Compound Concentrations in Vapor Intrusion Studies

Temporal and spatial variability of indoor air volatile organic compound (VOC) concentrations can complicate vapor intrusion (VI) assessment and decision-making. Indicators and tracers (I&T) of VI, such as differential temperature, differential pressure, and indoor radon concentration, are low-cost lines of evidence to support sampling scheduling and interpretation of indoor air VOC sampling results. This study compares peak indoor air chlorinated VOC concentrations and I&T conditions before and during those peak events at five VI sites. The sites differ geographically and in their VI conceptual site models (CSM). Relative to site-specific baseline values, the results show that cold or falling outdoor temperatures, rising cross slab differential pressures, and increasing indoor radon concentrations can predict peak VOC concentrations. However, cold outdoor air temperature was not useful at one site where elevated shallow soil temperature was a better predictor. Correlations of peak VOC concentrations to elevated or rising barometric pressure and low wind speed were also observed with some exceptions. This study shows how the independent variables that control or predict peak indoor air VOC concentrations are specific to building types, climates, and VI CSMs. More I&T measurements at VI sites are needed to identify scenario-specific baseline and peak related I&T conditions to improve decision-making.     

Emissions of 1,2-Dichloroethane from Holiday Decorations as a Source of Indoor Air Contamination

Groundwater contamination associated with an industrial facility in Utah has led to concerns about potential vapor intrusion into residences outside the facility boundary. Trichloroethylene (TCE) is the main contaminant of concern with 1,2-dichloroethane (1,2-DCA) present in some areas. An air-monitoring program implemented to detect vapor intrusion of these compounds found 1,2-DCA in homes outside areas of groundwater contamination, suggesting indoor sources in these cases. Investigative indoor air and product sampling were conducted to isolate consumer products emitting 1,2-DCA and to quantify the emission rates of identified products. The combination of room-by-room air sampling and emission measurements was successfully used to identify molded plastic holiday ornaments, having measured emission rates as high as 0.3 µg 1,2-DCA/min. Subsequent testing of seven comparable retail items found similar 1,2-DCA emissions. Screening-level calculations show that the measured emission rates of 1,2-DCA from these items can lead to indoor concentrations high enough to be of regulatory concern (0.094 to 9.4 µg/m³ based on 10^–6 to 10^–4 cancer risk levels).     

Improving Risk-Based Screening at Vapor Intrusion Sites in California

The attenuation factor (AF) of 0.03 recommended by the U.S. Environmental Protection Agency (USEPA) is increasingly being used by regulatory agencies for the development of subsurface vapor screening levels for vapor intrusion (VI). There are concerns, however, over the database used to derive the AF and the AF's applicability to building types and geographies not included in USEPA database. To derive a more technically defensible AF for subsurface vapor screening in California, a database consisting of 8415 paired indoor and subsurface vapor samples collected from 485 buildings at 36 sites in California was compiled. Filtering was applied to remove data of suspect quality that were potentially affected by background (non-VI) sources. Filtering reduced the size of the database to 788 indoor air and subsurface vapor pairs, 80% of which were trichloroethylene (TCE) measurements. An AF of 0.0008 was derived from only TCE vapor data, based on the ability of the AF to reliably identify buildings with indoor air concentrations above screening levels in 95% of cases where subsurface vapor screening levels were exceeded. The AF derived from this study demonstrated limited sensitivity to the variables typically considered important in VI characterization, which was partially attributed to relatively weak correlation of indoor air and subsurface vapor concentration data. The results of this study can be used to improve VI screening in California and other states and help focus limited resources on sites posing the greatest potential risk.     

Evidence for Increasing Indoor Sources of 1,2-Dichloroethane Since 2004 at Two Colorado Residential Vapor Intrusion Sites

Groundwater contamination associated with two former industrial facilities in Denver, Colorado, has led to concerns about vapor intrusion into residences adjacent to the facilities. 1,1,1-Trichloroethane (1,1,1-TCA), 1,1-dichloroethene (1,1-DCE), and trichloroethene (TCE) are the main contaminants of concern in groundwater, with trace levels of 1,2-dichloroethane (1,2-DCA) present at one of the sites. Indoor air monitoring programs have been ongoing at these two sites since 1998 and recent results have suggested that background, indoor source, 1,2-DCA has been increasing in the frequency of detection, and median and maximum concentration over the past several years. A lines of evidence evaluation was undertaken for both sites in order to document the predominance of indoor sources of 1,2-DCA. Evidence utilized included spatial evaluation of 1,2-DCA in indoor air; comparison of 1,2-DCA concentrations in mitigated and unmitigated homes; a phone survey to evaluate the potential for smoking to contribute to indoor air 1,2-DCA levels; evaluation of mitigation system effluent data; and an evaluation of volatile organic compound (VOC) ratios in groundwater and indoor air. The results of this evaluation indicated that smoking had no demonstrable influence on measured indoor air concentrations. In addition, it appears that consumer products have had a markedly increased influence on indoor air concentrations since 2005. Data from one of the industrial facilities at one of the sites also indicated that polyvinyl chloride (PVC) and vinyl composite floor adhesive used in a building remodel in 2005 apparently generated elevated levels of indoor 1,2-DCA and vinyl chloride, which have been sustained up to the present time.     

Sewer Gas: An Indoor Air Source of PCE to Consider During Vapor Intrusion Investigations

The United States Environmental Protection Agency (USEPA) is finalizing its vapor intrusion guidelines. One of the important issues related to vapor intrusion is background concentrations of volatile organic compounds (VOCs) in indoor air, typically attributed to consumer products and building materials. Background concentrations can exist even in the absence of vapor intrusion and are an important consideration when conducting site assessments. In addition, the development of accurate conceptual models that depict pathways for vapor entry into buildings is important during vapor intrusion site assessments. Sewer gas, either as a contributor to background concentrations or as part of the site conceptual model, is not routinely evaluated during vapor intrusion site assessments. The research described herein identifies an instance where vapors emanating directly from a sanitary sewer pipe within a residence were determined to be a source of tetrachloroethylene (PCE) detected in indoor air. Concentrations of PCE in the bathroom range from 2.1 to 190 µg/m³ and exceed typical indoor air concentrations by orders of magnitude resulting in human health risk classified as an “Imminent Hazard” condition. The results suggest that infiltration of sewer gas resulted in PCE concentrations in indoor air that were nearly two orders of magnitude higher as compared to when infiltration of sewer gas was not known to be occurring. This previously understudied pathway whereby sewers serve as sources of PCE (and potentially other VOC) vapors is highlighted. Implications for vapor intrusion investigations are also discussed.     

Examining the Use of USEPA's Generic Attenuation Factor in Determining Groundwater Screening Levels for Vapor Intrusion

A value of 0.001 is recommended by the United States Environmental Protection Agency (USEPA) for its groundwater‐to‐indoor air Generic Attenuation Factor (GAFG), used in assessing potential vapor intrusion (VI) impacts to indoor air, given measured groundwater concentrations of volatile chemicals of concern (e.g., chlorinated solvents). The GAFG can, in turn, be used for developing groundwater screening levels for VI given target indoor air quality screening levels. In this study, we examine the validity and applicability of the GAFG both for predicting indoor air impacts and for determining groundwater screening levels. This is done using both analysis of published data and screening model calculations. Among the 774 total paired groundwater‐indoor air measurements in the USEPA's VI database (which were used by that agency to generate the GAFG) we found that there are 427 pairs for which a single groundwater measurement or interpolated value was applied to multiple buildings. In one case, up to 73 buildings were associated with a single interpolated groundwater value and in another case up to 15 buildings were associated with a single groundwater measurement (i.e., that the indoor air contaminant concentrations in all of the associated buildings were influenced by the concentration determined at a single point). In more than 70% of the cases (390 of 536 paired measurements in which horizontal building‐monitoring well distance was recorded) the monitoring wells were located more than 30 m (and one up to over 200 m) from the associated buildings. In a few cases, the measurements in the database even improbably implied that soil gas contaminant concentrations increased, rather than decreased, in an upward direction from a contaminant source to a foundation slab. Such observations indicate problematic source characterization within the data set used to generate the GAFG, and some indicate the possibility of a significant influence of a preferential contaminant pathway. While the inherent value of the USEPA database itself is not being questioned here, the above facts raise the very real possibility that the recommended groundwater attenuation factors are being influenced by variables or conditions that have not thus far been fully accounted for. In addition, the predicted groundwater attenuation factors often fall far beyond the upper limits of predictions from mathematical models of VI, ranging from screening models to detailed computational fluid dynamic models. All these models are based on the same fundamental conceptual site model, involving a vadose zone vapor transport pathway starting at an underlying uniform groundwater source and leading to the foundation of a building of concern. According to the analysis presented here, we believe that for scenarios for which such a “traditional” VI pathway is appropriate, 10^?4 is a more appropriately conservative generic groundwater to indoor air attenuation factor than is the EPA‐recommended 10^?3. This is based both on the statistical analysis of USEPA's VI database, as well as the traditional mathematical models of VI. This result has been validated by comparison with results from some well‐documented field studies.     

Influence of Fluctuating Groundwater Table on Volatile Organic Chemical Emission Flux at a Dissolved Chlorinated-Solvent Plume Site

Groundwater elevation fluctuation has been recognized as one mechanism causing temporal indoor air volatile organic chemical (VOC) impacts in vapor intrusion risk assessment guidance. For dissolved VOC sources, groundwater table fluctuation shortens/lengthens the transport pathway, and delivers dissolved contaminants to soils that are alternating between water saturated and variably saturated conditions, thereby enhancing volatilization potential. To date, this mechanism has not been assessed with field data, but enhanced VOC emission flux has been observed in lab-scale and modeling studies. This work evaluates the impact of groundwater elevation changes on VOC emission flux from a dissolved VOC plume into a house, supplemented with modeling results for cyclic groundwater elevation changes. Indoor air concentrations, air exchange rates, and depth to groundwater (DTW) were collected at the study house during an 86-d constant building underpressurization test. These data were used to calculate changes in trichloroethylene (TCE) emission flux to indoor air, during a period when DTW varied daily and seasonally from about 3.1 to 3.4 m below the building foundation (BF). Overall, TCE flux to indoor air varied by about 50% of the average, without any clear correlation to changes in DTW or its change rate. To complement the field study, TCE surface emission fluxes were simulated using a one-dimensional model (HYDRUS 1D) for conditions similar to the field site. Simulation results showed time-averaged surface TCE fluxes for cyclic water-table elevations were greater than for stationary water-table conditions at an equivalent time-averaged water-table position. The magnitudes of temporal TCE emission flux changes were generally less than 50% of the time-averaged flux, consistent with the field site observations. Simulation results also suggested that TCE emission flux changes due to groundwater fluctuation are likely to be significant at sites with shallow groundwater (e.g., < 0.5 m BF) and permeable soil types (e.g., sand).     

Evaluation of Vapor Intrusion Risk from Ethylene Dibromide Using the Vertical Screening Distance Approach

Vapor intrusion (VI) occurs when volatile contaminants in the subsurface migrate through the vadose zone into overlying buildings. The 2015 U.S. EPA petroleum VI guidance recommends that additional investigation of the VI risk from gasoline hydrocarbons at the underground storage tank (UST) sites is not necessary where the vertical distance between a building and a vapor source exceeds a recommended vertical screening distance. However, due to the lack of soil-gas data on the attenuation of ethylene dibromide (EDB), additional VI investigations to evaluate VI risk from EDB are recommended at UST sites with leaded gasoline releases containing EDB. We analyzed soil-gas and groundwater concentrations of EDB from eight petroleum UST sites using a new analytical method with soil-gas detection limit <0.16 μg/m³ EDB (VI screening level at the 10⁻⁶ risk level). The analysis included (1) assessing the frequency of EDB detections ≤0.16 μg/m³ at various vertical separation distances and (2) predicting vertical screening distances for EDB using the U.S. EPA PVIScreen model for different soil types in the vadose zone above dissolved-phase and LNAPL sources. Ranges of estimated aerobic biodegradation rate constants for EDB, air exchange rates for residential buildings, and source vapor concentrations for other constituents were combined with conservative estimates of EDB source concentrations as model inputs. Concentrations of EDB in soil-gas indicated that the U.S. EPA recommended vertical screening distances are protective of VI risk from EDB. Conversely, vertical screening distances predicted by modeling were >6 ft (1.8 m) for sites with sand and loam soil above dissolved phase sources and >15 ft (4.6 m) for sites with sand soil above LNAPL sources. This predicted dependence on the vapor source type and soil type in the vadose zone highlights the importance of soil characterization for VI screening at sites with EDB sources.     

Vertical Screening Distance Criteria to Evaluate Vapor Intrusion Risk from 1,2-Dichloroethane (1,2-DCA)

Vapor intrusion (VI) involves migration of volatile contaminants from subsurface through unsaturated soil into overlying buildings. In 2015, the US EPA recommended an approach for screening VI risks associated with gasoline releases from underground storage tank (UST) sites. Additional assessment of the VI risk from petroleum hydrocarbons was deemed unnecessary for buildings separated from vapor sources by more than recommended vertical screening distances. However, these vertical screening distances did not apply to potential VI risks associated with releases of former leaded gasoline containing 1,2-dichloroethane (1,2-DCA), because of a lack of empirical data on the attenuation of 1,2-DCA in soil gas. This study empirically evaluated 144 paired measurements of 1,2-DCA concentrations in soil gas and groundwater collected at 47 petroleum UST sites combined with BioVapor modeling. This included (1) assessing the frequency of 1,2-DCA detections in soil gas below 10⁻⁶ risk-based screening levels at different vertical separation distances and (2) comparing the US EPA recommended vertical screening distances with those predicted by BioVapor modeling. Vertical screening distances were predicted for different soil types using aerobic biodegradation rate constants estimated from the measured soil-gas data combined with conservative estimates of source concentrations. The modeling indicates that the vertical screening distance of 6 feet (1.8 m) recommended for dissolved-phase sources is applicable for 1,2-DCA below certain threshold concentrations in groundwater, while 15 feet (4.6 m) recommended for light nonaqueous phase liquid (LNAPL) sources is applicable for sites with clay and loam soils in the vadose zone, but not sand, if 1,2-DCA concentrations in groundwater exceed 150 μg/L. This dependence of the predicted vertical screening distances on soil type places added emphasis on proper soil characterization for VI screening at sites with 1,2-DCA sources. The soil-gas data suggests that a vertical screening distance of 15 feet (4.6 m) is necessary for both dissolved-phase and LNAPL sources.     

Effect of NAPL Source Morphology on Mass Transfer in the Vadose Zone

The generation of vapor‐phase contaminant plumes within the vadose zone is of interest for contaminated site management. Therefore, it is important to understand vapor sources such as non‐aqueous‐phase liquids (NAPLs) and processes that govern their volatilization. The distribution of NAPL, gas, and water phases within a source zone is expected to influence the rate of volatilization. However, the effect of this distribution morphology on volatilization has not been thoroughly quantified. Because field quantification of NAPL volatilization is often infeasible, a controlled laboratory experiment was conducted in a two‐dimensional tank (28 cm × 15.5 cm × 2.5 cm) with water‐wet sandy media and an emplaced trichloroethylene (TCE) source. The source was emplaced in two configurations to represent morphologies encountered in field settings: (1) NAPL pools directly exposed to the air phase and (2) NAPLs trapped in water‐saturated zones that were occluded from the air phase. Airflow was passed through the tank and effluent concentrations of TCE were quantified. Models were used to analyze results, which indicated that mass transfer from directly exposed NAPL was fast and controlled by advective‐dispersive‐diffusive transport in the gas phase. However, sources occluded by pore water showed strong rate limitations and slower effective mass transfer. This difference is explained by diffusional resistance within the aqueous phase. Results demonstrate that vapor generation rates from a NAPL source will be influenced by the soil water content distribution within the source. The implications of the NAPL morphology on volatilization in the context of a dynamic water table or climate are discussed.     

Estimation of Generic Subslab Attenuation Factors for Vapor Intrusion Investigations

Generic indoor air:subslab soil gas attenuation factors (SSAFs) are important for rapid screening of potential vapor intrusion risks in buildings that overlie soil and groundwater contaminated with volatile chemicals. Insufficiently conservative SSAFs can allow high‐risk sites to be prematurely excluded from further investigation. Excessively conservative SSAFs can lead to costly, time‐consuming, and often inconclusive actions at an inordinate number of low‐risk sites. This paper reviews two of the most commonly used approaches to develop SSAFs: (1) comparison of paired, indoor air and subslab soil gas data in empirical databases and (2) comparison of estimated subslab vapor entry rates and indoor air exchange rates (IAERs). Potential error associated with databases includes interference from indoor and outdoor sources, reliance on data from basements, and seasonal variability. Heterogeneity in subsurface vapor plumes combined with uncertainty regarding vapor entry points calls into question the representativeness of limited subslab data and diminishes the technical defensibility of SSAFs extracted from databases. The use of reasonably conservative vapor entry rates and IAERs offers a more technically defensible approach for the development of generic SSAF values for screening. Consideration of seasonal variability in building leakage rates, air exchange rates, and interpolated vapor entry rates allows for the development of generic SSAFs at both local and regional scales. Limitations include applicability of the default IAERs and vapor entry rates to site‐specific vapor intrusion investigations and uncertainty regarding applicability of generic SSAFs to assess potential short‐term (e.g., intraday) variability of impacts to indoor air.     

Long‐Term Impacts on Groundwater and Reductive Dechlorination Following Bioremediation in a Highly Characterized Trichloroethene DNAPL Source Area

High‐resolution soil and groundwater monitoring was performed to assess the long‐term impacts of bioremediation using bioaugmentation with a dechlorinating microbial consortium (and sodium lactate as the electron donor) in a well‐characterized trichloroethene (TCE) dense nonaqueous phase liquid (DNAPL) source area. Monitoring was performed up to 3.7 years following active bioremediation using a high‐density monitoring network that included several discrete interval multi‐level sampling wells. Results showed that despite the absence of lactate, lactate fermentation transformation products, or hydrogen, biogeochemical conditions remained favorable for the reductive dechlorination of chlorinated ethenes. In locations where soil data showed that TCE DNAPL sources persisted, local contaminant rebound was observed in groundwater, whereas no rebound or continuous decreases in chlorinated ethenes were observed in locations where DNAPL sources were treated. While ethene levels measured 3.7 years after active treatment suggested relatively low (2 to 30%) dechlorination of the parent TCE and daughter products, carbon stable isotope analysis showed that the extent of complete dechlorination was much greater than indicated by ethene generation and that the estimated first‐order rate constant describing the complete dechlorination of TCE at 3.7 years following active bioremediation was approximately 3.6 y–1. Overall, results of this study suggest that biological processes may persist to treat TCE for years after cessation of active bioremediation, thereby serving as an important component of remedial treatment design and long‐term attenuation.     

Portable GC/MS Methods to Evaluate Sources of cVOC Contamination in Indoor Air

A portable gas chromatograph‐mass spectrometer (GC/MS) was used to investigate sources of chlorinated volatile organic compound (cVOC) contamination in indoor air at 46 residences around Hill AFB, Utah, that were potentially affected by vapor intrusion. Analytical methods were developed to allow sample turnaround times of less than 10 min and method detection limits (MDLs) generally less than 1 μg/m³ for a selected list of cVOCs. Area‐by‐area sampling was used to identify the likely vapor source locations. In many cases, individual container/enclosure sampling and subsequent field emission rate measurements from isolated consumer products were used to determine if identified products were likely to be the primary source of vapors in the residence. The portable GC/MS was also used to characterize vapor intrusion in two residences. In one of these two residences, building pressure control was used to enhance vapor entry in order to facilitate the investigation resulting in confirmation of vapor intrusion and identification of a primary route of vapor entry. cVOCs were identified in 42 of the 46 homes investigated, subsurface vapor intrusion was identified in two homes, and two homes had inconclusive results.     

Prediction of Indoor Air Quality from Soil-Gas Data at Industrial Buildings

Subslab or shallow soil-gas data are often compared with indoor air concentration data in vapor intrusion (VI) evaluations. If no indoor air data are available or confounding sources are present, or if future scenarios are considered, the soil-gas data may be used to estimate the indoor air concentrations due to VI. The typical approach in risk assessments is to use the 95th percentile values from a set of concentration data. For VI studies, however, this rarely is an option because the data sets tend to be quite small. Therefore, various guidance documents urge the use of maximum soil-gas values. This may be reasonable for small residential buildings, but can lead to very conservatively biased estimates if applied to large industrial buildings with localized areas of contamination, especially given that the sampling locations may not be randomly selected and instead are biased toward worst-case locations. By this approach, VI guidance implicitly tolerates a large percentage of false positive decision errors to minimize the number of false negative decision errors. In this paper, implications of using maximum values are discussed and illustrated with data sets from a number of large industrial buildings at various sites. An alternative approach to using maximum soil-gas values is proposed that serves to reduce the number of false positive results while controlling the number of false negatives to an acceptable level.     

Tree Coring as a Complement to Soil Gas Screening to Locate PCE and TCE Source Zones and Hot Spots

Preliminary risk assessment for prioritisation of site investigations requires efficient screening to reveal type and level of contamination. The screening methods, tree coring and soil gas sampling were applied and compared at two forested sites contaminated with tetrachloroethylene (PCE) or trichloroethylene (TCE) to evaluate their ability to locate source zones and contaminant hot spots. One test site represented a relatively homogeneous sandy soil and aquifer, and the second a more heterogeneous geology with both sandy and less permeable clay till layers overlying a chalk aquifer. Tree cores from different tree species were sampled and analysed, and compared to soil gas measurements and existing soil gas data. Both methods were found useful as screening tools to locate hot spots of PCE and TCE in the shallow subsurface. Tree coring was found to be particularly beneficial as a complement to soil gas sampling at sites with low permeable soils, and where contamination was located in the capillary rise or shallow groundwater. The shorter time required for tree coring reduced the costs compared to soil gas sampling, but the sensitivity and precision of tree coring were lower. However, this did not affect the feasibility of using tree coring to locate the hot spots. Moreover, a combination of the two methods can help to focus any subsequent investigations like soil or groundwater sampling. The use of tree coring to complement soil gas sampling for pre‐screening is expected to result in higher certainty for revealing hot spots and source zones at contaminated sites.     

Vapor Intrusion Screening at Petroleum UST Sites

Detailed site investigations to assess potential inhalation exposure and risk to human health associated with the migration of petroleum hydrocarbon vapors from the subsurface to indoor air are frequently undertaken at leaking underground storage tank (UST) sites, yet documented occurrences of petroleum vapor intrusion are extremely rare. Additional assessments are largely driven by low screening‐level concentrations derived from vapor transport modeling that does not consider biodegradation. To address this issue, screening criteria were developed from soil‐gas measurements at hundreds of petroleum UST sites spanning a range of environmental conditions, geographic regions, and a 16‐year time period (1995 to 2011). The data were evaluated to define vertical separation (screening) distances from the source, beyond which, the potential for vapor intrusion can be considered negligible. The screening distances were derived explicitly from benzene data using specified soil‐gas screening levels of 30, 50, and 100 µg/m³ and nonparametric Kaplan‐Meier statistics. Results indicate that more than 95% of benzene concentrations in soil gas are ≤30 µg/m³ at any distance above a dissolved‐phase hydrocarbon source. Dissolved‐phase petroleum hydrocarbon sources are therefore unlikely to pose a risk for vapor intrusion unless groundwater (including capillary fringe) comes in contact with a building foundation. For light nonaqueous‐phase liquid (LNAPL) hydrocarbon sources, more than 95% of benzene concentrations in soil gas are ≤30 µg/m³ for vertical screening distances of 13 ft (4 m) or greater. The screening distances derived from this analysis are markedly different from 30 to 100 ft (10 to 30 m) vertical distances commonly found cited in regulatory guidance, even with specific allowances to account for uncertainty in the hydrocarbon source depth or location. Consideration of these screening distances in vapor intrusion guidance would help eliminate unnecessary site characterization at petroleum UST sites and allow more effective and sustainable use of limited resources.     

Demonstration of Enhanced Bioremediation in a TCE Source Area at Launch Complex 34, Cape Canaveral Air Force Station

The ability of bioremediation to treat a source area containing trichloroethene (TCE) present as dense nonaqueous phase liquid (DNAPL) was assessed through a laboratory study and a pilot test at Launch Complex 34, Cape Canaveral Air Force Center. The results of microcosm testing indicate that the indigenous microbial community was capable of dechlorinating TCE to ethene if amended with electron donor; however, bioaugmentation with a dechlorinating culture (KB-1; SiREM, Guelph, Ontario, Canada) significantly increased the rate of ethene formation. In microcosms, the activity of the dechlorinating organisms in KB-1 was not inhibited at initial TCE concentrations as high as 2 mM. The initially high TCE concentration in ground water (1.2 mM or 155 mg/L) did not inhibit reductive dechlorination, and at the end of the study, the average concentration of ethene (2.4 mM or 67 mg/L) was in stoichiometric excess of this initial TCE concentration. The production of ethene in stoichiometric excess in comparison to the initial TCE concentration indicates that the bioremediation treatment enhanced the removal of TCE mass (either sorbed to soil or present as DNAPL). Detailed soil sampling indicated that the bioremediation treatment removed greater than 98.5% of the initial TCE mass. Confirmatory ground water samples collected 22 months after the bioremediation treatment indicated that chloroethene concentrations had continued to decline in the absence of further electron donor addition. The results of this study confirm that dechlorination to ethene can proceed at the high TCE concentrations often encountered in source areas and that bioremediation was capable of removing significant TCE mass from the test plot, suggesting that enhanced bioremediation is a potentially viable remediation technology for TCE source areas. Dehalococcoides abundance increased by 2 orders of magnitude following biostimulation and bioaugmentation.     

Evaluation of Passive Diffusive-Adsorptive Samplers for Use in Assessing Time-Varying Indoor Air Impacts Resulting from Vapor Intrusion

Passive diffusive-adsorptive samplers are being considered for vapor intrusion (VI) pathway assessment, particularly where multi-week time-weighted average concentrations are desired. Recent studies have shown that passive samplers can produce accurate results under well-controlled steady concentration conditions, and field performance was also demonstrated at several sites. The objective of this study was to examine passive sampler performance in settings with time-varying indoor air concentrations, through a comparison of passive sampler results to concentrations determined by 24-h active sorbent tube sampling in a series of multi-week deployments. Sampling was performed in a well-instrumented residential building as well as industrial buildings, over periods of time ranging from 1 to 7 weeks. Strong linear correlations were noted between passive and active sampling concentration results for some passive samplers, with passive sampling results being similar to or lower than measured active sampling results by about 50% for those samplers in the residential study and about 25% higher in the industrial building study. Other samplers produced poor agreement. The conclusion from this study is that some passive samplers have great potential for use in multi-week indoor air quality monitoring. It was further determined that there is need for accepted procedures to validate and calibrate passive samplers for use in the field.