Evaluation of vapor intrusion using controlled building pressure

Municipal pipeline networks as preferential vapor intrusion pathways: A review

Recent reports show a rise in instances where municipal networks, such as sewer lines, serve as pathways for vapor intrusion (VI), enabling volatile organic compounds (VOCs) vapors to travel along these networks. These VOCs pose potential health risks to occupants of buildings connected to these networks. Currently, there's a lack of specific technical or regulatory guidance on identifying and assessing the VI risk associated with sewer as preferential VI pathways. This critical review summarizes key findings from studies and site investigations related to sewer VI pathways. These findings cover background VOCs concentration levels in sewers, updates to site conceptual models, advances in sewer sampling techniques, innovative tools for identifying and characterizing sewer VI, and practices for assessing and mitigating sewer VI risk. While significant improvements have been made towards understanding how municipal pipeline networks act as VI pathways, more research is still needed to develop strategies for investigating sites and assessing risks associated with "pipeline VI pathways". Future research could focus on the development of "pipeline VI pathways" data set, the improvement and validation of investigation tools, and improving the understanding of VOCs transportation mechanisms within these "pipeline VI pathways".

Characterization of vapor intrusion sites with a deep learning-based data assimilation method

Appropriate characterization of site soils is essential for accurate risk assessment of soil vapor intrusion (VI). In this study, we develop a data assimilation method based on deep learning (i.e., ES_(DL)) to estimate the distribution of soil properties with limited measurements. Two hypothetical VI scenarios are employed to demonstrate site characterization using the ES_(DL) method, followed by validation with a laboratory sandbox experiment and then one practical site application. The results show that the ES_(DL) method can provide reasonable estimates of the effective diffusion coefficient distributions and corresponding emission rates (into the building) in all four cases. The spatial heterogeneity of site soils can be characterized by considerably enough measurements (i.e., 15 sampling points in the first hypothetical case); otherwise, layered characterization is recommended at the cost of neglecting horizontal heterogeneity of site soils. This new method provides an alternative to characterize VI sites with relatively fewer sampling efforts.

Insights into vapour intrusion phenomena: Current outlook and preferential pathway scenario

Vapour intrusion (VI) is the phenomenon by which volatile organic compounds (VOCs) migrate from the subsurface source through the soil and enter into the overlying buildings, affecting the indoor air quality and ultimately causing health hazards to the occupants. Health risk assessments associated with hydrocarbon contaminated sites and recommendations of site closure are often made by quantifying the VI risks using mathematical models known as 'vapour intrusion models' (VIM). In order to predict the health risk, various factors such as the lithological and geochemical conditions of the subsurface, environmental conditions, building operational conditions etc. are commonly evaluated using VIMs. Use of these models can overlook the role of preferential pathways like highly permeable subsurface layers and utility lines which act as the path of least resistance for vapour transport, which can increase the VI risks. The extensive networks of utility lines and sanitary sewer systems in urban areas can significantly exacerbate the uncertainty of VI investigations. The backfill materials like sand and gravel surrounding the utility lines can allow the vapours to easily pass through due to their high porosity as compared to natural formations. Hence, failure to understand the role of preferential pathways on the fate and transport of VOC in the vadose zone can result in more conservative predictions of indoor air vapour concentrations and wrong clean up approaches. This comprehensive review outlines the vapour transport mechanisms, factors influencing VI, VIMs and the role of preferential pathways in predicting indoor air vapour concentrations.

Numerical study of building pressure cycling to generate sub-foundation aerobic barrier for mitigating petroleum vapor intrusion

In this study, the role of building pressure cycling (BPC) method in generating a subslab aerobic barrier at petroleum contaminated sites was examined numerically. The numerical model was first validated with field observations and then used to simulate BPC applications in petroleum vapor intrusion scenarios. The results indicated that, after a long-term BPC operation (60 days), a subslab aerobic barrier could be generated with an adequate air injection rate (10 L/min in this study). The effects on hydrocarbon soil gas concentration profiles are expected to last for weeks even after the BPC system is turned off. Moreover, our investigations showed that the performances of the BPC application are virtually independent of hydrocarbon's reaction rate constant. The simulated sub-foundation aerobic conditions expected during BPC were comparable to those observed in a field study where a subsurface pipe system at the same air injection rate was used to create a subslab aerobic barrier. Thus, BPC application can represent an interesting alternative approach to the subsurface delivery systems as it is expected to achieve similar performance but with lower installation costs.

Numerical study of the building pressure cycling method for evaluating vapor intrusion from groundwater contamination

Vapor intrusion (VI) risk assessments determine the cleanup level of groundwater in the absence of ingestion. In recent VI investigations, the building pressure cycling (BPC) method has been applied to help minimize ambiguity caused by temporal variability of indoor air samples that are important to risk assessments, and, consequently, determine groundwater cleanup level accurately. In this study, we use a three-dimensional numerical model to examine the dynamic migration of VOCs from groundwater after the application of BPC. First, we validated the numerical model with field measurements. Then, the verified model is used to investigate the effects of site-specific features in determining the performance of BPC operation. At last, we summarize past field applications of BPC to examine the simulated results. Our study indicates that the BPC-induced indoor depressurization can increase the building loading rate in the first 2-3 h, which would then drop to 2-3 times of that with natural conditions in most cases of groundwater contamination. In some cases involving a strong source, e.g., a vapor source above the capillary fringe or a groundwater source with sandy soil above the groundwater level, the normalized building loading rates can be maintained as high as 4-9 without decrease after the first 2-3 h. Significantly higher increase in building loading rate may indicate a potential presence of a preferential pathway between the groundwater contamination and concerned building.

Vapor Intrusion Investigations and Decision-Making: A Critical Review

At sites impacted by volatile organic compounds (VOCs), vapor intrusion (VI) is the pathway with the greatest potential to result in actual human exposure. Since sites with VI were first widely publicized in late 1990s, the scientific understanding of VI has evolved considerably. The VI conceptual model has been extended beyond relatively simple scenarios to include nuances, such as biological and hydrogeological factors that may limit the potential for VI and alternative pathways, such as preferential pathways and direct building contact/infiltration that may enhance VI in some cases. Regulatory guidance documents typically recommend initial concentration- or distance-based screening to evaluate whether VI may be a concern, followed by a multiple-lines-of-evidence (MLE) investigation approach for sites that do not screen out. These recommendations for detailed evaluation of VI currently focus on monitoring of VOC concentrations in groundwater, soil gas, and indoor air and can be supplemented by other lines of evidence. In this Critical Review, we summarize key elements important to VI site characterization, provide the status and current understanding, and highlight data interpretation challenges, as well as innovative tools developed to help overcome the challenges. Although there have been significant advances in the understanding of VI in the past 20 years, limitations and knowledge gaps in screening, investigation methods, and modeling approaches still exist. Potential areas for further research include improved initial screening methods that account for the site-specific role of barriers, improved understanding of preferential pathways, and systematic study of buildings and infrastructure other than single-family residences.

Development and Validation of a Controlled Pressure Method Test Protocol for Vapor Intrusion Pathway Assessment

Controlled pressure method (CPM) testing is a building-specific diagnostic tool for vapor intrusion (VI) pathway assessment which offers advantages over traditional pathway assessment approaches. By manipulating the building pressure conditions, the CPM creates the worst-case VI impact and provides rapid insight into the type of vapor source(s). The primary barrier to general acceptance and use of this tool is the need for definitive guidance on test design parameters, such as the indoor-outdoor pressure difference (or exhaust fan flow rate), CPM test duration, exhaust fan location, and air sampling location(s) and conditions. This study focused on a systematic evaluation of each of these factors, which then led to the formulation of proposed CPM testing guidelines. The results suggest that CPM tests should be conducted with both negative and positive pressure indoor-outdoor differentials of about 10-15 Pa, and the tests should last for at least nine indoor air exchanges for negative pressure difference testing and four indoor air exchanges for positive pressure difference testing. Although exhaust fan intake sampling is sufficient to provide critical information to assess impacts during negative pressure testing, adding room-specific indoor air sampling to both negative and positive pressure difference testing can provide insight into vapor entry locations and indoor source contributions.

An examination of the building pressure cycling technique as a tool in vapor intrusion investigations with analytical simulations

The building pressure cycling (BPC) technique has been developed and applied by vapor intrusion (VI) site investigators to obtain estimates of reasonable maximum exposures and to identify possible background sources of contaminant vapors. This method assumes that by application of consistent indoor depressurization one can increase the average contaminated soil gas entry rate into a building of interest. In this study, a one-dimensional analytical model was developed to examine this assumption and explore the mechanism of BPC application. We have established that contaminant entry rate can typically reach a new pseudo-steady state on a time scale of one day following the imposition of enhanced indoor depressurization. Considering the traditional source-soil-building pathway, the results indicate that BPC can increase building loading rate in the first 3-5 hours, to an extent linearly related to the strength of depressurization, and after half a day, the normalized rate would reach a pseudo-steady state of about twice the value before application of depressurization. More significant and substainble increases in building loading rate indicate alternative pathways such as land drain or sewer pipeline. These findings are fully consistent with available field observations, and could help investigators optimize the performance of the BPC operation.

High-frequency fluctuations of indoor pressure: A potential driving force for vapor intrusion in urban areas

In this study, we examine the impact of a building's indoor pressure fluctuations in drawing subsurface volatile contaminants into the building, and how the presence of an impervious pavement surrounding the building influences this. Even in the absence of communication between the subsurface soil gas and ambient air fluctuations of building indoor pressure can cause upward advection of contaminated soil gas from the subfoundation zone into a building. For cases with the paved ground surface, the simulated volumetric soil gas entry rates are lower than steady-state cases with constant -5 indoor-outdoor pressure difference, by at least half an order of magnitude. When the indoor pressure fluctuation rate exceeds about 5 Pa/h (which corresponds a sinusoidal fluctuation with a period of 2 h), the predicted indoor air concentration of paved scenarios will be higher than the conventional case. When both the building foundation and surrounding pavement block diffusional escape of the volatile soil gas contaminants to the atmosphere, high subfoundation soil gas contaminant concentrations can exist, and contaminant entry into the building through foundation breaches is enhanced beyond what would be expected from diffusion as the building undergoes normal pressure cycling. Upward advection into the building may be induced even when the indoor pressure appears, based on limited measurements, to be higher than that in the subslab, particularly when the indoor pressure in the building quickly fluctuates. This represents a limitation on VI mitigation approaches that rely on indoor pressurization, if those approaches cannot at the same time control significant fluctuation of indoor pressure.

Comparison of modeled and measured indoor air trichloroethene (TCE) concentrations at a vapor intrusion site: influence of wind, temperature, and building characteristics

There is a lack of vapor intrusion (VI) models that reliably account for weather conditions and building characteristics, especially at sites where active alternative pathways, such as sewer connections and other preferential pathways, are present. Here, a method is presented to incorporate freely-available models, CONTAM, and CFD0, to estimate site-specific building air exchange rates (AERs) and indoor air contaminant concentrations by accounting for weather conditions and building characteristics at a well-known VI site with a land drain preferential pathway. To account for uncertainty in model input parameters that influence indoor air chlorinated volatile organic compound (CVOC) concentration variability, this research incorporated Monte Carlo simulations and compared model results with retrospective field data collected over approximately 1.5 years from the study site. The results of this research show that mass entry rates for TCE are likely influenced by indoor air pressures that can be modeled as a function of weather conditions (over seasons) and building characteristics. In addition, the results suggest that temporal variability in indoor air TCE concentrations is greatest (modeled and measured) due to the existence of a land drain, which acts as a preferential pathway, from the subsurface to the granular fill beneath the floor slab. The field data and modeling results are in good agreement and provide a rare comparison of field data and modeling results for a VI site. The modeling approach presented here offers a useful tool for decision makers and VI practitioners as they assess these complex and variable processes that have not been incorporated within other VI models.

A conceptual model for vapor intrusion from groundwater through sewer lines

The role of sewer lines as preferential pathways for vapor intrusion is poorly understood. As a result, these pathways are often not considered when developing vapor intrusion investigation or mitigation plans. Neglecting this pathway can complicate data interpretation, which can result in repeated, and potentially unnecessary, rounds of sampling. Although a number of recent studies have highlighted the importance of sewers as preferential pathways at individual buildings, there is currently little specific technical or regulatory guidance on how to address it. The purpose of our study, therefore, was to conduct systematic testing to better understand the sewer vapor intrusion conceptual model. Through sampling at >30 different sites, the degree of interaction between impacted groundwater and the sewer lines were identified as the main factor when determining the degree of risk for sewer vapor intrusion at a given site. Higher risk sites are those with direct interaction between the subsurface volatile organic compound (VOC) source, such as groundwater, and the sewer line itself. This information can be used to prioritize sites and buildings to test for this particular exposure pathway.

Differential toxicity of water versus gavage exposure to trichloroethylene in rats

Trichloroethylene (TCE) is a persistent environmental contaminant that causes male reproductive toxicity. We investigated whether transient increases in TCE exposure modulated male reproductive toxicity by exposing rats via daily oral to repeated gavage exposures (1000 mg/kg/day) and through drinking water (0.6% TCE) for 14 weeks. The gavage route resulted in reversible reduction of epididymis weight, and reduced body weight that persisted for up to 12-weeks after cessation of exposure. Physiologically-based pharmacokinetic modeling predicted that the gavage route results in higher C_max and AUC exposure of TCE compared to drinking water exposure, explaining the observed differences in toxicity between dosing regimens.

A source depletion model for vapor intrusion involving the influence of building characteristics

If volatile organic compound (VOC)-contaminated soil exists underneath a building, vapors may migrate upwards and intrude into the interior air of the building. Most previous models used to simulate vapor intrusion (VI) were developed by assuming that the source was constant, although a few recent models, such as the Risk-Based Corrective Action (RBCA) Tool Kit (TK) model, have been developed to consider source depletion (SD). However, the RBCA TK model ignores the effects of building characteristics due to its assumption that the ground is not covered by the actual building it models, which leads to incorrect results since the presence of the building affects the SD. In this study, a SD model is developed based on the three processes of VI while considering the impact of key building parameters on SD. The proposed model (i.e., the SD model) still follows the law of mass conservation, and the sensitivity analysis shows that the soil-building pressure differential (dP) is an important building characteristic that affects SD. Taking trichloroethylene (TCE) for simulation in the case of a soil concentration below the saturation concentration, as the soil permeability decreases, the differences in the results between the SD model and RBCA TK model decrease; as the Peclet number decreases, the effect of the dP on the results of the SD model decreases. The new model only accounts for the migration of contaminants at the source of depletion; therefore, the model is more applicable for these contaminants, which are considered to have low-biodegradable characteristics. Furthermore, since the model emphasizes the impact of buildings on the source, it is applicable when there is a considerable building area above the source, such as large commercial buildings or residential communities with underground parking lots, which exist in most cities.

Key Design Elements of Building Pressure Cycling for Evaluating Vapor Intrusion-A Literature Review

Building pressure cycling (BPC) is becoming an increasingly important tool for studying vapor intrusion. BPC has been used to distinguish subslab and indoor sources of vapor intrusion as well as to define reasonable worst case volatile organic compound mass discharge into a structure. Analyses have been performed both semi-quantitatively with concentration trends and quantitatively with more rigorous flux calculation and source attribution methods. This paper reviews and compares the protocols and outcomes from multiple published applications of this technology to define the key variables that control performance. Common lessons learned are identified, including those that help define the range of building size and type to which BPC is applicable. Differences in test protocols are discussed, recognizing that the complexity of the test protocol required depends on the particular objectives of each project. Research gaps are identified and tabulated for future validation studies and applications.

Intrusion of chlorinated hydrocarbons and their degradation products from contaminated soil. Measurement of indoor air quality and biomonitoring by analysis of end-exhaled air

A historic soil contamination of chlorinated hydrocarbons from a former dry cleaning shop caused intrusion of vapors into a building currently used as bookshop. The aim of this study was to determine the indoor air quality and the uptake of soil contaminants and their degradation products. Samples of indoor air were collected over one week in the warm and one week in the cold season. Pre-shift and post-shift samples of end-exhaled air were collected from two employees. Chlorinated hydrocarbons were analyzed in indoor air and exhaled air samples using thermal desorption gas chromatography mass spectrometry (TD-GC-MS). Tetrachloroethylene (PER), and its degradation products trichloroethylene (TRI), 1,1-dichloroethylene (1,1-DCE), 1,2-cis-dichloroethylene (1,2-cis-DCE), 1,2-trans-dichloroethylene (1,2-trans-DCE), methylene chloride (MC) and vinyl chloride (VC) were determined in ambient air. PER was the prime contaminant with a week average (±sd) of 805.2 ± 598.6 μg/m³ in June 2016 and 1031 ± 499.3 μg/m³ in December 2017. MC, 1,2-cis-DCE and TRI were detected at concentrations below 2.3 μg/m³. 1,1-DCE and VC were not detected. In exhaled air PER, 1,1-DCE, and MC were detected in both June and December, whereas TRI, 1,2-cis-DCE and 1,2-trans-DCE were only detected on one or two days in the cold season. VC was not detected in exhaled air. For PER, the mean concentrations (±sd) in end-exhaled air increased from a five days (Mon-Fri) average pre-shift value of 22.2 ± 8.0 to a post-shift value of 52.6 ± 15.5 ng/L in the male shop owner (p < 0.01) and in the female cashier these values were 26.0 ± 3.6 and 63.6 ± 12.7 ng/L, respectively (p < 0.01). Intrusion of chlorinated soil contaminants resulted in contamination of indoor air above the current accepted indoor air level for PER of 250 μg/m³. For PER in end-exhaled air an accumulation over the workweek was not observed.

Examining the role of sub-foundation soil texture in chlorinated vapor intrusion from groundwater sources with a two-layer numerical model

In this study we investigate the role of the sub-foundation soil texture in determining groundwater source-to-indoor air attenuation factors. A three-dimensional numerical model was used to simulate a series of two-layer scenarios, involving different sub-foundation and deep soil textures, foundation types and groundwater source depths. The results indicate that if the sub-foundation soil permeability is larger than 10^-11 m², the convection dominates the soil gas transport into the building, and the indoor air concentration increases by half an order of magnitude with one order of magnitude increase of the sub-foundation soil permeability. Otherwise, diffusion plays a more important role and the sub-foundation soil texture does not cause significant variation of indoor air concentration. We found that, independently from the deep soil texture, the capillary fringe offers the main resistance to vapor transport. In these cases, the deep soil texture could induce at most half an order of magnitude of variation in total effective diffusion coefficient in deep soil as well as groundwater source-to-indoor air attenuation factors. Finally, we found that, as the thin capillary zone represents the higher resistance to upward soil gas flow, groundwater source depth has little influence in determining the chlorinated vapor intrusion risk.

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.

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 meters (and some up to over 200 meters) 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 dataset 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.

Recent advances in vapor intrusion site investigations

Our understanding of vapor intrusion has evolved rapidly since the discovery of the first high profile vapor intrusion sites in the late 1990s and early 2000s. Research efforts and field investigations have improved our understanding of vapor intrusion processes including the role of preferential pathways and natural barriers to vapor intrusion. This review paper addresses recent developments in the regulatory framework and conceptual model for vapor intrusion. In addition, a number of innovative investigation methods are discussed.

Evidence of a sewer vapor transport pathway at the USEPA vapor intrusion research duplex

The role of sewer lines as preferential pathways for vapor intrusion is poorly understood. Although the importance of sewer lines for volatile organic compound (VOC) transport has been documented at a small number of sites with vapor intrusion, sewer lines are not routinely sampled during most vapor intrusion investigations. We have used a tracer study and VOC concentration measurements to evaluate the role of the combined sanitary/storm sewer line in VOC transport at the USEPA vapor intrusion research duplex in Indianapolis, Indiana. The results from the tracer study demonstrated gas migration from the sewer main line into the duplex. The migration pathway appears to be complex and may include leakage from the sewer lateral at a location below the building foundation. Vapor samples collected from the sewer line demonstrated the presence of tetrachloroethene (PCE) and chloroform in the sewer main in front of the duplex and at multiple sample locations within the sewer line upstream of the duplex. These test results combined with results from the prior multi-year study of the duplex indicate that the sewer line plays an important role in transport of VOCs from the subsurface source to the immediate vicinity of the duplex building envelope.

Three-Dimensional Simulation of Land Drains as a Preferential Pathway for Vapor Intrusion into Buildings

Preferential pathways can be significant vapor intrusion (VI) contributors, causing potentially higher inhalation risk to residents of affected buildings than that arising through traditional intrusion pathways. To assess land drains as a preferential pathway, a three-dimensional model, validated using data from a 4-yr field study, was used to study the roles of subfoundation soil permeability on soil gas flow and indoor depressurization. Results indicated that it is almost impossible for an indirect preferential pathway like a land drain ending in subfoundation soils with a permeability <10 m to affect indoor air quality if the land drain connects to a source with the same vapor concentration as that of the groundwater source beneath the building. An equation was developed to estimate the threshold permeability. We also found that even after the preferential pathway was identified using indoor depressurization (also known as controlled pressure method [CPM]) and then turned off, the influence of the preferential pathway and indoor depressurization on indoor concentration might last for months, although it may not be significant (i.e., may not exceed one order of magnitude, in this study). In the absence of such a preferential VI pathway, CPM may actually reduce indoor air concentrations of contaminants below those present under natural indoor pressure conditions, due to the emission rate limit determined by the upward diffusion rate from the vapor source. Our study highlights the role of measuring subfoundation soil permeability to soil gas flow in site investigations and warns practitioners about the possible mischaracterization of indoor air concentration after applying CPM in the absence of a preferential pathway.

Investigating the Role of Soil Texture in Vapor Intrusion from Groundwater Sources

Soil texture is believed to play a significant role in the migration of subsurface volatile chemicals into buildings at contaminated sites, an exposure process known as vapor intrusion (VI). In this study, we investigated the role of soil texture in determining the attenuation of contaminant soil gas concentration from groundwater source to receptor building. We performed soil column experiments, numerical simulations, and statistical analysis of the USEPA's VI database. The soil column experiments were conducted with commercial sand and soils with sand and sandy loam textures. Measured one-dimensional soil gas concentration profiles were compared with numerical predictions. Good agreement between experiments and model results supports the use of the classical multiphase chemical transport equation for simulating contaminant vapor transport from groundwater through the vadose zone. A full three-dimensional numerical model was then used to simulate typical VI scenarios with groundwater sources. Results indicate that, although soil particle texture can play a role in determining subslab-to-indoor air concentration attenuation, there is no obvious relationship between soil particle size and groundwater source-to-subslab except in the case of a shallow contaminant source. This conclusion is consistent with results reported in USEPA's VI database, in which variation in soil particle size does not affect source-to-subslab attenuation factors but does influence subslab-to-indoor air concentration attenuation factors by an average of about 0.4 order of magnitude. This finding suggests that an appropriate focus of VI site investigation should include the shallow soil beneath the building foundation.

A two-dimensional analytical model of vapor intrusion involving vertical heterogeneity

In this work, we present an analytical chlorinated vapor intrusion (CVI) model that can estimate source-to-indoor air concentration attenuation by simulating two-dimensional (2-D) vapor concentration profile in vertically heterogeneous soils overlying a homogenous vapor source. The analytical solution describing the 2-D soil gas transport was obtained by applying a modified Schwarz-Christoffel mapping method. A partial field validation showed that the developed model provides results (especially in terms of indoor emission rates) in line with the measured data from a case involving a building overlying a layered soil. In further testing, it was found that the new analytical model can very closely replicate the results of three-dimensional (3-D) numerical models at steady state in scenarios involving layered soils overlying homogenous groundwater sources. By contrast, by adopting a two-layer approach (capillary fringe and vadose zone) as employed in the EPA implementation of the Johnson and Ettinger model, the spatially and temporally averaged indoor concentrations in the case of groundwater sources can be higher than the ones estimated by the numerical model up to two orders of magnitude. In short, the model proposed in this work can represent an easy-to-use tool that can simulate the subsurface soil gas concentration in layered soils overlying a homogenous vapor source while keeping the simplicity of an analytical approach that requires much less computational effort.

US residential building air exchange rates: new perspectives to improve decision making at vapor intrusion sites

Vapor intrusion (VI) is well-known to be difficult to characterize because indoor air (IA) concentrations exhibit considerable temporal and spatial variability in homes throughout impacted communities. To overcome this and other limitations, most VI science has focused on subsurface processes; however there is a need to understand the role of aboveground processes, especially building operation, in the context of VI exposure risks. This tutorial review focuses on building air exchange rates (AERs) and provides a review of literature related building AERs to inform decision making at VI sites. Commonly referenced AER values used by VI regulators and practitioners do not account for the variability in AER values that have been published in indoor air quality studies. The information presented herein highlights that seasonal differences, short-term weather conditions, home age and air conditioning status, which are well known to influence AERs, are also likely to influence IA concentrations at VI sites. Results of a 3D VI model in combination with relevant AER values reveal that IA concentrations can vary more than one order of magnitude due to air conditioning status and one order of magnitude due to house age. Collectively, the data presented strongly support the need to consider AERs when making decisions at VI sites.

Measuring Vapor Intrusion: From Source Science Politics to a Transdisciplinary Approach

Investigation of indoor air quality has been on the upswing in recent years. In this article, we focus on how the transport of subsurface vapors into indoor air spaces, a process known as "vapor intrusion," (VI) is defined and addressed. For environmental engineers and physical scientists who specialize in this emerging indoor environmental exposure science, VI is notoriously difficult to characterize, leading the regulatory community to seek improved science-based understandings of VI pathways and exposures. Yet despite the recent growth in VI science and competition between environmental consulting companies, VI studies have largely overlooked the social and political field in which VI problems emerge and are experienced by those at risk. To balance and inform current VI studies, this article explores VI science and policy and develops a critique of what we call "source science politics." Drawing inspiration from the creative synthesis of social and environmental science/engineering perspectives, the article offers a transdisciplinary approach to VI that highlights collaboration with social scientists and impacted communities and cultivates epistemic empathy.

Indoor Air Contamination from Hazardous Waste Sites: Improving the Evidence Base for Decision-Making

At hazardous waste sites, volatile chemicals can migrate through groundwater and soil into buildings, a process known as vapor intrusion. Due to increasing recognition of vapor intrusion as a potential indoor air pollution source, in 2015 the U.S. Environmental Protection Agency (EPA) released a new vapor intrusion guidance document. The guidance specifies two conditions for demonstrating that remediation is needed: (1) proof of a vapor intrusion pathway; and (2) evidence that human health risks exceed established thresholds (for example, one excess cancer among 10,000 exposed people). However, the guidance lacks details on methods for demonstrating these conditions. We review current evidence suggesting that monitoring and modeling approaches commonly employed at vapor intrusion sites do not adequately characterize long-term exposure and in many cases may underestimate risks. On the basis of this evidence, we recommend specific approaches to monitoring and modeling to account for these uncertainties. We propose a value of information approach to integrate the lines of evidence at a site and determine if more information is needed before deciding whether the two conditions specified in the vapor intrusion guidance are satisfied. To facilitate data collection and decision-making, we recommend a multi-directional community engagement strategy and consideration of environment justice concerns.

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.

Evaluation of site-specific lateral inclusion zone for vapor intrusion based on an analytical approach

In 2002, U.S. EPA proposed a general buffer zone of approximately 100 feet (30 m) laterally to determine which buildings to include in vapor intrusion (VI) investigations. However, this screening distance can be threatened by factors such as extensive surface pavements. Under such circumstances, EPA recommended investigating soil vapor migration distance on a site-specific basis. To serve this purpose, we present an analytical model (AAMLPH) as an alternative to estimate lateral VI screening distances at chlorinated compound-contaminated sites. Based on a previously introduced model (AAML), AAMLPH is developed by considering the effects of impervious surface cover and soil geology heterogeneities, providing predictions consistent with the three-dimensional (3-D) numerical simulated results. By employing risk-based and contribution-based screening levels of subslab concentrations (50 and 500 μg/m(3), respectively) and source-to-subslab attenuation factor (0.001 and 0.01, respectively), AAMLPH suggests that buildings greater than 30 m from a plume boundary can still be affected by VI in the presence of any two of the three factors, which are high source vapor concentration, shallow source and significant surface cover. This finding justifies the concern that EPA has expressed about the application of the 30 m lateral separation distance in the presence of physical barriers (e.g., asphalt covers or ice) at the ground surface.

Passive sampling for volatile organic compounds in indoor air-controlled laboratory comparison of four sampler types

This article describes laboratory testing of four passive diffusive samplers for assessing indoor air concentrations of volatile organic compounds (VOCs), including SKC Ultra II, Radiello®, Waterloo Membrane Sampler (WMS) and Automated Thermal Desorption (ATD) tubes with two different sorbents (Tenax TA and Carbopack B). The testing included 10 VOCs (including chlorinated ethenes, ethanes, and methanes, aromatic and aliphatic hydrocarbons), spanning a range of properties and including some compounds expected to pose challenges (naphthalene, methyl ethyl ketone). Tests were conducted at different temperatures (17 to 30 °C), relative humidities (30 to 90% RH), face velocities (0.014 to 0.41 m s(-1)), concentrations (1 to 100 parts per billion by volume [ppbv]) and sampling durations (1 to 7 days). The results show that all of the passive samplers provided data that met the success criteria (relative percent difference [RPD] ≤ 45% of active sample concentrations and coefficient of variation [COV] ≤ 30%) in the majority of cases, but some compounds were problematic for some samplers. The passive sampler uptake rates depend to varying degrees on the sampler, sorbent, target compounds and environmental conditions, so field calibration is advantageous for the highest levels of data quality.

Vapor intrusion attenuation factors relative to subslab and source, reconsidered in light of background data

The basis upon which recommended attenuation factors for vapor intrusion (VI) have been derived are reconsidered. By making a fitting curve to the plot showing the dependence of observed indoor air concentration (c(in)) on subslab concentration (c(ss)) for residences in EPA database, an analytical equation is obtained to identify the relationship among c(in), css and the averaged background level. The new relationship indicates that subslab measurements may serve as a useful guide only if c(ss) is above 500 μg/m(3). Otherwise, c(in) is independent of c(ss), with a distribution in good agreements with other studies of background levels. Therefore, employing this screening value (500 μg/m(3)), new contaminant concentration attenuation factors are proposed for VI, and the values for groundwater-to-indoor and subslab-to-indoor air concentration attenuation factors are 0.004 and 0.02, respectively. The former is applied to examining the reported temporal variations of c(in) obtained during a long-term monitoring study. The results show that using this new groundwater-to-indoor air concentration attenuation factor also provides a reasonably conservative estimate of c(in).

Community perspectives on the risk of indoor air pollution arising from contaminated groundwater

The migration of volatile contaminants into overlying buildings, known as vapor intrusion, is a health concern for people living above contaminated groundwater. As public health and environmental agencies develop protocols to evaluate vapor intrusion exposure, little attention has been paid to the experiences and opinions of communities likely to be affected by vapor intrusion. Using a community-driven research approach and qualitative interviews, we explored community perspectives on the vapor intrusion pathway and the perceived impact on community health and well-being among neighbors living atop a large, shallow-chlorinated solvent plume in San Antonio, TX. Most participants associated vapor intrusion with health risks, expressing concern about the unavoidable and uncontrollable nature of their exposure. Few were satisfied with the responsiveness of public officials. Above all, participants wanted more accurate, transparent information and additional independent scientific investigations.

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.

Spatiotemporal variability of tetrachloroethylene in residential indoor air due to vapor intrusion: a longitudinal, community-based study

The migration of volatile contaminants from groundwater and soil into indoor air is a potential health threat at thousands of contaminated sites across the country. This phenomenon, known as vapor intrusion, is characterized by spatial and temporal heterogeneity. This study examined short-term fluctuations in concentrations of tetrachloroethylene (PCE) in the indoor air of residential homes due to vapor intrusion in a community in San Antonio, Texas, that sits atop an extensive, shallow plume of contaminated groundwater. Using a community-based design, we removed potential indoor sources of PCE and then collected twelve 3-day passive indoor air samples in each of the 20 homes. Results demonstrated a one-order-of-magnitude variability in concentration across both space and time among the study homes, although all measured concentrations were below risk-based screening levels. We found that within any given home, indoor concentrations increase with the magnitude of the barometric pressure drop (P=0.048) and humidity (P<0.001), while concentrations decrease as wind speed increases (P<0.001) and also during winter (P=0.001). In a second analysis to examine sources of spatial variability, we found that indoor air PCE concentrations between homes increase with groundwater concentration (P=0.030) and a slab-on-grade (as compared with a crawl space) foundation (P=0.028), whereas concentrations decrease in homes without air conditioners (P=0.015). This study offers insights into the drivers of temporal and spatial variability in vapor intrusion that can inform decisions regarding monitoring and exposure assessment at affected sites.

Analytical Quantification of the Subslab Volatile Organic Vapor Concentration from a Non-uniform Source

The transport of volatile organic vapors from subsurface to building involves complex processes. Since the release of the draft subsurface vapor intrusion guidance by the U.S. EPA in 2002, great progress has been made in understanding these processes in various field and modeling studies. In these studies, the importance of analyzing and predicting the subslab volatile organic vapor concentration was noted. To quantitatively predict subslab vapor concentration is, however, complicated, especially for sites located over non-uniform vapor sources. This manuscript provides a method to estimate the vapor concentration beneath the subslab using a closed-form analytical solution that can approximate full three-dimensional modeling results, but does not require the use of advanced numerical simulation. This method allows prediction of the subslab vapor concentration profile beneath the slab for various source configurations, given inputs of building slab dimension and source depth. The interaction of the influences of non-uniform source and the slab capping effect on the subslab vapor concentration is addressed.

Analytical modeling of the subsurface volatile organic vapor concentration in vapor intrusion

The inhalation of volatile and semi-volatile organic compounds that intrude from a subsurface contaminant source into indoor air has become the subject of health and safety concerns over the last twenty years. Building subslab and soil gas contaminant vapor concentration sampling have become integral parts of vapor intrusion field investigations. While numerical models can be of use in analyzing field data and in helping understand the subslab and soil gas vapor concentrations, they are not widely used due to the perceived effort in setting them up. In this manuscript, we present a new closed-form analytical expression describing subsurface contaminant vapor concentrations, including subslab vapor concentrations. The expression was derived using Schwarz-Christoffel mapping. Results from this analytical model match well the numerical modeling results. This manuscript also explores the relationship between subslab and exterior soil gas vapor concentrations, and offers insights on what parameters need to receive greater focus in field studies.

Influence of Soil Moisture on Soil Gas Vapor Concentration for Vapor Intrusion

Mathematical models have been widely used in analyzing the effects of various environmental factors in the vapor intrusion process. Soil moisture content is one of the key factors determining the subsurface vapor concentration profile. This manuscript considers the effects of soil moisture profiles on the soil gas vapor concentration away from any surface capping by buildings or pavement. The "open field" soil gas vapor concentration profile is observed to be sensitive to the soil moisture distribution. The van Genuchten relations can be used for describing the soil moisture retention curve, and give results consistent with the results from a previous experimental study. Other modeling methods that account for soil moisture are evaluated. These modeling results are also compared with the measured subsurface concentration profiles in the U.S. EPA vapor intrusion database.

Estimation of Contaminant Subslab Concentration in Vapor Intrusion Including Lateral Source-Building Separation

Most current vapor-intrusion screening models employ the assumption of a subsurface homogenous source distribution, and groundwater data obtained from nearby monitoring wells are usually taken to reflect the source concentration for several nearby buildings. This practice makes it necessary to consider the possible influence of lateral source-building separation. In this study, a new way to estimate subslab (nonbiodegradable) contaminant concentration is introduced that includes the influence of source offset with the help of a conformal transform technique. Results from this method are compared with those from a three-dimensional numerical model. Based on this newly developed method, a possible explanation is provided here for the great variation in the attenuation factors of the soil vapor concentrations of groundwater-to-subslab contaminants found in the EPA vapor-intrusion database.

Examination of the U.S. EPA's vapor intrusion database based on models

In the United States Environmental Protection Agency (U.S. EPA)'s vapor intrusion (VI) database, there appears to be a trend showing an inverse relationship between the indoor air concentration attenuation factor and the subsurface source vapor concentration. This is inconsistent with the physical understanding in current vapor intrusion models. This article explores possible reasons for this apparent discrepancy. Soil vapor transport processes occur independently of the actual building entry process and are consistent with the trends in the database results. A recent EPA technical report provided a list of factors affecting vapor intrusion, and the influence of some of these are explored in the context of the database results.

Examination of the influence of environmental factors on contaminant vapor concentration attenuation factors using the U.S. EPA's vapor intrusion database

Those charged with the responsibility of estimating the risk posed by vapor intrusion (VI) processes have often looked to information contained in the U.S. Environmental Protection Agency (EPA)'s VI database for insight. Indoor air concentration attenuation factors have always been a key focus of this database, but the roles of different environmental factors in these attenuation processes are still unclear. This study aims to examine the influences of these factors in the context of the information in the VI database. The database shows that the attenuation factors vary over many orders of magnitude and that no simple statistical fluctuation around any typical mean value exists. Thus far, no simple explanation of this phenomenon has been presented. This paper examines various possible contributing factors to the enormous range of observed values, looking at which ones can plausibly contribute to explaining them.

A numerical investigation of vapor intrusion--the dynamic response of contaminant vapors to rainfall events

The U.S. government and various agencies have published guidelines for field investigation of vapor intrusion, most of which suggest soil gas sampling as an integral part of the investigation. Contaminant soil gas data are often relatively more stable than indoor air vapor concentration measurements, but meteorological conditions might influence soil gas values. Although a few field and numerical studies have considered some temporal effects on soil gas vapor transport, a full explanation of the contaminant vapor concentration response to rainfall events is not available. This manuscript seeks to demonstrate the effects on soil vapor transport during and after different rainfall events, by applying a coupled numerical model of fluid flow and vapor transport. Both a single rainfall event and seasonal rainfall events were modeled. For the single rainfall event models, the vapor response process could be divided into three steps: namely, infiltration, water redistribution, and establishment of a water lens atop the groundwater source. In the infiltration step, rainfall intensity was found to determine the speed of the wetting front and wash-out effect on the vapor. The passage of the wetting front led to an increase of the vapor concentration in both the infiltration and water redistribution steps and this effect is noted at soil probes located 1m below the ground surface. When the mixing of groundwater with infiltrated water was not allowed, a clean water lens accumulated above the groundwater source and led to a capping effect which can reduce diffusion rates of contaminant from the source. Seasonal rainfall with short time intervals involved superposition of the individual rainfall events. This modeling results indicated that for relatively deeper soil that the infiltration wetting front could not flood, the effects were damped out in less than a month after rain; while in the long term (years), possible formation of a water lens played a larger role in determining the vapor intrusion risk. In addition, soil organic carbon retarded the transport process, and damped the contaminant concentration fluctuations.