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Chloroform is frequently detected in indoor air in vapor intrusion (VI) studies. At many sites, initial screening shows potential indoor air impacts for chloroform at unacceptable levels. During follow‐up site characterization, however, the VI pathway for chloroform is rarely found to be the primary source of chloroform in indoor air. This paper sets forth a conceptual model for understanding these results and summarizes data from 10 field sites from various US Environmental Protection Agency regions to illustrate the concepts. Chloroform is a volatile organic compound (VOC) that is categorized as being a trihalomethane. It is well documented that it can be formed when chlorine is used to disinfect water. Chloroform also may be emitted directly into indoor air from treated water and may be formed and/or emitted from various cleaning products. The typical indoor air background concentration in residential buildings is approximately 0.5–1 µg/m3, but results will vary, and values of 10 µg/m3 or higher have been measured. The indoor air background concentrations presented in the literature are well above typical screening level concentrations for residential buildings. Field data are presented that demonstrate that the attenuation factor for chloroform often differs widely from attenuation factors of other VOCs that may pose a potential risk via the VI pathway. [ABSTRACT FROM AUTHOR]

The vapor intrusion (VI) of volatile organic compounds (VOCs) is a major concern due to their acute and chronic toxicities to indoor human. VI could be more problematic by pressure-driven advection and this could be particularly notable in residential areas. This study introduces a numerical model designed to simulate the transport of VOCs by diffusion as well as advection in unsaturated zone. The model is validated with analytical models suggesting its ability to simulate complex VI scenarios with high fidelity. Using the developed model, several VI scenarios are investigated by considering the episodic changes of atmospheric pressure as well as ventilation system induced indoor pressure changes in an TCE contaminated site. The model simulation shows that relatively lower atmospheric pressures compare to those in indoor environment facilitate TCE migration to the atmosphere leading to less TCE intrusion into indoor environment. This is the case when the indoor pressure is greater than those in atmosphere. On the other hand, when the indoor pressure in building is lower than those in atmosphere and when the pressure in atmosphere is higher than those in indoor, more TCE is intruded inside the indoor of building through the subsurface building cracks leading to the increased TCE levels and human exposure risks. This work not only advances our understanding of VI dynamics but also provides a robust tool for predicting indoor VOC concentrations under varying environmental conditions, especially when pressure driven advection becomes an important transport process in unsaturated zone. [ABSTRACT FROM AUTHOR]

The potential risks of vapor intrusion (VI) can arise from low bulk soil contaminant concentration existing in shallow soils beneath a building foundation. To assess VI risks for such a contamination scenario, a comprehensive study was conducted on a factory building located at a trichloroethylene (TCE)-contaminated site. This study involved the integration of various types of field data, including groundwater, bulk soil, soil gas and indoor air data, along with the utilization of the Vapor Intrusion Screening Level (VISL) calculator. Previously observed high TCE concentrations in soil gas are attributed to accumulation of TCE vapor within the unsaturated soil beneath the building floor, since ground surface is extensively paved at this site. These soil gas data do not directly correlate with the magnitudes of bulk soil and/or groundwater TCE concentration with the linear adsorption model. Soil gas TCE concentration exceeding 107 μg/m³ (or bulk soil concentration exceeding 18.9 mg/kg) observed in shallow soils (at a depth of less than 1 m) may pose health risk to the workers inside the building due to VI, as we have detected TCE vapor concentrations exceeding indoor air screening level several times in the past. This bulk soil TCE concentration, however, falls below soil pollution control standards for TCE, i.e., 60 mg/kg, in Taiwan. As a result, soil remediation is not considered at this site. Soil gas TCE concentrations have reduced to less than 106 μg/m³ after two years of groundwater remediation work at this site. However, we have observed significantly higher soil gas TCE levels at a depth of 0.5 m compared to other depths. This discrepancy raises suspicions that an amount of TCE may still be trapped within the shallow soils that are not reached by groundwater table. [ABSTRACT FROM AUTHOR]

Vapor intrusion is detrimental for indoor air quality. One of the most common sources of vapor intrusion is soil contaminated with petroleum hydrocarbons. To evaluate the long-term risk from individual exposure to hydrocarbons it is necessary to measure quantitively and reliably an average concentration level of individual pollutants on a monthly or yearly basis. Temporal variability of vapor intrusion from hydrocarbons poses a significant challenge to determination of average exposure and there is a need for reliable long-term integrative sampling. To this end, an analytical method for determination of 10 selected nonmethane hydrocarbons (NMHCs), including hexane, heptane, octane, decane, benzene, toluene, ethyl-benzene, m,p-xylene, o-xylene, and naphthalene, sampled on active triple-bed tubes filled with Carbograph 2, Carbograph 1, and Carboxen 1003 adsorbents was developed and validated. Extensive laboratory studies proved the absence of breakthrough at 50% HR and ambient temperature for experiments lasting up to 28 days and established a safe sampling time/volume of 20 days/114 L when sampling at a low flow rate of around 4 mL min−1. In addition, the developed method includes detailed uncertainty calculations for determination of concentrations. Finally, the method was tested by measuring NMHC concentrations in indoor air at a former industrial site during a 2-month-long field campaign in Lyon. The results of the field campaign suggest that 4-week integrated concentration measurements can be achieved by using active sampling on triple-bed tubes at 4.5 mL min−1. [ABSTRACT FROM AUTHOR]

This study investigates the influence of density-driven transport on chlorinated vapor intrusion through modeling and experiments. Density-driven transport involves downward vapor advection, potentially reducing vapor intrusion into buildings. A 1-D steady-state numerical model was developed using COMSOL Multiphysics, considering upward diffusion and downward density-driven advection in the subsoil and in the granular fill layer beneath building's foundations. Source vapor concentration and granular fill layer permeability emerged as the key factors affecting density-driven transport. Regardless of building characteristics, for permeabilities in the granular fill layer exceeding 10−7 m2, density-driven transport is expected to become relevant at vapor concentrations of 1 mg m−3, while for lower soil permeabilities (10−8-10−10 m2), density-driven transport impact is expected for vapor concentrations exceeding 1 g m−3. The results of laboratory column trichloroethylene (TCE) diffusion tests through sand and gravel supported these findings, showing a vertical stratification of TCE vapor concentrations consistent with the model. The trends expected by modeling also align with the findings of different field studies, where the source to building attenuation factors (AF) were found to decrease with increasing source vapor concentration. These outcomes highlight that the common approach adopted for vapor intrusion screening from soil gas data based on default AF values independent of source vapor concentration, may potentially lead to an overestimation of indoor concentrations in the presence of high vapor concentrations and highly permeable soils. Given the use of permeable granular fill layers in building construction, this study underscores the importance of accounting for density-driven transport to improve the accuracy of vapor intrusion risk assessment. [Display omitted] • Density-driven transport influence on chlorinated vapor intrusion was investigated. • TCE vapors diffusion tests were performed in sand and gravel-filled columns. • A numerical model considering density-driven for vapor transport was developed. • Subsurface vapor concentration and soil permeability emerged as determinant factors. • Neglecting density-driven effect may lead to overestimating vapor intrusion risk. [ABSTRACT FROM AUTHOR]

Future constructions in the context of the industrial wastelands reuse may be exposed to Vapor Intrusion (VI). VI can be evaluated by combining in-situ measures and analytical models to evaluate exposure risk in future indoor environments. However, the assumptions in the existing models may reduce their accuracy when they do not meet the characteristics of real situations. Wrong estimations of indoor concentration levels may lead to inappropriate solutions against VI. In this context, new semi-empirical models (SEM) are proposed in order to better specify pollution scenarios and thus increase the accuracy of VI estimations. This development is based on a parametric study (numerical CFD) and a dimensionless analysis combined to existing VI models that consider a continuous source distribution in the soil. These expressions allow to better take into account the source position in the soil (i.e. depth and lateral source/building separation), soil properties (air permeability, diffusion coefficient of the pollutant, ...) and building features (building foundation, indoor pressure, air exchange rate, ...) in the estimation of indoor concentration levels. The obtained results with the proposed SEM were compared with a numerical CFD model and available experimental data, showing good accuracy in the estimation of VI. Given the advantages of these new models, they can provide better precision in the health risk assessments associated with VI. Furthermore, these expressions can be easily integrated into building ventilation codes allowing to consider air exchange rate and indoor pressure variations over time. [ABSTRACT FROM AUTHOR]

While it is thought that some per- and polyfluoroalkyl substances (PFAS) may volatilize from aqueous solutions, experimentally measured Henry’s Law constants (kH, synonymous with air : water partition coefficient) are scarce. This leads to a lack of understanding of the partitioning of PFAS and an inability to predict concentrations above contaminated groundwater (e.g., vapor intrusion). We measured kH for 27 PFAS via headspace analysis and manipulations of the gas to liquid phase ratio. Fifteen PFAS produced mass spectrometry signals suitable for kH measurements. At 25 °C the experimentally measured dimensionless kH were: 0.31 – 2.82 for four fluorotelomer alcohols (FTOHs), 0.09 – 0.18 for three fluorotelomer sulfonates (FTSs), 0.30 – 1.01 for three iodinated PFAS, 0.43 – 0.92 for two sulfonamides, 3.86 for 6:2 fluorotelomer olefin, 0.69 for 8:2 fluorotelomer carboxylic acid, and 0.32 for 8:2 fluorotelomer acrylate. Longer fluoroalkyl chain length resulted in increased kH for FTOHs and FTSs, the only two groups in which chain length was studied. Perfluorinated sulfonates and carboxylates were generally not volatile enough to be measured, even at pH as low as 1, although fluorotelomers of both functional groups were measurably volatile. Temperature effects were well described by the van’t Hoff equation. kH was not significantly different in various environmentally relevant matrices demonstrating the broad applicability of the produced constants.

Traditional soil vapor intrusion (VI) models usually rely on preset conceptual scenarios, simplifying the influences of limiting environmental covariates in determining indoor attenuation factors relative to subsurface sources. This study proposed a technical framework and applied it to predict VI attenuation factors based on site-specific parameters recorded in the United States Environmental Protection Agency (USEPA)’s and the California Environmental Protection Agency (CalEPA)’s VI databases, which can overcome the limitations of traditional VI models. We examined the databases with multivariate analysis of variance to identify effective covariates, which were then employed to develop VI models with three machine learning algorithms. The results of multivariate analysis show that the effective covariates include soil texture, source depth, foundation type, lateral separation, surface cover, and land use. Based on these covariates, the predicted attenuation factors by these new models are generally within one order of magnitude of the observations recorded in the databases. Then the developed models were employed to generate the generic indoor attenuation factors to subsurface vapor (i.e., the 95th percentile of selected dataset), the values of which are different between the USEPA’s and CalEPA’s databases by one order of magnitude, although comparable to recommendations by the USEPA and literature, respectively. Such a difference may reflect the significant regional disparity in factors such as building structures or operational conditions (e.g., indoor air exchange rates), which necessitates generating generic VI attenuation factors on a state-specific basis. This study provides an alternative for VI risk screens on a site-specific basis, especially in states with a good collection of datasets. Although the proposed technical framework is used for the VI databases, it can be equally applied to other environmental science problems.

The fate of volatile organic compounds (VOC) vapors in the unsaturated zone is the basis for evaluating the natural attenuation potential and vapor intrusion risk. Microcosm and column experiments were conducted to study the effects chemical speciation and soil types/properties on the fate of petroleum VOCs in unsaturated zone. The biodegradation and total attenuation rates of the seven VOCs obtained by microcosm experiments in black soil and yellow earth were also generally higher than those in floodplain soil, lateritic red earth, and quartz sand. The VOC vapors in floodplain soil, lateritic red earth, and quartz sand showed slow total attenuation rates (<0.3 d−1). N-pentane, methylcyclopentane, and methylcyclohexane showed lower biodegradation rates than octane and three monoaromatic hydrocarbons. Volatilization into the atmosphere and biodegradation are two important natural attenuation paths for VOCs in unsaturated soil columns. The volatilization loss fractions of different volatile hydrocarbons in all five unsaturated soils were generally in the order: n-pentane (93.5%–97.8%) > methylcyclopentane (77.2%–85.5%) > methylcyclohexane (53.5%–69.2%) > benzene (17.1%–73.3%) > toluene (0–45.7%) > octane (1.9%–34.2%) > m-xylene (0–5.7%). The fractions by volatilization into the atmosphere of all seven hydrocarbons in quartz sand, lateritic red earth, and floodplain soil were close and higher compared to the yellow earth and black soil. Overall, this study illustrated the important roles chemical speciation and soil properties in determining the vapor-phase transport and natural attenuation of VOCs in the unsaturated zone. [Display omitted] • Volatilization and biodegradation are two main vapor natural attenuation pathways. • Soil texture and SOM content affect vapor fate and attenuation in the vadose zone. • Hydrocarbons with less carbon atoms have stronger volatilization loss. • Strong sorption and biodegradation enhanced vapor mass loss. [ABSTRACT FROM AUTHOR]

Conventional analytical methods determine the concentration of compounds used to estimate the presence of environmental contamination capable of posing risk to human and ecological receptors. While these target analytes help regulators and environmental professional identify and manage potentially harmful conditions, these compounds can be generated by many natural and anthropogenic (man-made) sources and conventional methods alone fail to compositionally differentiate subsurface sources of contamination in complex environments. This case study demonstrates the advantage of using both conventional and forensic testing methods to accurately identify the source(s) of volatile hydrocarbon contamination in soil vapor samples for the development of an accurate conceptual site model in a neighbourhood near industrial facilities in Rio de Janeiro State, Brazil. The goal of this study was to differentiate potential impacts from a local steelmaking plant from fugitive hydrocarbons associated with more generic human activity in urban settings. This case study demonstrated the advantage of constructing chemical fingerprints from conventional volatile organic compound (VOC) method TO 15 and a forensic volatile hydrocarbon method using an enhanced version of method TO 15 (PIANO). The chemical fingerprints of vapor samples collected from subsurface soil gas and sewers were analysed and compared to laboratory reference samples. These data determined that the VOCs detected in neighbourhood soil vapor samples were associated with fugitive petroleum products migrating in the sewer pipelines and not with the steelmaking wastes emplaced near the residential area. This article discusses the forensic data and chemical signatures that support these findings, and the use of environmental forensic techniques to evaluate environmental data associated with complex scenarios, involving multiple contamination sources. [ABSTRACT FROM AUTHOR]

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.

Trichloroethylene (TCE) in groundwater has the potential to volatilize through soil into indoor air where it can be inhaled. The purpose of this study was to determine whether individuals living above TCE-contaminated groundwater are exposed to TCE through vapor intrusion. We examined associations between TCE concentrations in various environmental media and TCE concentrations in residents. For this assessment, indoor air, outdoor air, soil gas, and tap water samples were collected in and around 36 randomly selected homes; blood samples were collected from 63 residents of these homes. Additionally, a completed exposure survey was collected from each participant. Environmental and blood samples were analyzed for TCE. Mixed model multiple linear regression analyses were performed to determine associations between TCE in residents' blood and TCE in indoor air, outdoor air, and soil gas. Blood TCE concentrations were above the limit of quantitation (LOQ; ≥ 0.012 µg L(-1)) in 17.5% of the blood samples. Of the 36 homes, 54.3%, 47.2%, and >84% had detectable concentrations of TCE in indoor air, outdoor air, and soil gas, respectively. Both indoor air and soil gas concentrations were statistically significantly positively associated with participants' blood concentrations (P = 0.0002 and P = 0.04, respectively). Geometric mean blood concentrations of residents from homes with indoor air concentrations of >1.6 µg m(-3) were approximately 50 times higher than geometric mean blood TCE concentrations in participants from homes with no detectable TCE in indoor air (P < .0001; 95% CI 10.4-236.4). This study confirms the occurrence of vapor intrusion and demonstrates the magnitude of exposure from vapor intrusion of TCE in a residential setting.

Organic contaminant diffusion through a novel multilayer coextruded vapor barrier is examined for benzene, toluene, ethylbenzene, xylenes (BTEX), trichloroethylene (TCE), and tetrachloroethylene (PCE). The vapor barrier considered is composed of five materials: linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), ethylene vinyl alcohol (EVOH), a tie layer (TL), and a degradation layer (DL). Parameters for the LLDPE, HDPE, TL, and DL are developed using material-specific diffusion tests. Contaminant-specific permeation coefficients (Pg) for these materials range from 1.4 to 9.2×10−11 m2/s. The diffusion parameters of the EVOH are inferred from testing of a thin (0.0889-mm) coextruded LLDPE/TL/EVOH/TL/EVOH membrane, and the contaminant specific Pg values range from 1.9 to 7.0×10−14 m2/s. These individual layer parameters are used to develop a single set of parameters for each contaminant for the entire vapor barrier, with overall contaminant specific Pg values of 2.7−13.8×10−13 m2/s. The parameters are used to model various vapor intrusion scenarios which show that the multilayer vapor barrier results in significant predicted decreases in airspace concentrations compared to HDPE or no barrier. [ABSTRACT FROM AUTHOR]

Per- and polyfluoroalkyl substances (PFAS) are used in various industrial products; however, they pose serious health risks. In this study, soil, soil gas, and groundwater samples were collected at a PFAS manufacturing facility in New Jersey, USA, to determine the presence and distribution of PFASs from the soil surface to groundwater and at various distances from the presumed source. Fluorotelomer alcohols (FTOHs) were detected in soil (< 0.26–36.15 ng/g) and soil gas (160–12,000 E µg/m3), while perfluorinated carboxylic acids (PFCAs) were found in soil (4.3–810 ng/g), soil gas (<0.10–180 µg/m3), and groundwater (37–49 µg/L). FTOH and PFCA concentrations decreased as the distance from the presumed source increased, suggesting that PFCAs are likely to migrate in groundwater, whereas FTOHs primarily move in the vapor phase. The presence of PFAS in the groundwater, soil, and soil gas samples indicate its potential for vapor intrusion; thus, some PFAS may contribute to indoor air inhalation exposure. To the best of our knowledge, this is the first report on the quantification of volatile PFAS in soil gas at a PFAS manufacturing facility. [Display omitted] • Per- and polyfluoroalkyl substances (PFAS) pose serious health risks. • PFAS in soil, soil gas, and groundwater at a site in New Jersey were analyzed. • Fluorotelomer alcohols were detected in soil and soil gas samples. • PFCAs were found in soil, soil gas, and groundwater samples. • PFAS are capable of vapor intrusion. [ABSTRACT FROM AUTHOR]

Two of the most prevalent Superfund-site contaminants are carcinogenic solvents PCE (perchloroethylene) and TCE (trichloroethylene). Because their cleanup is difficult and costly, remediators have repeatedly falsified site-cleanup data, as Tetra Tech apparently did recently in San Francisco. Especially for difficult-to-remediate toxins, this paper hypothesizes that scientific misrepresentations occur in toxic-site assessments, before remediation even begins. To begin to test this hypothesis, the paper (1) defines scientific-data audits (assessing whether published conclusions contradict source data), (2) performs a preliminary scientific-data audit of toxic-site assessments by consultants Ninyo and Moore for developer Trammell Crow. Trammel Crow wants to build 550 apartments on an unremediated Pasadena, California site – once a premier US Navy weapons-testing/development facility. The paper (3) examines four key Ninyo-and-Moore conclusions, that removing only localized metals-hotspots will (3.1) remediate TCE/PCE; (3.2) leave low levels of them; (3.3) clean the northern half of soil, making it usable for grading, and (3.4) ensure site residents have lifetime cancer risks no greater than 1 in 3,000. The paper (4) shows that source data contradict all four conclusions. After summarizing the benefits of routine, independent, scientific-data audits (RISDA), the paper (5) argues that, if these results are generalizable, RISDA might help prevent questionable toxic-site assessments, especially those of expensive-to-remediate toxins like PCE/TCE. [ABSTRACT FROM AUTHOR]

At contaminated sites, it is common to observe volatile organic compounds rising from subsurface sources and migrating into overlying buildings through cracks or other openings present in foundation slabs and basement walls. Such process, called vapor intrusion, is usually the most critical exposure pathway at sites contaminated by chlorinated solvents. In this study we present a chlorinated vapor intrusion tool implemented in Microsoft® Excel® using Visual Basic for Applications and integrated within a graphical interface that helps users to visualize two-dimensional (2D) soil gas concentration profiles and indoor air concentration in scenarios involving subsurface vertical heterogeneity. Vertical heterogeneity in soil gas concentration can be induced by a variable soil moisture profile caused by layering (geological barriers) or by the presence of the capillary fringe. This tool can be used in the risk assessment procedure to assess expected indoor air concentrations in conjunction with other lines of evidence, such as the evaluation of the 2D soil gas concentration profiles below and beyond the building footprint. Moreover, this tool allows users to predict indoor air concentration by employing either the traditional perimeter crack entry pathway or the empirical subslab-to-indoor attenuation factor. After a brief description of the developed tool, we show some practical applications to highlight the potential benefits in using this tool compared with the U.S. Environmental Protection Agency (U.S. EPA) tool that implements a simplified form of the Johnson and Ettinger model. Based on our testing, we found that the two-layer approach (capillary fringe and vadose zone) employed in the U.S. EPA tool, can lead to an overestimation of subslab vapor concentrations by more than an order of magnitude. [ABSTRACT FROM AUTHOR]

• Surface pavement does not affect deep soil gas concentration profiles. • Laterally extensive source requires a "half footprint size" deeper exterior sample. • Distance source requires 1–3 m deeper exterior sample for uniform soil moisture. In this study, we investigate the soil gas concentration attenuation with diffusive transport in lateral and vertical transport in cases with surface pavements with a pilot-scale tank. Three scenarios were investigated, one with a completely open soil surface and the other two involving partially paved soil surface. The results, on the one hand, indicate that the soil gas concentration generally decreases linearly and exponentially in the vertical and horizontal transport directions, respectively, generally in accordance with available modeling studies. On the other hand, our experiment shows that low-permeability ground covers can increase shallow soil gas concentrations beneath the pavement by at most 3–4 times, inducing higher subslab concentration than that below open ground surface, even if the latter is obtained at a closer location to vapor source. For cases with uniform soil properties, our study suggests exterior soil gas sample should be taken at a depth below the building foundation by half of the building footprint size, if the vapor source is laterally extensive relative to the building footprint, or by 1 m and 2–3 m for slab-on-grade and basement scenarios, respectively, if the vapor source is located laterally away from the building. [ABSTRACT FROM AUTHOR]

Abstract 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. Graphical abstract Image 1 Highlights • A source depletion model for vapor intrusion was proposed. • The model considers the impact of building characteristics. • The model still follows the law of mass conservation. • The model is more applicable when the area of building is large. A source depletion model for vapor intrusion that takes into account the impact of building characteristics was proposed. [ABSTRACT FROM AUTHOR]

The potential human health risks associated with soil vapor intrusion and volatile organic compounds (VOCs) exposure were characterized at an industrialized site by the quantification of gaseous VOCs in soil pores using a passive sampling technique. The gaseous tetrachloroethene (PCE) in soil pores varied between 12 and 5,400 μg m−3 showing 3 orders of magnitude variation with dependence on groundwater PCE concentrations. Though the PCE concentration in the air only varied between 0.45 and 1.5 μg m−3 showing negligible variations compared to the variation observed in soil pores. The PCE concentration in the air varied between 0.45 and 1.5 μg m−3. The calculation of fugacity suggested that the PCE in the test site originated from groundwater. Measured PCE in groundwater ranged from 14 to 2,400 times higher than PCE in soil gas. This indicates that conducting a vapor intrusion risk assessment using passive soil gas sampling is critical for accurate risk characterization and assessment. Estimated PCE inhalation cancer risks for street cleaners and indoor residents varied between 10−6 and 10−4 with a low plausible hazard, and between 10−3 and 10−2 with a high risk, respectively. The results of this study demonstrate that passive sampling offers a significantly lower cost and labor-intensive approach compared to traditional methods for assessing pollution distribution in contaminated sites and characterizing risks. This highlights the potential for wider application of passive sampling techniques in environmental studies. • Soil vapor intrusion risk and VOCs exposure at contaminated site were characterized. • VOCs concentrations in air, groundwater, and soil gas were quantified. • The fate of VOCs in the soil was evaluated by calculating the fugacity fraction. • Lateral diffusion from the source was more important than vertical diffusion. • Results imply that passive sampling soil gas is crucial for precise risk assessment. [ABSTRACT FROM AUTHOR]

Most of the proposed Vapor Intrusion (VI) models are developed assuming a steady indoor environment (i.e., building pressure and air exchange rate). To account for variations in these building conditions, these models are coupled with multizone codes to enable more precise modeling of indoor air pollution. In this paper, semi-empirical VI models are integrated into MATHIS-QAI, a multi-zone ventilation software. This coupled tool allows consideration not only of the impact of the building ventilation system characteristics, airtightness, and foundation type, but also the computation of more realistic pollution scenarios by specifying the lateral separation between the pollution source in the soil and the building. A sensitivity analysis was conducted to quantify the influence of these parameters on the indoor concentration of pollutants. The results showed that the main driving parameter in this event is the source location in the soil. However, a significant impact of the building characteristics and weather conditions on the indoor pollutant concentration was also observed. These characteristics vary significantly from one building to another, necessitating specific and appropriate calculations. The proposed tool, based on nodal modeling, offers an easy-to-use simulation that does not require significant computational resources compared to Computational Fluid Dynamics approaches. This coupling can be utilized for optimal management and reduction of uncertainties in risk assessment. Ultimately, it can serve as a relevant tool in the design and conception of more efficient buildings against VI. • Semi-empirical vapor intrusion (VI) models were integrated in a ventilation code. • A parametric study was conducted considering several VI scenarios. • VOCs source position in the soil remains the key parameter on VI process. • Specifying pollution scenario features increases the quality of the VI modelling. • Higher precision of VI estimations enhances building protection management. [ABSTRACT FROM AUTHOR]

Abstract The reduction of trichloroethylene (TCE) in gas phase by different types of granular zero-valent iron (Fe0) was examined in anaerobic batch vapor systems performed at room temperature. Concentrations of TCE and byproducts were determined at discrete time intervals by analysis of the headspace vapors. Depending on the type of iron used, reductions of TCE gas concentration from 35% up to 99% were observed for treatments of 6 weeks. In line with other experimental studies performed with aqueous solutions, the particle size was found to play a key role in the reactivity of the iron. Namely an increase of the TCE removal up to almost 3 times was observed using iron powders with particle size lower than 425 μm compared to iron powders with particle size lower than 850 μm. The manufacturing process of the iron powder was instead found to play only a limited role. Namely, no significant differences were observed in the TCE reduction by Fe0 obtained using an iron powder attained by water atomization and sieving compared to the removal achieved using an iron powder subjected to a further annealing processes to reduce the content of oxides. Conversely, the pretreatment of the iron powder with HCl was found to enhance the reactivity of the iron. In particular, by washing the iron powder of 425 μm with HCl acid 0.1 M the reduction of TCE after 6 weeks of treatment increase from approximately 80% for the as received material to >99% for the pretreated iron powder. We also performed tests at different humidity of the iron observing that not statistical differences were obtained using a water content of 10% or 50% by weight. In all the experiments, the only detectable byproducts of the reactions were C4–C6 alkenes and alkanes that can be attributed to a hydrogenation of the C Cl bond. Graphical abstract Unlabelled Image Highlights • We examined the reduction of TCE vapors by granular zero-valent iron. • The particle size of the granular material plays a key role in the iron reactivity. • The manufacturing process of iron was found to not affect the TCE removal. • Using an acid washed iron a 99% reduction of TCE was observed after 6 weeks. [ABSTRACT FROM AUTHOR]

We have utilized the California GeoTracker database to evaluate field duplicate variability and the significance of sample contamination for groundwater and vapor samples collected from contaminated sites in California. Vapor duplicates are more variable than water duplicates with median percent difference in concentration of 25% compared to 7% for water samples. In addition, large differences in concentration were more common in vapor duplicates. For vapor analyte pairs, 20% of pairs had a percent difference in concentration of >300% while, for groundwater analyte pairs, only 3% had a percent difference of >300%. Contamination of samples during collection or analysis is also more significant for vapor samples. For water samples, sample contamination appears unlikely to result in false positive exceedances of drinking water standards; however, for vapor samples, sample contamination may result in false positive exceedances of indoor air screening values. For vapor samples, the use of reusable canisters and flow controllers is likely an important source of sample contamination. [ABSTRACT FROM AUTHOR]

Highlights • A two-layer numerical model was used to simulate chlorinated vapor intrusion. • Sub-foundation soil plays a key role for soil permeability higher than 10−11 m2. • Capillary fringe represents the higher resistance to upward soil gas flow. • Groundwater source depth only slightly affects chlorinated vapor intrusion. Abstract 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. [ABSTRACT FROM AUTHOR]

The distribution of the subslab soil gas attenuation factor α for residential houses was simulated using a mathematical relationship that quantifies the influence of stack and wind effects on vapor intrusion. The simulations presented in this article account for variations in leak distribution on the building envelope, effective leakage area distribution in the U.S. residential housing stock, and temperature and wind speed conditions for 20 U.S. cities in seven temperature-based climate zones. For the most air-tight type of houses (“energy-efficient”) in these climate zones, the 95th percentile of the annual mean α ranges from 0.001 to 0.004 as the annual mean temperature goes from 20°C to 3°C. The less air-tight types of houses (“low-income” and “conventional”) have α distributions with lower values. Utility of the simulations in assessing short-term vapor intrusion exposures is also demonstrated by constructing distributions of the maximum 21-day average for the year. The 95th percentile of these short-term α distributions for energy-efficient houses ranges from 0.002 to 0.005. The results presented in this article support the development of vapor intrusion screening levels that are based on values of α appropriate for a climate zone and housing type. [ABSTRACT FROM AUTHOR]

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

One of the goals of the French national campaign called “Etablissements Sensibles (Sensitive Establishments)” is to evaluate indoor air degradation in schools because of vapor intrusion of volatile compounds from soil gases towards the indoor air, related to the presence of former industrial sites on or near the establishment. During this campaign, as recommended by the United States of Environmental Protection Agency (US EPA), indoor air quality was evaluated from soil gas concentrations using generic attenuation factors, and extra investigations into soil gases and indoor air were performed when the estimated values exceeded target indoor air concentrations. This study exploits matched data on subsurface soil gases and indoor air that came from the “Sensitive Establishments” campaign. It aims to consolidate and refine the use of attenuation factors as a function of environmental variables acquired routinely during environmental assessments. We have been able to select the measured environmental variables that have the most influence on vapor intrusion using Principal Components Analysis and hypotheses tests. Since the collected data are mainly related to weak sources (only 15% schools required risk management measures related to vapor intrusion), halogenated volatile organic compounds (HVOC) were selected as tracer compounds for vapor intrusion for this study. This choice enables the exclusion or minimization of background sources contributions. From the results we have calculated the descriptive statistics of the attenuation factors distribution for the subslab-to-indoor air pathway and refined the attenuation factors for this pathway through an easily obtained parameter, building age. Qualitative comparison of attenuation factors according to the building age shows that attenuation factors observed for building less than 50 years are lower than attenuation factors for buildings 50 years old and above. These results show the utility of creating databases for consolidating and refining attenuation factors and therefore improving their use. [ABSTRACT FROM AUTHOR]

The current work is an attempt to illustrate the importance of using passive soil gas as an innovative investigation technique in the assessment of soil and groundwater pollutions that emanates from volatile hydrocarbon activities in newly emerging countries. Bazian Oil Refinery as one of the largest refinery in Iraqi Kurdistan Region produces 40,000 barrels a day and provides a wide range of petroleum products for daily consumption. The types and scale of different process that happen in this industrial site have led to concerns with regard to its impact on both the soil and groundwater the vicinity of the factory. The researcher conducted a combined sampling design with a dual-phased extraction procedure for soil vapor and groundwater samples in order to assess the susceptibility of the subsurface to pollution with hydrocarbon. The aims were to characterize potential source(s), map the areal extent of the site which is at risk to be affected with the identified9 hydrocarbon compounds and vapor. A collection kit from Beacon Environmental Service was used to collect a total number of 50 passive soil vapors in the first step of work. To extrapolate results, five shallow boring for soils and six for water sampling were carefully observed. The selection of the sampling points was based on the results revealed by the PSG survey that showed significant quantities of analyzed organic hydrocarbon for a follow-up investigation. The matrices were analyzed by ALS Laboratory to target more than 40 VOCs and SVOCs. The plan was to make the mass to concentration tie-in for the selected analyzed compounds (Benzene, Toluene, and Total Petroleum Hydrocarbons) from the PSG in mass (nanograms) with both the soil and water samples in concentration. The results revealed that the PSG technique is unique in identifying the source and extent of soil and groundwater pollutions plume. [ABSTRACT FROM AUTHOR]

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Sewer systems have been recently recognized as potentially important exposure pathways to consider during vapor intrusion assessments; however, this pathway has not been well-characterized and there is need for additional information about the occurrence of volatile organic compounds (VOCs) in sewer systems. This paper reports the results of sewer gas sampling conducted in a sanitary sewer over the years of 2014–2017. Sewer gas samples were collected and analyzed using several different techniques, including TO-15 (grab), TO-17 (passive), Radiello® (passive) and a novel continuous monitoring technique, the Autonomous Rugged Optical Multigas Analyzer (AROMA). The applicability of each of the different approaches used in this study is discussed in the context of investigating sanitary sewers as a vapor intrusion alternative pathway. The data confirmed that trichloroethylene (TCE) concentrations in sewer gas were detected adjacent to and extending hundreds of feet away from a previously defined vapor intrusion area, where TCE was a primary contaminant. TCE concentrations detected in sewer gas ranged from non-detect to 1600 μg/m 3 . Temporal variability was observed in TCE concentrations over timescales that ranged from minutes to months to years at discrete sampling locations. Spatial variability in sewer gas concentrations was also observed throughout the study area. Temporal and spatial variability may be caused by groundwater contamination sources in the study area, as well as sewer gas transport mechanisms. [ABSTRACT FROM AUTHOR]

Traditional natural attenuation studies focus on aqueous process in the saturated zone while vapor-phase biodegradation and natural attenuation in the unsaturated zone received much less attention. This study used microcosm experiments to explore the vapor-phase biodegradation and natural attenuation of 23 petroleum VOCs in the unsaturated zone including 7 monoaromatic hydrocarbons, 6 n-alkanes, 4 cycloalkanes, 3 alkylcycloalkanes and 3 fuel ethers. We found that monoaromatic hydrocarbon vapors were easily attenuated with significantly high first-order attenuation rates (9.48 d−1-43.20 d−1) in live yellow earth, of which toluene and benzene had the highest rates (43.20 d−1 and 28.32 d−1, respectively). The 13 aliphatic hydrocarbons and 3 fuel ethers all have relatively low attenuation rates (<0.54 d−1) in live soil and negligible biodegradation contribution. We explored the effects of soil types (black soil, yellow earth, lateritic red earth and quartz sand), soil moisture (2, 5, 10, and 17 wt%) contents and temperatures (4, 15, 25, 35 and 45 °C) on the vapor attenuation. Results showed that increasing soil organic matter (SOM) content, silt content, porosity and soil microorganism numbers enhanced contaminant attenuation and remediation efficiency. Increasing moisture content reduced the apparent first-order biodegradation rates of monoaromatic hydrocarbon vapors. The vapor-phase biodegradation had optimal temperature (∼25 °C in yellow earth) and increasing or decreasing temperature slowed down biodegradation rate. Overall, this study enhanced our understanding of vapor-phase biodegradation and natural attenuation of petroleum VOCs in the unsaturated zone, which is critical for the long-term management and remediation of petroleum contaminated site. [Display omitted] • 23 VOC vapors natural attenuation rates were explored in four unsaturated soils. • Increasing soil moisture content decreased the natural attenuation rates. • The optimal temperature for vapor-phase biodegradation were at around 25 °C. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Vapor intrusion of synthetic fuel additives represents a critical yet still neglected problem at sites contaminated by petroleum fuel releases. This study used an advanced numerical model to investigate the vapor intrusion potential of fuel ether oxygenates methyl tert -butyl ether (MTBE), tert -amyl methyl ether (TAME), and ethyl tert -butyl ether (ETBE). Simulated indoor air concentration of these compounds can exceed USEPA indoor air screening level for MTBE (110 μg/m 3 ). Our results also reveal that MTBE has much higher chance to cause vapor intrusion problems than TAME and ETBE. This study supports the statements made by USEPA in the Petroleum Vapor Intrusion (PVI) Guidance that the vertical screening criteria for petroleum hydrocarbons may not provide sufficient protectiveness for fuel additives, and ether oxygenates in particular. In addition to adverse impacts on human health, ether oxygenate vapor intrusion may also cause aesthetic problems ( i.e. , odour and flavour). Overall, this study points out that ether oxygenates can cause vapor intrusion problems. We recommend that USEPA consider including the field measurement data of synthetic fuel additives in the existing PVI database and possibly revising the PVI Guidance as necessary. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Vapor intrusion (VI) is the migration of subsurface vapors, including radon and volatile organic compounds (VOCs), from the subsurface to indoor air. The VI exposure pathway extends from the contaminant source, which can be impacted soil or groundwater, to indoor air-exposure points. VOC contaminants of concern typically include halogenated solvents as well as petroleum hydrocarbons. Radon, a colorless radioactive gas that is released by radioactive decay of radionuclides in rock and soil, migrates into homes through VI in a similar fashion to VOCs. This project focused on the performance of permanent versus temporary sub-slab sampling ports for the determination of VI of halogenated VOCs and radon into an unoccupied house. VOC and radon concentrations measured simultaneously in soil gas using collocated temporary and permanent ports appeared to be independent of the type of port. The variability between collocated temporary and permanent ports was much less than the spatial variability between different locations within a single residential duplex. Post sampling leak test results suggested that the temporary SSP desiccation and cracking of the clay portion of the seal were not as detrimental to the port seal performance as would have been expected, this suggests that the Teflon tape portion of the seals served an important function. Pre and post sampling leak tests are advisable when temporary ports are used to collect a time-integrated sample. These results suggest that temporary sub-slab sampling ports can provide data equivalent to that collected from a permanent sub-slab sampling port. [ABSTRACT FROM PUBLISHER]

Vapor intrusion (VI) is a term used to describe indoor air (IA) contamination that occurs due to the migration of chemical vapors in the soil and groundwater. The overall vapor transport process depends on several factors such as contaminant source characteristics, subsurface conditions, building characteristics, and general site conditions. However, the classic VI conceptual model does not adequately account for the physics of airflow around and inside a building and does not account for chemical emissions from alternative "preferential" pathways (e.g. sewers and other utility connections) into IA spaces. This mini-review provides information about recent research related to building air exchange rates (AERs) and alternative pathways to improve the accuracy of VI exposure risk assessment practices. First, results from a recently published AER study for residential homes across the United States (US) are presented and compared to AERs recommended by the US Environmental Protection Agency (USEPA). The comparison shows considerable differences in AERs when season, location, building age, and other factors are considered. These differences could directly impact VI assessments by influencing IA concentration measurements. Second, a conceptual model for sewer gas entry into buildings is presented and a summary of published field studies is reported. The results of the field studies suggest that alternative pathways for vapors to enter indoor spaces warrant consideration. Ultimately, the information presented in this mini-review can be incorporated into a multiple-lines-of-evidence approach for assessing site-specific VI exposure risks. [ABSTRACT FROM AUTHOR]

An equilibrium passive sampler made of polydimethylsiloxane (PDMS) fiber was developed to measure volatile organic compounds (VOCs) in soil vapor. Expanded polytetrafluoroethylene (ePTFE) was used to protect PDMS from pollution and direct contact with soil components. For all tested VOCs, equilibrium was reached after 7 days at 5 °C. The equilibrium partition coefficients of VOCs between PDMS, gas, and water were measured at three different temperatures. The analyte concentrations in PDMS exposed to gas and water separately were almost the same, which suggests that C gas and C water in soil pores can be accurately deduced from C PDMS after equilibrium at various temperatures. To evaluate the passive sampler, active sampling measurements were performed simultaneously. Concentrations of VOCs deduced from the passive sampler were consistent with the concentrations measured by active sampling near the 1:1 line. Tests with artificial soils were conducted to observe the effects of soil components on passive sampling. The results suggest that the effect of water saturation can be ignored; in other words, the developed passive sampler can be applied in the vadose zone, which has fluctuating water saturation. With a holder for the sampler made of stainless steel, the developed in situ passive sampler can measure VOCs in contaminated soil vapor. The developed passive sampler was proven to be an alternative for measuring VOCs in soil vapor, which can be helpful for soil risk assessment and for observing the diffusion of VOCs in contaminated sites. [Display omitted] • An equilibrium passive sampler was developed to measure VOCs in the soil vapor. • C gas and C water in soil pores can be accurately deduced from C PDMS. • To evaluate the passive sampler, active sampling was performed simultaneously. • Tests were conducted to observe the effects of soil components on passive sampling. • A holder for the sampler made of stainless steel was used for field applications. [ABSTRACT FROM AUTHOR]

The vadose zone is a very dynamic and active environment that directly affects natural attenuation and vapor intrusion of volatile organic compounds (VOCs). Therefore, it is important to understand the fate and transport of VOCs in the vadose zone. A column experiment combined with model study was conducted to investigate the influence of soil type, vadose zone thickness, and soil moisture content on benzene vapor transport and natural attenuation in the vadose zone. Vapor-phase biodegradation and volatilization to atmosphere for benzene are two main natural attenuation mechanism in the vadose zone. Our data showed that biodegradation in black soil is the main natural attenuation mechanism (82.8%) while volatilization is the main natural attenuation mechanism in quartz sand, floodplain soil, lateritic red earth and yellow earth (>71.9%). The R-UNSAT model-predicted soil gas concentration profile and flux were close with four soil column data except for yellow earth. Increasing the vadose zone thickness and soil moisture content significantly reduced the volatilization contribution while increased biodegradation contribution. The volatilization loss decreased from 89.3% to 45.8% when the vadose zone thickness increased from 30 cm to 150 cm. The volatilization loss decreased from 71.9% to 10.1% when the soil moisture content increased from 6.4% to 25.4%. Overall, this study provided valuable insights into clarifying the roles of soil type, moisture, and other environmental conditions in vadose zone natural attenuation mechanism and vapor concentration. [Display omitted] • Detailed understanding of vadose zone vapor transport and fate. • Vapor concentration and natural attenuation mechanism depend on soil type. • Increasing the vadose zone thickness reduced the volatilization contribution. • Increased soil moisture reduced the volatilization loss and prevented vapor migration. [ABSTRACT FROM AUTHOR]