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Building modeling approach for IAQ assessment: Influence of the main drivers of vapor intrusion from the subsurface
Abstract
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.
Les sites pollués (sol ou eaux souterraines) représentent un potentiel de risque pour la santé humaine et l’environnement. Il existe des outils d’aide à la gestion, en complément des mesures in-situ, qui permettent d’estimer rapidement et à moindre coût les risques sanitaires associés à l’exposition des polluants gazeux du sol dans les espaces intérieurs afin d’établir des mesures de prévention et/ou correction. Cependant, et malgré leur intérêt, il a été montré dans la littérature qu’il existe des différences importantes entre les concentrations intérieures mesurées et les estimations des outils existants. Ces incertitudes reposent principalement sur trois aspects : une mauvaise caractérisation du site, une modélisation incomplète des voies et mécanismes de transfert, ou bien du fait de négliger l’influence de certains paramètres sur le transfert. Par exemple, le fait de négliger la latéralité de la source reste une explication plausible des limites des modèles classiques de transfert. Les auteurs conviennent que la migration latérale joue un rôle important sur l’atténuation de la concentration intérieure en polluant, contrairement aux scénarios de source homogène ou continue, où les vapeurs migrent uniquement de manière verticale vers le bâtiment. Ainsi, lorsque la source est latéralement décalée vis-à-vis du bâtiment, les vapeurs vont migrer préférentiellement vers l’atmosphère et moins vers le bâtiment générant une atténuation plus importante de la concentration intérieure. Dans ce contexte, l’objectif principal de ces travaux de thèse est la contribution à l’amélioration des outils d’aide à la gestion afin d’élargir leur plage d’application. Pour ce faire, des nouveaux modèles ont été développés permettant de tenir compte de la latéralité de la source dans l’estimation de la concentration intérieure en polluant. Le développement de ces modèles est réalisé à partir de l’expérimentation numérique et l’analyse adimensionnelle sur la base des outils existants (modèles semi-empiriques construits en considérant une source continue). La combinaison de ces deux approches permet d’une part, de garder la capacité des modèles source continue de tenir compte des propriétés physiques du sol (perméabilité, coefficient de diffusion, …) et des caractéristiques du bâtiment (typologie de soubassement, dépression, volume, …), et d’une autre part, de mieux préciser la position de la source dans le sol en considérant l’influence de sa latéralité dans les estimations. Ces nouveaux modèles ont été issus d’une analyse comparative permettant de vérifier la cohérence et la précision des estimations vis-à-vis d’un modèle numérique (CFD), de données expérimentales et de modèles existants dans la littérature. Finalement, ces expressions ont été intégrées dans un code de ventilation (MATHIS-QAI) permettant de mieux préciser les caractéristiques des environnements intérieurs (système de ventilation, perméabilité à l’air de l’enveloppe, volume du bâtiment, …) et de réaliser des estimations des niveaux de concentration en fonction des variations temporelles (vitesse du vent, température extérieure, …) au cours du temps. À partir d’une étude paramétrique il a été montré que malgré l’impact non-négligeable des caractéristiques du bâtiment, l’influence de la latéralité de la source sur l’atténuation de la concentration intérieure en polluant reste prédominante (atténuation de plusieurs ordres de grandeur quand la source est décalée latéralement du bâtiment en comparaison à une source continue). Cependant, préciser les caractéristiques du bâtiment (soubassement, système de ventilation, perméabilité à l’air de l’enveloppe,…), ainsi que les conditions météorologiques uniques de chaque projet de construction, permet d’augmenter la précision des estimations en évitant la mise en œuvre de solutions extrêmes ou bien encore, de mesures inadaptées.
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 paper, 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.
The aim of this study is to check the accuracy of a nodal model to predict correctly the flow fields involved inside a building by wind-induced pressure. The model is confronted to experimental tests involving a four-storey dwelling at a reduced scale of 1/20 placed in a wind tunnel facility. Different configurations are tested considering openings of different sizes for outside openings as well as for internal doors and the presence or not of a collective duct connecting the kitchens to the outside at roof level. For each configuration, various wind incidences are studied. Internal pressure coefficient obtained in each room is compared between the experiment and the model. The effects of the variation of the windows discharge coefficient as function of wind incidence is discussed.
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.
This paper concerns modeling and simulation of coupled heat and air flow in buildings. A brief overview of the current state in modeling this issue is included. Starting from a zonal mass balance approach, the paper describes a method used for the simultaneous solution of the associated nonlinear equations, and the solution coupling of the heat and mass conservation equation sets. By means of a case study involving a case of strongly coupled heat and air flow, this paper aims to quantify the differences--in terms of accuracy and computer resources--resulting from coupled and decoupled solution methods. The main conclusion from the case study is that the coupled solution method will be able to generate accurate results, even with simulation time steps of one hour. Reducing the time step will increase the computing resources used considerably, with a relatively small improvement in the accuracy. For equal length of time step a coupled solution method will use more computer resources than a decoupled solution. In the case of the decoupled method it is necessary to reduce the time step, to ensure the accuracy. For the current case study, the decoupled solution method using a simulation time step of 360 s was less accurate than the coupled solution method with a time step of one hour. However, the computer resources used were more than double. Based on the case study it may be concluded that the coupled solution gives the best overall results in terms of both accuracy and computer resources used.
Matlab/Simulink is known in a large number of fields as a powerful and modern simulation tool. In the field of building and HVAC simulation its use is also increasing. However, it is still believed to be a tool for small applications due to its graphical structure and not to fit well for the simulation of multizone buildings. This paper presents the development of a new multizone building model for Matlab/Simulink environment, implemented into the SIMBAD Building and HVAC Toolbox. It's general enough to model a variety of useful cases. Conforming to the Simulink philosophy, the model is modular and structured into blocks to represent the modelled phenomena. To simplify the description of the simulated building, a graphical user interface SIMBDI is developed in parallel, generating an xml building description file. This file can be read directly by the SIMBAD multizone building model. Finally, a simulation case is presented in order to compare the new model with the Trnsys multizone building model.
Domestic pollution is relevant to health because people spend most of their time indoors. One half of the world's population is exposed to high concentrations of solid fuel smoke (biomass and coal) that are produced by inefficient open fires, mainly in the rural areas of developing countries. Concentrations of particulate matter in kitchens increase to the range of milligrams per cubic meter during cooking. Solid fuel smoke possesses the majority of the toxins found in tobacco smoke and has also been associated with a variety of diseases, such as chronic obstructive pulmonary disease in women, acute respiratory infection in children and lung cancer in women (if exposed to coal smoke). Other tobacco smoke-associated diseases, such as tuberculosis, asthma, respiratory tract cancer and interstitial lung diseases, may also be associated with solid fuel smoke inhalation, but evidence is limited. As the desirable change to clean fuels is unlikely, efforts have been made to use efficient, vented wood or coal stoves, with varied success due to inconsistent acceptance by the community.
The CSOIL exposure model was developed for derivation of ntervention Values for soil and groundwater clean-up. These Intervention Values are based on potential risks to humans exposed to soil contaminants. Theoretical evaluation of the CSOIL's volatilization module has shown to be not suitable for actual risk-assessment. For this reason, the VOLASOIL model has been developed for actual risk assessment in case of volatile soil contaminants. The VOLASOIL model calculates the indoor air concentration for the Dutch situation in buildings situated on soils contaminated with volatile compounds. The VOLASOIL model can be seen as an optimum between scientifically sound and applicable in practice. The model can be used for site-specific risk assessment because of the possibility of flexible combination of modelling and measurements and calculations being made for several specific contamination cases. Some of these are floating contaminant layers, pure contaminant in the open capillary zone, contaminated groundwater in crawl spaces, et cetera. The VOLASOIL model could be used as a decision-support tool within the framework of soil clean-up priority (Soil Protection Act), construction permit issues (Housing Act), and soil quality management (spatial planning). A user-friendly Windows-application has been developed for using the VOLASOIL model in practice. This computer program can be obtained at the RIVM. Het blootstellingsmodel CSOIL is ontwikkeld voor de afleiding van de Interventiewaarden voor bodem- en grondwatersanering. Deze Interventiewaarden zijn gebaseerd op het potentiele risico voor de mens bij blootstelling aan bodemverontreiniging. Theoretische evaluatie van de vervluchtigingsmodule uit het CSOIL-model heeft aangetoond dat de module niet geschikt is voor actuele risico-analyse. Daarom is het VOLASOIL-model ontwikkeld voor de actuele risico-analyse in het geval van bodemverontreiniging met vluchtige verbindingen. Het VOLASOIL-model berekent, voor de Nederlandse situatie, de binnenluchtconcentratie in huizen gebouwd op dergelijk vervuilde bodems. Het model kan beschouwd worden als een optimum tussen degelijke theoretische onderbouwing en toepasbaarheid in de praktijk van het bodemonderzoek. Doordat een flexibele combinatie mogelijk is tussen meten en rekenen en omdat het model berekeningen kan uitvoeren voor verschillende verontreinigingssituaties (drijflaag, puur produkt in de onverzadigde zone, gecontamineerd grondwater in de kruipruimte, etc.), kan het model gebruikt worden voor locatie-specifieke risico-analyse. Het VOLASOIL-model zou gebruikt kunnen gaan worden als een beslissingsondersteunend instrument in het kader van de saneringsurgentiesystematiek (Wet Bodembescherming), de systematiek voor de beoordeling van de bodemkwaliteit bij bouwvergunningsaanvragen (Woningwet) en Actief Bodembeheer (Ruimtelijke Ordening). Ten behoeve van het gebruik in de praktijk is een gebruikersvriendelijke windows-applicatie ontwikkeld. Dit computerprogramma is bij het RIVM te verkrijgen.
Details of a three-dimensional finite element model of soil vapor intrusion, including the overall modeling process and the stepwise approach, are provided. The model is a quantitative modeling tool that can help guide vapor intrusion characterization efforts. It solves the soil gas continuity equation coupled with the chemical transport equation, allowing for both advective and diffusive transport. Three-dimensional pressure, velocity, and chemical concentration fields are produced from the model. Results from simulations involving common site features, such as impervious surfaces, porous foundation sub-base material, and adjacent structures are summarized herein. The results suggest that site-specific features are important to consider when characterizing vapor intrusion risks. More importantly, the results suggest that soil gas or subslab gas samples taken without proper regard for particular site features may not be suitable for evaluating vapor intrusion risks; rather, careful attention needs to be given to the many factors that affect chemical transport into and around buildings.
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.
Various screening-level and analytical models have been proposed in order to evaluate Vapor Intrusion (VI) and provide assessment tools for exposure risk in indoor environments. However, many in situ investigations show important differences between predicted and measured indoor concentrations generally associated with inappropriate conceptual modelling, incomplete VI process or by ignoring critical parameters in the evaluations. In this study, a numerical model is developed to better understand how polluted site characteristics as source position, soil properties, building pressure and type of construction may affect VI process from non-degrading chemicals. The results confirm that source location plays a critical role on VI compared to soil properties and building features. Increasing lateral distance from a building decreases indoor concentration about 5 orders of magnitude when the source is shallow and 2 to 3 orders of magnitude for deeper sources. However, despite the main influence of the position of the source, soil properties and building characteristics impacts are not insignificant: building pressure (−4 Pa) may increase VI by a factor of 2 compared to building at atmospheric pressure, slab on grade construction types increase vapor attenuation of 80% compare to a bare ground configuration and permeable soils may allow vapors to migrate more easily to the building by generating an indoor concentration up to 10 times higher compared to impermeable soils. Current VI models including lateral separation, generally adopted in polluted site engineering, are unable to consider those influencing parameters, especially building features, and thus need to be extended to improve the management of contaminated land before building constructions.
This paper reviews vapor intrusion (VI) modeling studies published in the last decade (2010–2020). Compared to research carried out in the late 1990s and the early 2000s that were mainly focused on basic vapor transport phenomena and attenuation in the subsurface, the topics addressed in recent years focused on more complex scenarios, including the blocking effect of building footprint and surface pavements, soil and source heterogeneity, the role of capillary fringe, weather conditions such as rain, indoor‐outdoor pressure differences, and temperature, building features, screening distances, and building pressure cycling. A brief description of these models and the main findings are reported in this paper. Generally, recent modeling works give more care about the influence of natural factors, which are relatively easy to be quantified and included in the model. Much less attention was given to factors involving human activities, such as preferential pathways, indoor environment structure, and background sources. The latter, however, may play a key role in determining the exposure to people of concern at sites contaminated by volatile contaminants. Thus, future modeling studies should be oriented to address these issues. This article is protected by copyright. All rights reserved.
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.
Indoor air concentrations are susceptible to temporal and spatial variations and have long posed a challenge to characterize for vapor intrusion scientists, in part, because there was a lack of evidence to draw conclusions about the role that building and weather conditions played in altering vapor intrusion exposure risks. Importantly, a large body of evidence is available within the building science discipline that provides information to support vapor intrusion scientists in drawing connections about fate and transport processes that influence exposure risks. Modeling tools developed within the building sciences provide evidence of reported temporal and spatial variation of indoor air contaminant concentrations. In addition, these modeling tools can be useful by calculating building air exchange rates (AERs) using building specific features. Combining building science models with vapor intrusion models, new insight to facilitate decision-making by estimating indoor air concentrations and building ventilation conditions under various conditions can be gained. This review highlights existing building science research and summarizes the utility of building science models to improve vapor intrusion exposure risk assessments.
Vapor intrusion exposure risks are difficult to characterize due to the role of atmospheric, building and subsurface processes. This study presents a three-dimensional VI model that extends the common subsurface fate and transport equations to incorporate wind and stack effects on indoor air pressure, building air exchange rate (AER) and indoor contaminant concentration to improve VI exposure risk estimates. The model incorporates three modeling programs: 1) COMSOL Multiphysics to model subsurface fate and transport processes, 2) CFD0 to model atmospheric air flow around the building, and 3) CONTAM to model indoor air quality. The combined VI model predicts AER values, zonal indoor air pressures and zonal indoor air contaminant concentrations as a function of wind speed, wind direction and outdoor and indoor temperature. Steady state modeling results for a single-story building with a basement demonstrate that wind speed, wind direction and opening locations in a building play important roles in changing the AER, indoor air pressure, and indoor air contaminant concentration. Calculated indoor air pressures ranged from approximately -10Pa to +4Pa depending on weather conditions and building characteristics. AER values, mass entry rates and indoor air concentrations vary depending on weather conditions and building characteristics. The presented modeling approach can be used to investigate the relationship between building features, AER, building pressures, soil gas concentrations, indoor air concentrations and VI exposure risks.
The adoption of source to building separation distances to screen sites that need further field investigation is becoming a common practice for the evaluation of the vapor intrusion pathway at sites contaminated by petroleum hydrocarbons. Namely, for the source to building vertical distance, the screening criteria for petroleum vapor intrusion have been deeply investigated in the recent literature and fully addressed in the recent guidelines issued by ITRC and U.S.EPA. Conversely, due to the lack of field and modeling studies, the source to building lateral distance received relatively low attention. To address this issue, in this work we present a steady-state vapor intrusion analytical model incorporating a piecewise first-order aerobic biodegradation limited by oxygen availability that accounts for lateral source to building separation. The developed model can be used to evaluate the role and relevance of lateral vapor attenuation as well as to provide a site-specific assessment of the lateral screening distances needed to attenuate vapor concentrations to risk-based values. The simulation outcomes showed to be consistent with field data and 3-D numerical modeling results reported in previous studies and, for shallow sources, with the screening criteria recommended by U.S.EPA for the vertical separation distance. Indeed, although petroleum vapors can cover maximum lateral distances up to 25–30 m, as highlighted by the comparison of model outputs with field evidences of vapor migration in the subsurface, simulation results by this new model indicated that, regardless of the source concentration and depth, 6 m and 7 m lateral distances are sufficient to attenuate petroleum vapors below risk-based values for groundwater and soil sources, respectively. However, for deep sources (> 5 m) and for low to moderate source concentrations (benzene concentrations lower than 5 mg/L in groundwater and 0.5 mg/kg in soils) the above criteria were found extremely conservative as the model results indicated that for such scenarios the lateral screening distance may be set equal to zero.
A complete vapor intrusion (VI) model, describing vapor entry of volatile organic chemicals (VOCs) into buildings located on contaminated sites, generally consists of two main parts- one describing vapor transport in the soil and the other its entry into the building. Modeling the soil vapor transport part involves either analytically or numerically solving the equations of vapor advection and diffusion in the subsurface. Contaminant biodegradation must often also be included in this simulation, and can increase the difficulty of obtaining a solution, especially when explicitly considering coupled oxygen transport and consumption. The models of contaminant building entry pathway are often coupled to calculations of indoor air contaminant concentration, and both are influenced by building construction and operational features. The description of entry pathway involves consideration of building foundation characteristics, while calculation of indoor air contaminant levels requires characterization of building enclosed space and air exchange within this. This review summarizes existing VI models, and discusses the limits of current screening tools commonly used in this field.
The intrusion into and subsequent accumulation of contaminant vapors in buildings and family dwellings is of concern for health and safety reasons. When preparing environmental and health risk assessments, one must be able to quantify this exposure pathway in order to decide if site-specific conditions correspond to unacceptable indoor contaminant vapor concentrations. For cases in which contaminated-site specific conditions, a related problem is the determination of residual contaminant levels below which associated adverse health effect risks are deemed negligible. Unfortunately, there are currently no accepted models for predicting vapor intrusion rates, and there is considerable debate over which transport mechanisms govern the process. This paper presents a heuristic model for screening-level calculations. It incorporates both convective and diffusive mechanisms, as well as contaminant soil, and building foundation properties. Sample calculations are presented for a range of parameter values to illustrate use of the model and the relative contributions of individual transport mechanisms.
There has been a great deal of recent interest in evaluating the potential for vapor transport from subsurface contamination into nearby buildings. There does not exist, however, any method for calculating whether buildings that are not directly over the source of contamination may be impacted. For simplicity, typical modeling approaches for estimating vapor migration through soil into buildings assume that the contaminated plume or soil lies directly underneath the building. The models do not take into account the lateral distance, if any, between the source of VOC emissions and the indoor space. This paper presents an approach to calculate diffusion-limited emission fluxes as a function of lateral distance and evaluates the significance of these emissions over relatively short lateral distances from the source. A theoretical, mathematical approach was used. The results show that soil-gas concentration and emission flux are both a decreasing exponential function of the lateral distance from the edge of the contaminant plume. Based on our calculations, the emission flux and the soil-gas concentration are insignificant within a relatively short lateral distance from the source (e.g., 30 m). © 2004 American Institute of Chemical Engineers Environ Prog, 23: 52–58, 2004
This paper presents model simulation results of vapor intrusion into structures built atop sites contaminated with volatile or semi-volatile chemicals of concern. A three-dimensional finite element model was used to investigate the importance of factors that could influence vapor intrusion when the site is characterized by non-homogeneous soils. Model simulations were performed to examine how soil layers of differing properties alter soil gas concentration profiles and vapor intrusion rates into structures. The results illustrate difference in soil gas concentration profiles and vapor intrusion rates between homogeneous and layered soils. The findings support the need for site conceptual models to adequately represent the site's geology when conducting site characterizations, interpreting field data and assessing the risk of vapor intrusion at a given site. For instance, in layered geologies, a lower permeability and diffusivity soil layer between the source and building often limits vapor intrusion rates, even if a higher permeability layer near the foundation permits increased soil gas flow rates into the building. In addition, the presence of water-saturated clay layers can considerably influence soil gas concentration profiles. Therefore, interpreting field data without accounting for clay layers in the site conceptual model could result in inaccurate risk calculations. Important considerations for developing more accurate conceptual site models are discussed in light of the findings.
A three-dimensional numerical model of the soil vapor-to-indoor air pathway is developed and used as a tool to anticipate not-yet-measured relationships between the vapor attenuation coefficient, alpha (indoor air concentration/source vapor concentration), and vapor source-building lateral separation, vapor source depth, and building construction characteristics (depth of building foundation) for nondegrading chemicals. The numerical model allows for diffusive and advective transport, multicomponent systems and reactions, spatially distributed foundation cracks, and transient indoor and ambient pressure fluctuations. Simulations involving different lateral separations between the vapor source and building show decreasing alpha values with increasing lateral separation. For example, alpha is 2 orders of magnitude less when a 30 m x 30 m source located 8 m below ground surface is displaced from the edge of the building by 20 m. The decrease in alpha with increasing lateral separation is greater for shallower source depths. For example, alpha is approximately 5 orders of magnitude less when a 30 m x 30 m source located 3 m below ground surface is displaced from the edge of the building by 20 m. To help visualize the effects of changing vapor source-building separations, normalized vapor concentration contour plots for both horizontal and vertical cross sections are presented for a sequence of lateral separations ranging from the case in which the 30 m x 30 m source and 10 m x 10 m building footprint centers are collocated to shifting of the source positioning by 50 m. Simulations involving basement and slab-on-grade constructions produce similar trends. In addition, when buildings are overpressurized to create outflow to soil gas on the order of 1-3 L/min, emissions to indoor air are reduced by over 5 orders of magnitude relative to intrusion rates at zero building underpressurization. The results are specific to simulations involving homogeneous soil properties, nondegrading chemicals, steady source concentrations and building underpressurizations, and the geometries studied in this work.
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