Effect of Building-Source Separation and Preferential Pathways for Steady State Vapor Intrusion Simulations in Non-Homogenous Geologies

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1 Submitted to Environmental Science & Technology Effect of Building-Source Separation and Preferential Pathways for Steady State Vapor Intrusion Simulations in Non-Homogenous Geologies Journal: Environmental Science & Technology Manuscript ID: es--0 Manuscript Type: Article Date Submitted by the Author: -Apr- Complete List of Authors: Bozkurt, Ozgur; Lawrence Berkeley National Laboratory Pennell, Kelly; Brown Univeristy, Engineering Suuberg, Eric; Brown University, Division of Engineering

2 Page of Submitted to Environmental Science & Technology Effect of Building-Source Separation and Preferential Pathways for Steady State Vapor Intrusion Simulations in Non-Homogenous Geologies Ozgur Bozkurt, Kelly G. Pennell, Eric M. Suuberg* Brown University, Division of Engineering, Hope Street, Box D, Providence, RI 0. Eric_Suuberg@brown.edu RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) *Corresponding Author Phone: (0) -; Fax (0) -; Eric_Suuberg@brown.edu ABSTRACT. A three-dimensional finite element model of soil vapor intrusion described by Pennell et al. is used to examine questions related to building (receptor)-source separation for evaluating vapor intrusion scenarios. Simulations for non-degrading chemicals in nonhomogenous soils are discussed in conjunction with the potential for vapor intrusion to pose health risks via indoor air contamination. The results suggest that relying on suggested lateral and vertical building/source separation distances, without proper regard for particular site features, may not be conservative. The results also demonstrate that vapor intrusion risks are

3 Submitted to Environmental Science & Technology Page of not substantially increased by preferential pathways unless these intersect areas where highly contaminated soil gas exists and are located in close proximity to the building foundation where steep pressure gradients exist. Even then, the ability of a preferential pathway to affect vapor intrusion rates may still be limited by the nature of the native soil surrounding the pathway. The combined importance of soil gas concentration and soil gas flow rates is discussed in relation to the model simulations. The findings emphasize that vapor intrusion risks are best estimated using contaminant mass transport potential, rather than the commonly employed soil gas concentrations. Suggestions for developing qualitative predictions of the mass transport potential using typical site characterization data (i.e. geological features and soil gas concentrations) are highlighted. KEYWORDS. Vapor Intrusion, modeling, hazardous waste. Introduction Vapor intrusion involves indoor air contamination as a result of hazardous material volatilization from subsurface sources into above ground structures. Vapor intrusion risks have been the subject of much controversy, partly because of reported spatial and temporal variability. Despite the complexities of vapor intrusion, regulatory agencies are tasked with developing regulations and guidelines to ensure protection of human health. Recent vapor intrusion guidance (e. g. ITRC and NJDEP ) default to the USEPA recommendation of m (0ft) lateral and vertical building/source separation for non-degrading compounds. USEPA s guidance (as well as ITRC and NJDEP ) allows for interpretation of the roughly m recommended separation distance. For instance, provisions are included in each of the guidance documents requiring evaluation of preferential pathways, which may influence the

4 Page of Submitted to Environmental Science & Technology vapor intrusion process. Using experience and/or professional judgment to evaluate the influence of preferential pathways or other geologic heterogeneities on building source separation distances is difficult since the effects of these geologic features on vapor intrusion have not undergone quantitative evaluation. In fact, most commonly, field data are interpreted using a homogenous site conceptual model. This is the scenario for which most of the theoretical analysis to date has been conducted, but which may not be relevant to a particular situation. The results presented in this paper represent an initial effort to evaluate the effect that various geologic heterogeneities have on building-source separation distances and to investigation how various preferential pathways may or may not change vapor intrusion rates, as compared to a homogenous site conceptual model. The USEPA recommendation of m source-building separation distance is supported by the theoretical estimate reported by Lowell and Eklund for homogenous soils. The results of Abreu and Johnson also support that the m estimate is fairly conservative for the given set of homogenous cases that those authors investigated. However, Lowell and Eklund, and Abreu and Johnson both call for additional research to consider non-homogenous soils. The present paper extends consideration of the vapor intrusion processes to non-homogenous soils and to situations than have not been considered in the above referenced studies. Nonhomogeneities are common in the field, and the purpose of this paper is to identify some particular features of heterogeneity that may or may not be important, particularly as regulators consider the importance of source-receptor separation. Building on the contributions of many other modeling studies (e.g.,,,), Lowell and Eklund examined building-separation distances using a two-dimensional (-D) vapor intrusion model. Their physical picture was similar to that shown as Scenario (b) of Figure, which will be discussed below in connection with the subject research. Although the Lowell and Eklund model neglected advective gas transport (which is a transport process that is generally most

5 Submitted to Environmental Science & Technology Page of important near the building foundation ), they report steep falloff in soil gas contaminant concentration with increasing lateral distance from the source. More recently, Abreu and Johnson examined building-source separation distances using the same basic set of -D equations to describe vapor intrusion as are employed in this paper. For similar scenarios (homogenous soils, basement foundations, etc.), our approach gives essentially the same results as obtained by Abreu and Johnson. Abreu and Johnson clearly showed that there is an important effect of soil gas flow rate on indoor air concentration, and that deeper sources give lower indoor air concentrations for an infinite source underlying the structure. For a finite source with lateral displacement from the building, they predicted that subslab contaminant concentration falls rapidly as the source moves out from under the building, in qualitative agreement with the Lowell and Eklund prediction, despite the difference in transport processes included by the model. Abreu and Johnson also showed that there is an important interaction between lateral and vertical separation; for a source offset from the building footprint, the subslab contaminant concentration decreases with shallower sources. Our results agree with these earlier results. However, the importance of non-homogenous geologies on vapor intrusion potentials must be considered, and the present results provide additional insight about when, these factors might need to be taken into account in evaluating vapor intrusion risks at a given site. Methods and Materials The model used in this paper is presented by Pennell et al. and Bozkurt et al. Supporting Information contains the main working equations. The physical situations modeled are shown in Figure, the details of which are described in supporting information. The estimate of indoor air contaminant concentration (Supporting Information - Table ) is an extreme simplification, but is consistent with vapor intrusion modeling convention.

6 Page of Submitted to Environmental Science & Technology Mass flow rate of the contaminant into the building, M ck, (the numerator of Equation ) is a better indicator of contaminant vapor intrusion potential than indoor air concentration itself. The latter will depend upon the assumed air exchange rate within the building, background sources of chemicals in indoor air, etc. However, to be consistent with convention employed by the vapor intrusion community, indoor air concentration results are presented herein. Because Equation was consistently applied for all scenarios included in this research, the conclusions drawn using the modeled indoor air concentration data are the same as those that would be drawn from contaminant mass flow entry rate results. The simulations presented in this paper are obtained at steady state and do not consider the many time-dependent processes that may be of importance when evaluating vapor intrusion risks. Time-dependent processes of potential importance can have widely different timescales. Additional research is being conducted to consider time dependent phenomena; however, the purpose of this paper is to evaluate the steady-state vapor intrusion process for non-degrading compounds in non-homogenous soils to augment the work previously conducted by others,. Results and Discussion Soil gas concentration profiles are typically determined by diffusive transport. In rare cases, when advective transport rates are high, the soil gas concentration profiles may be affected by advection. For typical situations, the upward diffusion of the contaminant from the source towards the atmosphere determines the profiles. However, advective transport can often be important in establishing the rate at which contaminant mass enters the building. The contaminant flux into the building is a combination of diffusive transport of contaminant to the foundation and advective plus diffusive transport into the building; however for geologies with k>~ - m, the contribution of diffusive transport into the building is generally smaller than advective transport (i.e., diffusion through a foundation crack is much slower than the

7 Submitted to Environmental Science & Technology Page of pressure-induced advection into the building). Preferential pathways Figures,, and Table, show the results for preferential pathway scenarios, each with various soil permeabilities. Figure shows the normalized indoor air concentration (C indoor /C source ) values for the homogeneous scenario (a), and the preferential pathways Scenarios (f) to (j). For comparison purposes, Figure also presents normalized indoor air concentration values for different vertical building-source separations for homogenous soils, Scenario (a). Normalized indoor air concentration values for different lateral building-source separations for homogenous soils, Scenario (b), are presented in supporting information. These results are in agreement with previously reported results by Abreu and Johnson.Note that these results emphasize how high permeability sandy soils always offer the greatest potential for vapor intrusion, all else being equal. The change in source depth has a relatively modest effect on indoor air concentration (as compared to the influence of soil properties) for the range of source depths examined. In addition, the effect of preferential pathways also has modest effect (for the scenarios modeled). The most substantial effect was observed for Scenario (i) (for k= - m and k= - m ) where the source located at m depth and vertical preferential pathways are connected to the foundation. For these simulations, the indoor air concentrations are similar to (or slightly great than) those that result from a m source without preferential pathways. As the permeability of the soil surrounding the preferential pathways decreases (and diffusive transport into the building becomes the dominant transport mechanism), the influence of the preferential pathway becomes negligible. Even for higher permeability soils, the effect of the preferential pathway may not be considered significant; for k= - m, the indoor air concentration for Scenario (i), for some extremely high permeability preferential pathways was only

8 Page of Submitted to Environmental Science & Technology approximately a factor of greater than the case with no preferential pathways, Scenario (a). To fully predict the effect of a preferential pathway, one needs to consider the combined roles of soil gas concentration in the immediate vicinity of the preferential pathway and the potential for the preferential pathway to affect soil gas air flow rates. This is illustrated by the results summarized in Figures and. Figure shows both a normalized color concentration plot and a normalized pressure field contour line plot for homogeneous soil containing a man made preferential pathway (conceptualized as a septic system ) located atop an infinitely large contaminant source, Scenario (j). Plane E-E shows the normalized concentration contour lines and plane F-F shows the normalized pressure field contour lines at.m bgs, i.e., just below the system pipes. The red dashed lines represent the septic system. This scenario is intended to represent a worst-case scenario in which the septic system is connected to the building of interest and air flows directly into the building through the preferential pathway defined by the septic system. The soil gas concentration profile in this scenario is similar to that for a homogenous site model involving a single building at the center of an open field (Pennell et al. and Abreu and Johnson ). However, the influence of the pressure field now extends to a much larger area because the porous filling material assumed to be used around the septic system transmits the indoor negative pressure disturbance to a larger area (bottom right panel of Figure ). The negative pressure field created by the septic system actually promotes airflow through the path of least resistance, which is the soil surface, causing local dilution in the contaminant concentration near the septic system (plane E-E of Figure ). So here, in a scenario which intuition might suggest would enhance the ability of contaminant to enter a structure, the indoor air contaminant concentration is found to be unaffected or even lower than in the absence of the septic system (see Table, discussed further below). It needs to be

9 Submitted to Environmental Science & Technology Page of emphasized that for a preferential pathway to increase vapor intrusion rates, the pathway needs to intersect a zone of high contaminant concentration (or the airflow rate associated with the pathway needs to be high and involve a zone of significant contaminant concentration). Table presents predicted air flow rates, soil gas concentrations at the foundation crack and indoor air concentration values for a single house in a base case of Scenario (a) and that house with preferential pathways, Scenarios (f) (j). The results show that the presence of a preferential pathway can increase soil gas flow rates into the building. However, for some cases, the increase in soil gas flow rates diluted the contaminant concentration at the crack. The combined effect determines whether or not vapor intrusion rates will be increased, or decreased. When advective transport plays an important role in vapor intrusion rates (high permeability soils, k>~ - m ), vapor intrusion rates generally increased (typically less than a factor of two). The exception is seen in the contribution of the septic system as a preferential pathway for the contaminant transport as already noted. Again, this is because the septic system does not intersect a high concentration zone, even though it does result in increased soil gas flow rates. Figure further illustrates the influence of soil gas pressure field and soil gas concentration profiles. While a horizontal preferential pathway (Scenario (g)), and a preferential pathway at an angle with respect to the water table (Scenario (h)) distribute the building depressurization over a larger area, vertical preferential pathways (Scenario (f)) concentrate the negative pressure beneath the building foundation. When a preferential pathway does not intersect the foundation (Scenarios (a), (f), (g), (h)) there is a steep pressure gradient near the foundation crack entrance. Pressure gradients indicate indicates where a significant inward flow of soil gas is possible. Scenario (i) shows that when the preferential pathway carries the pressure disturbance deep into the soil, gas flow takes place from deeper in

10 Page of Submitted to Environmental Science & Technology the soil (nearer the source). Therefore, since advective transport is an important contributor of the total mass entering the building, these types of preferential pathways are those that are likely to increase vapor intrusion potentials. As shown on Table, when the surrounding geology is not permeable enough to allow advective transport to significantly enhance mass transport into the preferential pathway (k= - m and - m ), the effect of the preferential pathway is greatly diminished. Hence, it is not only the permeability of the soil around the building, but also the permeability of the soil surrounding a preferential pathway that can significantly impact vapor intrusion potential. Source Building Separation Figure presents normalized contaminant concentration color contour and line plots for layered soil scenarios with different lateral building-source separations (Scenario (c)). The building center-source center separation is 0m, m, and m (m from the edge of the source to the edge of the building) for the top, middle, and bottom plots respectively in Figures a and b. Figure a shows color contaminant concentration profiles where the top layer is low permeability and the bottom layer is high permeability. The indicated A-A plane for the middle plot of Figure a shows the normalized contaminant concentration contour lines.m bgs, and dashed red lines represent the location of the source. In the various source locations shown in Figure a, the low permeability soil in the top layer acts as a cap limiting the contaminant transport towards the soil surface, and allowing the contaminant beneath it to diffuse laterally. Figure b shows contaminant concentration profile plots for the scenario with high permeability top layer and low permeability bottom layer. The B-B plane illustrates the contaminant concentration contour lines. bgs for the middle panel of Figure b, and dashed red lines again represent the location of the source. It can be seen that contaminant

11 Submitted to Environmental Science & Technology Page of concentrations beneath the building foundation for the Figure b scenarios are lower than those in Figure a. The difference visible in contaminant concentration plots (Figure a and b) illustrates the impact of lateral diffusion, which is most apparent in the bottom panels of a and b. Although the source is m away from the building wall, some of the contaminant in the bottom panel of Figure a reaches the building foundation whereas in Figure b the contaminant is lost to upward diffusion before it reaches the building foundation. The soil gas concentration beneath the building foundation is higher in the scenarios with low top layer permeability (Figure a) than with a high permeability top layer (Figure b). However, due to lower predicted advective soil gas flow into the building in the low permeability top layer cases (Figure a) the indoor air concentrations are actually lower for this scenario compared to the scenario with high permeability top layer (Figure b). For example, the normalized indoor air concentration corresponding to the middle panel in a is approximately 0.0, whereas that corresponding to the middle panel of b is 0.. These results again emphasize how subslab measurements of contaminant concentrations can be very misleading in assessment of vapor intrusion potential. The subslab concentration of a are much higher than the subslab concentrations of b, and this is opposite the predicted indoor air concentration trend. The results presented in Figure can also be used to understand the effect of surface capping (e.g. paved surfaces, etc.). When the soil is capped, vertical contaminant migration is limited and therefore lateral diffusion is promoted (Figure a). At a site with extensive surface capping (and sufficiently permeable soils) high vapor intrusion potential exists even when the source is located beyond the EPA recommended distance of m. However, if low permeability soil near the foundation limits vapor transport into the building (as the case in Figure a), vapor intrusion may not pose a concern. If the surface cap (or low permeability

12 Page of Submitted to Environmental Science & Technology soil) does not extend to the foundation, then contaminant transport into the building could be a concern. As previous research has shown, the soil near the foundation affects the rate of soil gas entry into the building; however the geology beyond the foundation influences the concentration of the soil gas that enters the building and consequently the rate of vapor intrusion. This is apparent in the discontinuous clay layer scenario discussed below. Normalized soil gas contaminant concentration plots for a scenario involving a discontinuous clay layer, Scenario (d), are presented in Figure. The discontinuous clay layer covers half of the domain and acts as an impervious barrier blocking contaminant from diffusing directly upward towards the soil surface, while forcing the contaminant to diffuse laterally. Although the source is directly beneath the clay layer, only a portion of the contaminant travels upward from the end of the discontinuous clay layer in C-C and rest of the contaminant diffuses laterally into a much larger area under the clay layer in D-D. The results shown on Figure highlight the danger of using a homogenous site conceptual model to evaluate vapor intrusion risks at a site with a discontinuous clay layer. Depending where field data are collected, the soil gas concentration may not directly serve to indicate vapor intrusion risks. To illustrate the overall effect on vapor intrusion, Figures a to d show the normalized indoor air concentration for different vertical and lateral building-source separations combinations, and for different soil permeability values. The indoor air concentration increases, as expected, when the vertical distance between the building and the source decreases. Indoor air concentration is the highest for a m source depth for any given soil permeability, when the source is directly beneath the building. As the source moves laterally further away, the indoor air concentration becomes dependent on several factors. As lateral building center-source center separation becomes greater than m for k= - m (Figure a) and k= - m (Figure b), and greater than m for k= - m (Figure d),

13 Submitted to Environmental Science & Technology Page of indoor air concentration increases with deeper vertical building-source separation (consistent with a conclusion drawn by Abreu and Johnson ). This is more apparent on a log scale for the - m permeability case (Figure c). The log scale used on Figure c demonstrates the important role that a clay layer could have on vapor intrusion. As the source moves laterally away beneath the discontinuous clay layer, the indoor air concentration is nearly two orders of magnitude higher as compared to that predicted from a homogenous site conceptual model. On the other hand, for Scenario (e) (septic system), there is little change as compared to the base case Scenario (a). This is true even though the source is immediately beneath the preferential pathway. Together, these results suggest the often counter-intuitive nature of vapor intrusion. A clay layer is typically assumed to hinder vapor intrusion, while a preferential pathway is typically thought to promote vapor intrusion, yet the present results illustrate that the opposite may also possible. As previously discussed, it is the combined role of concentration and soil gas flow rates that will determine the extent to which vapor intrusion occurs. Figure e shows contaminant concentration profiles for source depths (m and m) for a single permeability case (k= - m ) with m lateral building-source separation. In the case of m vertical separation (top panel of e), the contaminant leaves the domain through the soil surface before it diffuses laterally towards the structure, whereas in the case of the m vertical building-source separation (lower panel of e) the contaminant diffuses laterally and reaches the building foundation. The results presented in this paper suggest that when calculating a safe vertical or lateral building-source distance, geological properties of the soil must be taken into account. EPA s estimate of m lateral separation is conservative for some cases. However, for certain situations, low permeability soil layers can promote lateral diffusion along a path of least

14 Page of Submitted to Environmental Science & Technology resistance, causing contaminant to spread over a wider area than might otherwise be expected. Regulators and practitioners should carefully consider the role that low (and high) permeability zones may play in the promoting or limiting vapor intrusion. The role of preferential pathways should be evaluated by considering their ability to increase mass transport of the contaminant into the building; recognizing that some preferential pathways may be effective at transporting soil gas, but may not substantially increase mass transport into the building. The present results suggest that greater emphasis should be placed on characterization of site geology, and that subsurface contaminant concentration measurements may be of more limited value in establishing risk than might presently be believed. Acknowledgments The project described was supported by Grant Number PES00 from the National Institute of Environmental Health Sciences. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute of Environmental Health Sciences or the National Institutes of Health.

15 Submitted to Environmental Science & Technology Page of References:. Pennell, K. G., Bozkurt, O., Suuberg, E. M. Development and Application of a -D Finite Element Vapor Intrusion Model Journal of the Air and Waste Management Association, 0, : -0.. United States Environmental Protection Agency (USEPA). Draft US EPA's Vapor Intrusion Database: Preliminary Evaluation of Attenuation Factors. Office of Solid Waste. March 0. ( --0b.pdf). Interstate Technology and Regulatory Council (ITRC), Vapor Intrusion Pathway: A Practical Guideline. VI-. Washington, D.C., 0. ( New Jersey Department of Environmental Protection (NJDEP), Vapor Intrusion Guidance, 0. ( United States Environmental Protection Agency (USEPA). Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils, 0, EPA -D Lowell, P.S., Eklund, B., VOC Emission Fluxes as a Function of Lateral Distance from the Source, Environmental Progress, 0, (), -.. Abreu, L.D., Johnson, P.C. Effect of Vapor source --Building Separation and Building Construction on Soil Vapor Intrusion as Studied with a Three-Dimensional Numerical

16 Page of Submitted to Environmental Science & Technology Model, Environmental Science and Technology, 0,, 0-.. Jury, W. A., Russo, D., Streile, G., Abd, H. E. Evaluation of Volatilization by Organic Chemicals Residing Below the Soil Surface, Water Resources Research. 0, ():-.. Johnson, P. C., Ettinger. R.A. Heuristic Model for Predicting the Intrusion Rate of Contaminant Vapors into Buildings, Environmental Science and Technology,, ():-.. Johnson, P. C. Identification of Application-Specific Critical Inputs for the Johnson and Ettinger Vapor Intrusion Algorithm, Ground Water Monitoring and Remediation. 0, (), -.. Hers, I., Zapf-Gilje, R., Johnson, P.C., Li, L. Evaluation of the Johnson and Ettinger Model for the Prediction of Indoor Air Quality, Ground Water Monitoring and Remediation, 0,, -.. Bozkurt, O. Pennell, K.G., Suuberg, E.M. Evaluation of the Vapor Intrusion Process for Non-Homogeneous Soils. Journal of Ground Water Monitoring & Remediation, 0, (), -.. Massman, J.W., Applying groundwater flow models in vapor extraction system design. Journal of Environmental Engineering,, () -.. Millington, R. J. Gas Diffusion in Porous Media; Science,, (), 0-.. Millington, R.J. Quirk, J.P. Permeability of Porous Solids Transactions Faraday Soc.,, -.. Driscoll, F. G. Groundwater and Wells, nd Edition, Johnson Filtration Systems, St. Paul

17 Submitted to Environmental Science & Technology Page of Minnesota,,.. Fetter, C. W., Applied Hydrogeology, Prentice Hall, Inc. Upper Saddle River, New Jersey 0 (0).

18 Page of Table : Comparison of indoor air concentrations of different preferential pathway scenarios. The contaminant source beneath building is assigned to be infinite in all cases. Contaminant source concentration was mg/m for all scenarios. Homogenous Soil - Building without septic system - Scenario (a) Permeability (m ) Q crack (µm /s) C crack (mg/m ) C indoor (mg/m ) x x x x Homogenous Soil - Building with septic system Scenario (j) Permeability (m ) Submitted to Environmental Science & Technology Q crack (µm /s) C crack (mg/m ) Q septic (µm /s) C septic (mg/m ) x x x x C indoor (mg/m ) Homogenous Soil - Vertical preferential pathways beneath building foundation Scenario (f) Permeability (m ) Q crack (µm /s) C crack (mg/m ) C indoor (mg/m ) x x x x Homogenous Soil - Horizontal preferential pathways beneath building foundation Scenario (g) Permeability (m ) Q crack (µm /s) C crack (mg/m ) C indoor (mg/m ) x x x x Homogenous Soil - Preferential pathways with an angle with water table between beneath building foundation and source Scenario (h) Permeability (m ) Q crack (µm /s) C crack (mg/m ) C indoor (mg/m ) x x x x Homogenous Soil - Vertical preferential pathways connected to building foundation Scenario (i) Permeability (m ) Q crack (µm /s) C crack (mg/m ) C indoor (mg/m ) x x x x

19 Submitted to Environmental Science & Technology Page of Q crack refers to volumetric soil gas infiltration rate through a foundation perimeter crack and Q septic to the infiltration rate ascribed to the septic system. C crack and C septic refer to average contaminant concentration adjacent the crack and septic system, respectively. C septic and Q septic represent values at the 0.m by 0.m trench connection to building. This area was filled with porous material. The area was unsealed and served as a preferential pathway for vapor transport.

20 Page of Submitted to Environmental Science & Technology Figure : Scenarios modeled (a) (j). H and L stand for shown vertical and lateral separation, respectively.

21 Submitted to Environmental Science & Technology Page of Figure : Effect of soil permeability on indoor air concentration for different source depths. Source is infinite in extent.

22 Page of Submitted to Environmental Science & Technology Figure : Normalized TCE soil gas concentration and normalized soil pressure field plots for a building with attached septic system, Scenario (j). Dashed lines in red represent the septic system. E-E' shows the normalized concentration lines at.m bgs and F-F' show the normalized pressure field contour lines at.m bgs. Details of this scenario are illustrated in Figure j.

23 Submitted to Environmental Science & Technology Page of Figure : Comparison of normalized soil gas pressure field contour plots and normalized soil gas concentration plots for domains with simple preferential pathways. Pressure field is normalized with disturbance pressure (P/P CER ), and concentration profile is normalized with the source concentration (C/C SOURCE ). Note: CER refers to the characteristic entrance region, which represents a perimeter crack.

24 Page of Submitted to Environmental Science & Technology 0 Figure : Normalized TCE soil gas concentration plots for layered soil and different lateral building source separation (Scenario c).

25 Submitted to Environmental Science & Technology Page of Figure : Normalized TCE soil gas concentration plots for soil containing a discontinuous clay layer for different lateral building source separations (Scenario d). Figures C-C and D-D represent the contour lines above (.m bgs) and below (.m bgs) the clay layer; dashed red lines represent the location of the source. Details of the geometry are illustrated in Figure d. C-C top view of the plane. m bgs D-D top view of the plane. m bgs

26 Page of Submitted to Environmental Science & Technology Figure : a, b, c, d normalized indoor air concentration for different lateral and vertical building-source separations. Figure e shows the normalized concentration contour lines for m and m source depth (top and bottom respectively) with m lateral building separation (k= - m ). a. c Lateral distance between the edge of the building and the edge of the source (m) 0 source depth m m m m m - disc. clay layer (k= - m ) Lateral distance between the center of the building and the center of the source (m) Lateral distance between the edge of the building and the edge of the source (m) source depth m m m m m - disc. clay layer m - septic system (k= - m ) (log scale) Lateral distance between the center of the building and the center of the source (m) b d. 0 Lateral distance between the edge of the building and the edge of the source (m) 0 m m m m m - disc. clay layer m - septic system Lateral distance between the center of the building and the center of the source (m) source depth (k= - m ) Lateral distance between the edge of the building and the edge of the source (m) Lateral distance between the center of the building and the center of the source (m) source depth m m m m m - disc. clay layer (k= - m )