Simulating the Effect of Aerobic Biodegradation on Soil Vapor Intrusion into Buildings

Evaluation of Low Strength Sources Associated with Dissolved Gasoline Plumes

API PUBLICATION 4775

APRIL 2009

Simulating the Effect of Aerobic Biodegradation on Soil Vapor Intrusion into Buildings

Evaluation of Low Strength Sources Associated with Dissolved Gasoline Plumes

Regulatory and Scientific Affairs Department

API PUBLICATION 4775

APRIL 2009

PREPARED UNDER CONTRACT BY:

LILIAN D. V. ABREU, ROBERT ETTINGER, AND TODD MCALARY GEOSYNTEC CONSULTANTS, INC.

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Copyright © 2009 American Petroleum Institute

Foreword

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Abstract 1

1. Introduction 1

2. Background 2

3. Approach 2

   1. Conditions Simulated 3

   2. Multi-Component Mixture Vapor Source 5

4. Results and Discussion 6

   1. Effect of Source Concentration 6

   2. Effect of First-Order Biodegradation Rate 12

   3. Effect of Source Depth 12

   4. Effect of Building Type 19

   5. Results and Discussion for Multi-Component Gasoline Sources 21

5. Evaluation of Additional Parameters 26

   1. Effect of Soil Type 26

   2. Effect of Foundation Crack Location 29

   3. Effect of a High Moisture-Content Soil Layer 31

6. Discussion 32

   1. Development of a Conceptual Model 32

   2. Preliminary Screening. 33

   3. Site-Specific Assessments 34

7. Conclusions and Recommendations 35

   1. Conclusions 35

   2. Recommendations 36

8. References 36

Appendix A Predicted Soil Gas Pressure Field and Air Flow Rate into the Building 38

Appendix B Plots of Attenuation Factors as a Function of Source Concentration, Depth and First-order Biodegradation Rates for Basement Scenarios 44

Appendix C Plots of Attenuation Factors as a Function of Source Concentration, Depth and First-order Biodegradation Rates for Slab-on-grade Scenarios 49

Figures

1. Vertical cross section of sample model domain showing the grid refinement for

   basement scenario 3

2. Vertical cross section of sample model domain showing the grid refinement for

   slab-on-grade scenario 3

3. Effect of low vapor source concentration (Cvs) on soil gas concentration distribution and vapor intrusion attenuation factors () for basement foundation scenarios and

   hydrocarbon biodegradation rate  = 0.79 h -1 9

4. Effect of low vapor source concentration (Cvs) on soil gas concentration distribution and vapor intrusion attenuation factors () for slab-on-grade foundation scenarios and

   hydrocarbon biodegradation rate  = 0.79 h -1 10

   
   

5. Influence of soil vapor source concentration and first-order biodegradation rates () on vapor intrusion attenuation factors () for basement scenarios, homogeneous sand

   soil and source depth (D) of 5 m bgs 11

6. Influence of soil vapor source concentration and first-order biodegradation rates ()

   on vapor intrusion attenuation factors () for slab-on-grade scenarios, homogeneous sand

   soil and source depth (D) of 5 m bgs 12

7. Effect of biodegradation rate ( on soil gas concentration distribution and vapor intrusion attenuation factors () for low vapor source concentration (4 mg/L) located at 4 m bgs

   (2 m below a basement foundation) 13

8. Effect of biodegradation rate ( on soil gas concentration distribution and vapor intrusion attenuation factors () for low vapor source concentration (4 mg/L) located at 4 m bgs

   (~4 m below the slab-on-grade foundation) 14

9. Effect of source depth on the soil gas concentration distribution and vapor intrusion attenuation factors () for basement scenarios with a low vapor source concentration

   of 1 mg/L and biodegradation rate  = 0.79 h -1 15

10. Effect of source depth on the soil gas concentration distribution and vapor intrusion attenuation factors () for slab-on-grade scenarios with a low vapor source concentration

    of 1 mg/L and biodegradation rate  = 0.79 h -1 16

11. Effect of source depth on the soil gas concentration distribution and vapor intrusion attenuation factors () for basement scenarios with a high vapor source concentration

    of 100 mg/L and biodegradation rate  = 0.79 h -1 17

12. Attenuation factors as a function of source depth below foundation and first-order biodegradation

    rate for basement scenarios with perimeter cracks and 10 mg/L vapor source concentration 18

13. Attenuation factors as a function of source depth below foundation and first-order biodegradation

    rate for slab-on-grade scenarios with perimeter cracks and 10 mg/L vapor source concentration 19

14. Effect of building type on soil gas concentration distribution for low vapor source concentration

    (4 mg/L) and biodegradation rate  = 0.79 h -1 20

15. Effect of building type on soil gas concentration distribution for high vapor source concentration

    (100 mg/L) and biodegradation rate  = 0.79 h -1 21

16. Effect of multi-component source on soil gas distribution and oxygen consumption in the

    subsurface for dissolved groundwater source scenario 23

17. Effect of multi-component source on soil gas distribution and oxygen consumption in the

    subsurface for NAPL source scenario 24

18. Normalized steady-state soil gas concentration distribution for oxygen and hydrocarbon

    with a vapor source concentration of 4 mg/L located at 5 m bgs (3 m below the foundation) 27

19. Attenuation factors as a function of soil type and vapor source concentration for a

    source located at 5 m bgs (3 m below a basement foundation) 28

20. Attenuation factors as a function of soil type and source depth below a basement

    foundation for a 10 mg/L source vapor concentration 29

21. Effect of crack positioning (perimeter vs center of foundation) on attenuation factors as a

    function of vapor source concentration located 1 m below a basement foundation 30

22. Effect of crack positioning (perimeter vs center of foundation) on attenuation factors as a

    function of vapor source concentration located 3 m below a slab-on-grade foundation 30

23. Normalized steady-state soil gas concentration distribution for oxygen and hydrocarbon with

    a vapor source concentration of 1 mg/L located at 4 m bgs (2 m below a basement foundation) 31

24. Normalized steady-state soil gas concentration distribution for oxygen and hydrocarbon with

a vapor source concentration of 10 mg/L located at 4 m bgs (2 m below a basement foundation) 32

A1 Normalized steady-state disturbance pressure distribution for a homogeneous soil permeability field (Kg=10-11 m2) surrounding basement and slab-on-grade foundations with perimeter cracks

and a lower boundary at depths of 3, 5 and 7 m bgs 39

A2 Plan view of the foundation crack distribution: a) perimeter crack; b) center-of-foundation cracks . . . 40 A3 Normalized steady-state disturbance pressure distribution for a homogeneous soil permeability

field (Kg=10-11 m2) surrounding basement foundations with cracks located on perimeter and

on center of the foundation slab 41

A4 Normalized steady-state disturbance pressure distribution for a homogeneous soil permeability field (Kg=10-11 m2) below slab-on-grade foundations with cracks located on perimeter and on

center of the foundation slab 42

A5 Effect of crack positioning (perimeter vs center of foundation) on soil gas flow (Qs) into the building for basement and slab-on-grade structures under-pressurized by 5 Pa and sand soil subsurface with air permeability of 1E-11 m2 43

Tables

1. Model Input Parameters 4

2. Values for Key Parameters Studied 4

3. Multiple Compound Mixture Vapor Source 5

4. Attenuation Factor Results for Single Component Source Basement Scenarios 7

5. Attenuation Factor Results for Single Component Source Slab-on-Grade Scenarios 8

6. Predicted Attenuation Factors for Dissolved Groundwater Multiple-Component Source and

   Equivalent Single Component Source for Slab-on-Grade Scenario and Source Depth of 4 m bgs 25

7. Predicted Attenuation Factors for NAPL Multiple-Component Source and Equivalent Single Component Source for Slab-on-Grade Scenario and Source Depth of 10 m bgs 26

8. Soil Physical Properties 26

9. Example Calculations 35

Simulating the Effect of Aerobic Biodegradation on Soil Vapor Intrusion into Buildings:

Evaluation of Low Strength Sources Associated with Dissolved Gasoline Plumes

Abstract

Aerobic biodegradation can contribute significantly to the attenuation of petroleum hydrocarbon vapors in the unsaturated zone; however, most regulatory guidance for assessing potential human health risks via vapor intrusion to indoor air either neglect biodegradation or only allow for one order of magnitude additional attenuation for aerobically degradable compounds, which may be overly conservative in many cases. This paper describes results from 3-dimensional numerical model simulations of vapor intrusion for petroleum hydrocarbons to assess the influence of aerobic biodegradation on the attenuation factor for a variety of source concentrations and depths for buildings with basements and slab-on-grade construction. Provided that oxygen is present in the vadose zone, aerobic biodegradation of petroleum hydrocarbon vapors in the unsaturated zone will reduce the soil gas concentrations and the potential risks from vapor intrusion to indoor air compared to non-degrading compounds. At lower source concentrations and/or deeper source depths, aerobic biodegradation may result in a reduction in vapor intrusion attenuation factors by many orders of magnitude. The magnitude of the reduction depends on site-specific conditions, which should be considered in the development of a conceptual site model for each site. However, oxygen supply and degradation rates are likely to be sufficient at many sites to mitigate potential risks from vapor intrusion for low vapor concentration sources (less than about 2 mg/L-vapor total hydrocarbons). The simulations conducted in this study provide a framework for understanding the degree to which bio-attenuation will occur under a variety of scenarios and provide insight into site conditions that will result in significant biodegradation. This improved understanding may be used to select site-specific attenuation factors for degradable compounds and develop soil vapor screening levels appropriate for particular combinations of source concentrations, source depth, and building characteristics, which should be defined as part of a site conceptual model.

1. Introduction

Subsurface migration of volatile compounds and vapor intrusion to indoor air is a potential exposure pathway for human occupants of buildings over or near contaminated soils and groundwater. In the past decade, there has been a significant increase in attention to vapor intrusion issues and several new regulatory guidance documents have been developed by State and Federal agencies for assessment and management of vapor intrusion risks. These guidance documents generally provide a framework for screening sites to assess whether vapor intrusion poses no significant risk or may require further evaluation, including assessment, remediation, or exposure controls.

Most regulatory guidance documents use conservative assumptions to account for uncertainties in the screening process. This results in decisions to further evaluate sites more frequently than may actually be necessary. It is expected that screening procedures will become less conservative as we learn more about the processes affecting vapor intrusion. To date, most vapor intrusion screening procedures either assume that biodegradation does not occur, or allow for an arbitrary 10-fold reduction in the predicted indoor air concentration for petroleum hydrocarbons.

Many petroleum hydrocarbons are metabolized by ubiquitous, naturally occurring soil microbes provided that sufficient oxygen is present in the subsurface. Several modeling studies and empirical data reviews have shown that aerobic biodegradation in the unsaturated zone can significantly attenuate vapor intrusion of petroleum hydrocarbons in some settings (i.e. DeVaull, 2007; Abreu and Johnson, 2006, Roggemans, et al. 2001). For example, the Abreu and Johnson (2006) study showed significant reduction in vapor intrusion for deeper and weaker sources and little to no reduction for shallower and stronger sources.

This work builds on the Abreu and Johnson (2006) study by focusing specifically on low-concentration petroleum hydrocarbon vapor sources, as this may be a common case for buildings down-gradient of petroleum source zones and overlying dissolved petroleum hydrocarbon plumes. Simulations were performed using the three-dimensional mathematical model developed by Abreu and Johnson (2006, 2005) for a range of scenarios to develop relationships between the site-specific conditions and the vapor intrusion attenuation factor α, which is defined as the ratio of the indoor air concentration of a chemical divided by its subsurface

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