1、Collecting and Interpreting Soil Gas Samples from the Vadose ZoneA Practical Strategy for Assessing the Subsurface Vapor-to-Indoor Air Migration Pathway at Petroleum Hydrocarbon SitesRegulatory Analysis and Scientific Affairs PUBLICATION NUMBER 4741 NOVEMBER 2005Collecting and Interpreting Soil Gas
2、Samples from the Vadose Zone A Practical Strategy for Assessing the Subsurface Vapor-to-Indoor Air Migration Pathway at Petroleum Hydrocarbon Sites Regulatory and Scientific Affairs PUBLICATION NUMBER 4741 NOVEMBER 2005 PREPARED UNDER CONTRACT BY: Lesley Hay Wilson, Ph.D. Sage Risk Solutions LLC Pau
3、l C. Johnson, Ph.D. Department of Civil and Environmental Engineering Arizona State University James R. Rocco Sage Risk Solutions LLC ii SPECIAL NOTES API publications necessarily address problems of a general nature. With respect to particular circumstances, local, state, and federal laws and regul
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12、tute iii Forward Nothing contained in any API publication is to be construed as granting any right, by implication or otherwise, for the manufacture, sale, or use of any method, apparatus, or product covered by letters patent. Neither should anything contained in the publication be construed as insu
13、ring anyone against liability for infringement of letters patent. Suggested revisions are invited and should be submitted to the Director of Regulatory Analysis and Scientific Affairs, API, 1220 L Street, NW, Washington, DC 20005. The information included in this publication is intended as general g
14、uidelines and not specific recommendations for all sites. Site-specific considerations, professional judgment and regulatory requirements will dictate the methods and procedures used at any particular site. This publication is not intended to replace the advice of qualified professionals. Trademarks
15、: Cali-5-bondis a registered trademark of Calibrated Instruments, Inc. Luer-lokis a registered trademark of the Becton, Dickinson and Company Corporation Tedlaris a registered trademark of the E. I. Du Pont De Nemours and Company Corporation Teflonis a registered trademark of the E. I. Du Pont De Ne
16、mours and Company Corporation Tenaxis a registered trademark of the Buchem B.V. Corporation iv Acknowledgements API and the authors would like to acknowledge and thank the following people for their contributions of time, comments, and expertise during this study and in the preparation of this repor
17、t: API STAFF CONTACT Harley Hopkins, Regulatory Analysis and Scientific Affairs Department (RASA) MEMBERS OF THE SOIL AND GROUNDWATER TECHNICAL TASK FORCE Curtis Stanley, Shell Global Solutions (U.S.), Chairman Brian Davis, Chevron Corporation Rick Greiner, ConocoPhillips Dan Irvin, ConocoPhillips V
18、ic Kremesec, Atlantic Richfield Company, A BP Affiliated Company Matt Lahvis, Shell Global Solutions (U.S.), Inc. Paul Lundegard, Chevron Corporation Mark Malander, ExxonMobil Corporation Tom Peargin, Chevron Corporation Todd Ririe, Chevron Corporation OUTSIDE REVIEWERS Bart Eklund, URS Corporation
19、Robert Ettinger, GeoSyntec Consultants Blayne Hartman, H MaDEP 2002; WDHFS 2003; NJDEP 2004; CRWQCB 2003; CSDDEH 2003). This document also does not address safety- and hazard-mitigation efforts to prevent fires or explosions resulting from the accumulation of hazardous vapors. It assumes that these
20、situations have been controlled by emergency or immediate response actions before the planning of a soil-gas-sampling program is initiated. If the results of the soil-gas-sampling program indicate that there is an immediate concern for human exposures to vapor-phase chemicals of concern, then emerge
21、ncy response or interim actions should be implemented as required under state or federal regulations. vii Contents 1.0 Introduction 1 2.0 Soil Gas Transport and Soil Gas Profiles at Petroleum Hydrocarbon Impacted Sites 3 2.1 Expectations for Soil Gas Profiles at Petroleum Hydrocarbon Impacted Sites
22、5 2.2 Measured Soil Gas Profiles at Petroleum Hydrocarbon Impacted Sites. 7 3.0 Conceptual Migration Model for Subsurface Vapor to Indoor Air. 14 4.0 Development of a Strategy for Soil Gas Sampling. 17 4.1 General Approach. 18 4.2 Point Sampling . 18 4.3 Transects and Vertical Profiles. 19 4.3.1 Sel
23、ection of Lateral Positions for Soil Gas Transects. 21 4.3.2 Vertical Profiles 22 4.4 Summary of Sampling Depth and Location Selection Considerations. 24 4.5 Some Comments on Sample Collection Adjacent to and Beneath Buildings. 29 4.6 Sampling Frequency. 31 4.7 Additional Considerations to Increase
24、Confidence in Data Sets and the Interpretation of Soil-Gas-Sampling Results 32 5.0 Soil Gas Sample Collection 35 5.1 Basic Monitoring Installation Options 35 5.1.1 Permanent Probes . 36 5.1.2 Temporary Driven Probes. 37 5.2 Comparison of Monitoring Installations. 37 5.3 Other Considerations for Samp
25、ling Probe Installations 39 5.4 Sample Collection Procedures 39 5.4.1 Soil Gas Equilibration 39 5.4.2 Sample Probe Purging 40 5.4.3 Sample Collection. 40 5.4.4 Sample Collection Vacuum 40 5.5 Ways to Avoid Common Problems with Soil Gas Sampling . 41 5.6 Alternatives to Soil Gas Sampling 43 5.6.1 Pas
26、sive Implant Samplers. 43 5.6.2 Flux Chambers 44 6.0 Analytical Methods 46 6.1 Analytical Method Selection 46 6.1.1 Field Analytical Methods . 48 6.1.2 Common Analytical Methods. 48 6.2 Data Quality 48 7.0 Analysis and Interpretation of Soil Gas Sampling Data . 50 7.1 Data Organization. 52 7.2 Data
27、Analysis 53 7.2.1 Data Quality Analysis.53 7.2.2 Data Consistency Analysis . 54 7.3 Exposure Pathway Assessment.55 7.3.1 Exposure Pathway Completeness . 55 7.3.2 Exposure Pathway Significance 56 7.4 Further Evaluation 56 8.0 References 58 9.0 Additional Reading. 61 9.1 Analytical Methods. 61 9.2 Bio
28、degradation . 61 9.3 Data Analysis 62 9.4 General . 62 9.5 Modeling 63 9.6 Sample Collection Methods 64 9.7 Site Characteristics and Conceptual Vapor-Migration Models 65 viii Appendix A. Characteristics Checklist Appendix B. Selection of Soil Gas Sample Locations Appendix C. Soil Gas Sample Collecti
29、on Appendix D. Analytical Methods Appendix E. Data Evaluation Figures Figure 2-1. Typical conventional conceptual model of soil gas migration. 3 Figure 2-2. Revised conceptual model of soil gas migration at petroleum hydrocarbon impacted sites. . 5 Figure 2-3. Soil gas profiles (Roggemans et al. 200
30、2). 9 Figure 2-4. Soil gas profile at a site with methane production in the source zone (Johnson et al. 2003). This figure shows the soil gas profiles for oxygen (circles) and methane (diamonds). 10 Figure 2-5. Normalized soil gas concentration distribution for oxygen and hydrocarbon undergoing aero
31、bic biodegradation with first-order rate = 0.18 (h-1) and vapor source at concentrations of 20 mg/L, 100 mg/L and 200 mg/L located underneath a basement foundation at a depth of 8 m below ground surface. Hydrocarbon and oxygen contours are normalized to the source and the atmospheric concentrations,
32、 respectively. From Abreu (2005).12 Figure 2-6.Normalized soil gas concentration distributions for oxygen and hydrocarbon undergoing aerobic biodegradation with a first-order rate = 0.18 (h-1) and a vapor source concentration of 200 mg/L located beneath a slab-on-grade foundation at depths of 1 m, 3
33、 m, 5 m and 8 m below ground surface. Hydrocarbon and oxygen contours are normalized to the source and the atmospheric concentrations, respectively. From Abreu (2005) 13 Figure 4-1. Considerations for vertical profiles at relatively flat sites (e.g., consistent distance between the ground surface an
34、d the vapor source depth) with consistent stratigraphy. . 23 Figure 4-2. Considerations for vertical profiles at sites with significant spatial variability in the distance between ground surface and the vapor source depth 24 Figure 4-3. Sub-slab-to-indoor-air attenuation. . 31 Figure 7-1. Flowchart
35、for data evaluation. . 51 Tables Table 4-1. Considerations for Samples Collected Immediately above the Vapor Source 25 Table 4-2. Considerations for Samples Collected Laterally Mid-Way between the Vapor Source and the Building Location . 26 Table 4-3. Considerations for Samples Collected Adjacent to
36、 the Base of an Existing Building Foundation or Basement 27 Table 4-4. Considerations for Samples Collected Immediately below the Building Foundation or Basement 28 Table 4-5. Considerations for Samples Collected within the Footprint of a Future Building Location . 29 Table 6-1. Common Analytical Me
37、thods . 48 Table 7-1. Example Comparisons of Biodegradation Stoichiometry and Fluxes. 55 1 1.0 Introduction Topic: This section provides general information about the subsurface-vapor-to-indoor-air exposure pathway. Purpose: To define the following activities for collecting and interpreting soil gas
38、 samples: Collecting information Identifying sample locations Determining sample collection methods Selecting sample analytical methods Interpreting results. Significance: Soil gas data can be used to assess for significant concentrations of petroleum hydrocarbon vapors in the subsurface, to determi
39、ne if vapor migration to a building is occurring, to estimate possible indoor air concentrations, and to identify significant attenuation of vapor transport by natural processes. Soil gas sampling has been conducted for many years as a tool for evaluating the distribution of chemicals of concern in
40、soil and groundwater, for guiding site characterizations, and for monitoring remedial action progress. However, soil gas data collected for a site characterization are generally focused on developing an understanding of the location and distribution of chemicals of concern in environmental media (e.
41、g., soil, groundwater) and not on determining the definitive concentrations of chemicals of concern in soil gas. As a result, the methodologies used for traditional site characterizations may not be appropriate for evaluating the subsurface-vapor-to-indoor-air exposure pathway. When assessing the su
42、bsurface-vapor-to-indoor-air pathway, an initial screen is conducted to identify those sites where further site-specific investigation and assessment are warranted. As the sampling of indoor air poses many practical and technical challenges, the ensuing site-specific pathway assessment often focuses
43、 on soil gas collection and analysis (as described in API 1998). For the evaluation of the subsurface-vapor-to-indoor-air exposure pathway, soil gas samples are collected to: Establish a snapshot of the concentrations of chemicals of concern in soil gas at a location along the exposure pathway betwe
44、en the source and the building location Analyze the potential for human receptors to be exposed in indoor environments Predict the expected indoor air concentration based on the soil gas concentrations using an estimated attenuation factor 2 Account for the fate and transport processes between a sam
45、pling location and the indoor environment. In the case of petroleum hydrocarbons, aerobic biodegradation has the potential to attenuate concentrations of chemicals of concern in soil gas and fluxes significantly as the vapors move toward buildings from soils or groundwater containing concentrations
46、of petroleum hydrocarbon chemicals of concern. Therefore, soil gas data collected for the subsurface-vapor-to-indoor-air exposure pathway need to be focused on the purpose and use of the data and be based on these specific data-quality objectives. The collection and use of soil gas data to evaluate
47、the subsurface-vapor-to-indoor-air exposure pathway is a relatively new approach for this exposure pathway, and limited information is available on the appropriate methodologies. To address this need, options for the collection, analysis, and interpretation of soil gas data are presented in this doc
48、ument. This document emphasizes conceptual models for vapor transport in the formulation of sampling plans and data analysis and presents information needed to support the selection of sample locations and depths. The overall process of collection and analysis of soil gas samples is supported throug
49、h important reminders and checklists at the end of each section. A systematic process is provided in the data analysis section to check the collected data for inconsistencies and for determination of situations requiring further study. Specifically, five basic activities have been identified for the collection and interpretation of soil gas samples. These activities include the following: 1. Collecting information to understand the characteristics of the site (Section 3.0, Appendix A) 2. Identifying the location or locations for soil gas sample collection (Secti
