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Assessing Vapor Intrusion

Collecting and Interpreting Soil Gas Samples from the Vadose Zone: A Practical Strategy for Assessing the Subsurface Vapor-to-Indoor Air Migration Pathway at Petroleum Hydrocarbon Sites

API Publication 4741

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Prepared under contract to API by:

  • Lesley Hay Wilson, Ph.D.
  • Sage Risk Solutions LLC
  • Paul C. Johnson, Ph.D.
  • Department of Civil and Environmental Engineering
  • Arizona State University
  • James R. Rocco
  • Sage Risk Solutions LLC


This document focuses on the collection of soil gas samples for assessing the significance of the subsurface-vapor-to-indoor-air exposure pathway. While soil gas collection is not the only means of assessing this pathway, soil gas data play a prominent role in recent guidance published by the American Petroleum Institute (API 1998) and the United States Environmental Protection Agency (USEPA 2002a). For example, these data can be used to help make decisions concerning:

  • Resource Conservation and Recovery Act (RCRA) corrective action environmental indicators (EI) for human health exposures
  • Current exposure scenarios in existing buildings
  • Future exposure scenarios in existing buildings
  • Future exposure scenarios in future buildings.

This document is intended to complement API 1998 and USEPA 2002a. It provides more in-depth information on issues associated with soil gas sampling and data interpretation as applied to pathway assessment. This document is specifically focused on petroleum hydrocarbon impacted sites. However, much of the information presented is applicable to all soil gas sampling. Soil gas sampling has been used for many years for site assessment and remediation system monitoring purposes. The user, however, will find that the data quality objectives and acceptable methods of sampling for pathway assessment are different from those that are commonly acceptable for using soil gas data for delineation, site assessment, or monitoring remediation systems.

This document is unique in that it emphasizes conceptual models for vapor transport, describes how to choose sample locations and depths, explains how to check the data for inconsistencies and also provides checklists on each of these topics to assist field project managers. This document allows for flexibility in the selection and refinement of practicable and defensible sampling methods. The focus here is on identifying key issues associated with soil gas sampling and data interpretation.

Field project managers should find this document useful when developing scope-of-work requirements for site-specific work plans and bid requests. To support preparation of site-specific work plans, scope-of-work action items are included at the end of Sections 4.0 through 7.0.

Section 1.0 provides a brief introduction. Section 2.0 discusses soil gas transport, with emphasis on petroleum hydrocarbon vapors, and presents a brief synopsis of expected soil gas profiles based on empirical analysis of existing data. Section 3.0 discusses the conceptual vapor-migration model. Section 4.0 focuses on sampling locations, depths, and sampling frequency. Section 5.0 focuses on monitoring installations and sample collection procedures. Section 6.0 discusses methods of soil gas analysis. Section 7.0 discusses interpretation of soil gas data. Appendix A provides a site information checklist. Appendix B provides worksheets for three typical scenarios that can be used for planning sampling locations. Appendix C provides more details on sample collection. Appendix D gives supporting information on analytical methods, and Appendix E provides tools to be used in the data evaluation.


  • API. 1998. Assessing the Significance of Subsurface Contaminant Vapor Migration to Enclosed Spaces, Site-Specific Alternatives to Generic Estimates. Publication No. 4674. Health and Environmental Sciences Department. Washington D.C.
  • United States Environmental Protection Agency (USEPA). 2002a. OSWER Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils (Subsurface Vapor Intrusion Guidance). November 29. Draft.