API PUBL 4741-2005

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 mangers. 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.

Recommendations presented here are based on the experience and professional judgment of the authors and are designed to be broadly applicable. This does not imply that the recommendations are universally applicable or that there are not situations for which other methods or procedures would be better suited. The user of this manual is cautioned to consider all of the site-specific information and to make decisions based on site-specific circumstances, experience, and professional judgment. In addition, some regulatory agencies have expressed preferences for sampling methods and techniques. The applicable regulatory preferences should be examined for each site.

This document does not address all potential strategies for assessing the vapor-to-indoor-air pathway. The user of this manual is directed to USEPA 2002a, and applicable state regulatory guidance for other methods to assess the vapor-to-indoor-air pathway (see PaDEP 2002; 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 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 emergency response or interim actions should be implemented as required under state or federal regulations.