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This document supersedes the 2003 Advisory—Active Soil Gas Investigation A Soil Gas Investigation Report (CayEPA,2003)and 1997 LARWQCB"Interim Guidance for Active Soil Gas Investigation"(CRWQCB,1997).Unless otherwise noted in this Advisory,active Workplan soil gas investigations should be performed in accordance with this document. Passive soil gas sampling methods are discussed briefly in Appendix A. Sites A workplan should be prepared and submitted to the regulating agency for review where sampling methods other than soil gas investigations are necessary should and approval at least 30 days prior to its proposed implementation schedule. be addressed separately with the regulating agency. Any variations or deviations from this Advisory should be specified in the workplan. The soil gas workplan may be incorporated as part of a 2.0 Project Scope comprehensive site investigation workplan or as a stand-alone document, depending on site-specific circumstances.The workplan should include DQOs . Study Purpose and Data Quality Objectives:A single soil gas investigation is and a Conceptual Site Model(CSM)that identifies the contaminant source(s), often undertaken for multiple purposes. Subsequently, each purpose will have pathway(s)and potential receptor(s)to aid in thejustification for the number, significantly different data quality objectives(DQOs),particularly the target location and frequency of samples. contaminant concentrations. Examples of different purposes for performing a soil gas investigation are provided below: Development of Conceptual Site Model • Transaction audit screening for potential leaks at aboveground storage tanks A CSM is an integral part of all site investigations.The purpose of a CSM is to (AST),underground storage tanks(USTs)or other underground pollution provide a conceptual understanding of the potential for exposure to hazardous sources contaminants at a site based on the sources of contamination,the release • Monitoring the performance of a SVE system mechanisms,the transport media,the exposure pathways,and the potential • Mapping soil vapor plumes to select houses for indoor-air monitoring receptors.The CSM should include a diagrammatic or schematic figure that • Stand-alone data set for performing a vapor intrusion risk assessment using relates the sources of contamination to receptors and the environment,and generic attenuation factors or a mathematical model to estimate indoor-air should identify all potential sources of contamination,the potentially concentrations from soil gas data contaminated media,and exposure pathways.The CSM organizes and • Designing mitigation measures and performance monitoring communicates information about the site characteristics and provides all interested parties with a conceptual understanding of the potential for exposure The DQO process is a systematic planning tool based on the scientific method to chemicals of potential concern(COPCs)at a site. Additional resources for establishing criteria for data quality and for developing data collection regarding CSMs include.(1)Cal/EPA,2005;(2)ITRC 2007;(3)Cal/EPA,1999; designs. By using the qQO process to plan environmental data collection efforts, (4)USEPA, 1988;(5)USEPA, 1989;and(6)USEPA,1994a. the effectiveness,efficiency,and defensibility of decisions can be improved in a ° resource-effective manner. DQOs should be established before the study is The basic components of a CSM are: conducted. Example input parameters to the DQOs include the site scenario (such as residential use versus commercial use),action levels for the site, (1) Type of contaminants,including VOCs,currently or previously stored or method reporting limits that need to be met,and the appropriate measurement handled at the site method to be used.The expected output is the most resource-effective design for (2) Known concentrations of COPCs in media such as soil gas,soil and the study. Information concerning DQOs is provided in USEPA(1994a,b, groundwater 2000a). (3) Identification of the primary and secondary sources of COPCs,including: location,depth,and phase(s)of COPCs Technical Reports (4) Primary release mechanism (5) Contaminant transport mechanism and pathway(s),including the The following reports should be completed and submitted for all soil gas identification of preferential pathways and vapor entry points investigations: (6) Exposure media such as surface soil,drinking water,and air (7) Potential receptors including human and ecological receptors,inhabitants • Workplan,including a detailed Conceptual Site Model(CSM) of residential building(s),inhabitants of commercial buildings,and • Sampling and Analysis Plan(SAP)(may be a component of the Workplan) groundwater 3/3/2010 - -11- 3/3/2010 -12-