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ADVISORY - ACTIVE SOIL GAS INVESTIGATIONS As a coordinated effort,this document is Issued by the California Regional Water Quality Control Board—Los Angeles Region(LARWQCB)and Department of Toxic Substances Control(DISC)as an Advisory subject to review and revision as necessary. Mention of trade names or commercial products does not constitute the Agency's endorsement or recommendation. The information in this Advisory should not be considered as regulations. In this Advisory,"Agency"should mean LARWQCB and/or DTSC. 1.0 INTRODUCTION Active soil gas investigations are useful to obtain vapor phase data at sites potentially affected by volatile organic compounds (VOCs), including chlorinated and aromatic hydrocarbons. Active soil gas investigations may also be used to investigate sites potentially affected by methane and hydrogen sulfide, and to measure fixed and biogenic gasses (e.g., oxygen, carbon dioxide, or carbon monoxide). Among other things, the data can be used to identify the source and determine the spatial distribution of VOC contamination at a site, or to estimate indoor air concentrations for risk assessment purposes. For site characterization,the Agency encourages both soil gas and soil matrix sampling. Typically, soil gas data are more representative of actual site conditions in coarse-grained soil formations while soil matrix data are more representative of actual site conditions in fine-grained soil formations. For evaluating the risk associated with vapor intrusion to indoor air, soil gas data are the preferred contaminant data set, where practicable. Flux chamber and passive sampling methods are not discussed in this Advisory. Any sites where such sampling methods are necessary will be addressed separately. On February 25, 1997, LARWQCB re-issued the"Interim Guidance for Active Soil Gas Investigation"(ASGI) as guidance for investigating sites with potential VOC contamination. Unless otherwise noted in this Advisory, the active soil gas investigation should be performed in accordance with the most current ASGL 2.0 SUPPLEMENTAL RECOMMENDATIONS The following sections supplement the ASGI in an effort to ensure that consistent methodologies are applied during soil gas investigations to produce reliable and defensible data of high quality. All sampling probe installation, sampling, and analytical procedures, whether or not discussed below, are subject to Agency review and approval. 2.1 Project Management 2.2 Soil Gas Sampling Probe Installation 2.3 Purge Volume Test 2.4 Leak Test 2.5 Purge/Sample Flow Rate 2.6 Soil Gas Sampling 2.7 Analysis of Soil Gas Samples 01/28/2003 1 -