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Soil Vapor Sampling Methodology • Gas sampling will be performed by hydraulically pushing soil gas probes to depths specified in the table above. • Dedicated vapor probes will be sampled at depths specified in the table above at specified areas of concern. Vapor samples will be collected into Tedlar bags or Summa canisters using a peristaltic pump, Teflon tubing, and a Masterflex tubing. • Use of a peristaltic pump will ensure that sampled air does not circulate through a pump, causing potential cross contamination and leakage. • All tubing will be discarded between sampling locations to eliminate the possibility of cross contamination. • Sample flow rate will be set from 0.5 to 1 liters per minute and measured using a flow meter. • Tedlar bags or Summa canisters used for sample collection will be analyzed by a state-certified laboratory, using a field-based gas chromatograph. • Soil vapor sampling will be conducted in accordance with DTSC Vapor Intrusion Guidance (2011a). Reporting Detectable results of soil matrix, soil vapor, and groundwater sampling will be compared to the San Francisco Bay Regional Water Quality Control Board's (SFBRWQCB) Environmental Screening Levels (ESLs) published in January 2019 and the California Department of Toxic Substances Control, Human and Ecological Risk Office (HERO) issued an updated Human Health Risk Assessment (HHRA) Note No. 3 in June 2020. In this note, DTSC makes recommendations regarding the methodology and use of the U.S. EPA Regional Screening Levels (RSLs) and DTSC-modified screening levels (jointly referred to herein as "DTSC Recommended SLs")for soil vapor screening under residential and commercial/industrial land use scenarios. The above screening levels were developed as tools to screen sites to evaluate the need for additional action in the form of additional assessment or evaluation. The screening levels are based on conservative levels derived from assessment of potential exposure pathways to the most sensitive property use. The screening levels are not default cleanup goals. For this property, comparison of detected contaminants in soil vapor with screening levels will be utilized to evaluate the human health risk from possible vapor intrusion under a commercial use scenario. It should be noted that the screening levels are guidelines and are not enforceable regulatory standards. The DTSC-Recommended SLs for evaluating soil vapor intrusion are calculated using indoor air screening levels and recommended attenuation factors. These calculated soil vapor screening levels are typically applied to samples collected no more than five feet below ground surface, and have been developed for both existing buildings and future buildings (DTSC and CaIEPA, October 2011). The values calculated using Note No. 3 recommendations are very conservative, and concentrations in excess of the calculated DTSC-Recommended SLs are not conclusive evidence of adverse risks to human health. Depending on VOC concentrations, their distribution, land use, and other factors, additional investigation — such as 6