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February 19, 2021 Mr. Olivier Theard Page 4 SLR International Corporation slrconsulting.com o At three locations outside the building, temporary dual-nested soil vapor probes will be installed at 5 and 10 ft bgs as shown on Figure 3. o At four locations inside the building, sub slab soil vapor monitoring points (Vapor Pins®) will be installed as shown on Figure 3. o Temporary dual-nested soil vapor probe installation schematics are shown in Figure 5 and sub slab soil vapor monitoring point details are shown in Figure 4. The installation and sampling will be performed in accordance with the Department of Toxic Substances Control (DTSC) Advisory for Active Soil Vapor Investigations, dated July 2015 and the DTSC Supplemental Guidance: Screening and Evaluating Vapor Intrusion (February 2020). o Soil vapor and sub slab soil vapor samples will be collected and analyzed for VOCs by EPA Method TO-15. Task 3: Data Evaluation and Reporting Following receipt of the MiHPT logs and the laboratory analytical results: • SLR will review the MiHPT logs of each boring, including separate charts of electrical conductivity, Electron Capture Detector (ECD), Halogenated Specific Detector (XSD), Photoionization Detector (PID), Flame Ionization Detector (FID), Hydraulic Profiling Tool (HPT) back pressure, HPT flow rate, and estimated hydraulic conductivity (K) to analyze for the presence and relative quantity of VOCs, as well as subsurface lithology data. • Compare the soil vapor and sub slab soil vapor sampling results to the California Human Health Screening Levels (CHHSLs) for Soil and Soil Gas (OEHHA, 2005), the Updated Human Health Risk Assessment (HHRA) Note 3 - DTSC-Modified Screening Levels (DTSC, 2020), and the SFRWQCB 2019 Screening Levels (SFRWQCB, 2019). • SLR will also compare the results of the MiHPT investigation and the laboratory analytical results with the results of the January 2021 Additional ESA to develop a more complete conceptual model of the Site. • SLR will then prepare a draft Supplemental ESA report, including a summary of the field activities, figures and tables presenting the analytical data and comparing the data to applicable screening levels, and a discussion of the sewer inspection, MiHPT logs, and soil vapor sampling results. Task 4: Indoor/Outdoor Air Sampling The sampling activities described below, including indoor and outdoor sampling, are proposed for the Tier II scope of work. Prior to implementing the scope of work, SLR may modify the proposed scope based on the results of the Tier I result presented in the draft Supplemental ESA report. • Perform a pre-sampling building survey of the Site in accordance with the DTSC Supplemental Guidance: Screening and Evaluating Vapor Intrusion (February 2020). Pre-sampling building survey activities can include, but are not limited to, visually observing the building (interior and exterior) and surrounding area, and reviewing the building layout and drawings. Real-time vapor screening