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INSTALLATION AND SAMPLING OF SUB-SLAB SOIL VAPOR PROBES AND ADDITIONAL SHALLOW SOIL VAPOR PROBES To provide additional observation points for vapor extraction pilot testing, and to provide additional assessment of soil vapor intrusion risk, FREY proposes to install and sample sub-slab soil vapor probes SS-1 and SS-2, and to install additional shallow (6 feet bgs) soil vapor probes SV-3 and SV-4, at the locations shown on Figure 3. Installation of soil vapor probes (6 feet bgs) and sub-slab soil vapor probes will be in accordance with the document titled,Advisory—Active Soil Gas Investigations, published by the California Environmental Protection Agency (Advisory)(Cal EPA, 2015). Installation of Sub-Slab and Shallow Soil Vapor Probes Sub-slab soil vapor probes SS-1 and SS-2 are proposed to be installed inside the Site building as Soil Vapor Pins®. The sub-slab soil vapor probes will be installed by first drilling a 0.625-inch diameter hole at each probe location to a depth of 1-inch below the bottom of the concrete building slab. A wire brush and vacuum will then be used to remove residual concrete dust and/or cuttings from the hole. The assembled soil vapor pins® and silicone sleeves will then be installed in the freshly drilled and cleaned holes in the slab. A cap will be placed on the Soil Vapor Pins® to prevent vapor loss. A diagram of a typical installed soil vapor pin® is included in Appendix D. Soil vapor probes SV-3 and SV-4 are proposed to be installed outside the building north of the former fuel UST in the sidewalk, and adjacent to the north building wall, respectively (Figure 3). The borings for the soil vapor probes will be drilled with a hand auger to a final depth of approximately 6.5 feet bgs. Soil vapor probe SV-3 will be drilled inside an existing "blank" wellbox in the sidewalk, in which a CPT boring (CPT-1) is planned to be advanced in the near future (FREY, 2022). Soil cuttings will be screened for the presence of undifferentiated volatile organic compounds (UVOCs) using a photoionization detector (PID), but soil samples are not proposed to be collected for laboratory analysis from either boring. Soil vapor probes SV-3 and SV-4 will be installed in their respective borings with vapor probe implants set at approximately 6 feet bgs. Each soil vapor probe will be constructed with approximately 6.5-foot long, 0.25-inch outer diameter Nylaflow tubing connected to a plastic vapor implant set at approximately 6 feet bgs. An air tight vinyl cap will be placed on the other end of the tubing, which will protrude above the ground surface. Approximately one-foot of screen washed sand will be placed in the borehole encasing the plastic vapor implant. One vertical foot of dry bentonite granules will be placed at the top of the sandpack. The remainder of the probe annulus will be backfilled with neat cement to approximately 2-inches. Soil vapor probe SV-3 will be covered with the existing traffic-rated wellbox in which it is to be installed. SV-4 will be covered with a traffic-rated wellbox. A diagram of a typical installed soil vapor probe is included in Appendix D. FREY Environmental, Inc. 6 1092-01