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2. 2021 Soil Vapor Sampling Events In 2021,soil vapor sampling was conducted in a manner consistent with previous sampling events following the procedures described in the Revised Soil Vapor and Groundwater Investigation Work Plan (Revised Work Plan; Haley&Aldrich, 2019). In first quarter(Q1) 2021, a total of thirty-six primary soil vapor samples were collected from eleven locations at depths ranging from five to twenty-five feet below ground surface (bgs). In third quarter(Q3) 2021,twenty-six primary soil vapor samples were collected from nine locations at depths ranging from five to fifteen feet bgs.Sampling records are provided in Appendix A. Figure 2 presents the soil vapor sample locations. During both 2021 sampling events, soil vapor samples were collected in one-liter stainless steel canisters, each provided and certified by the laboratory and submitted to the same California-certified laboratory(Eurofins Air Toxics in Folsom,California) under chain-of-custody protocol for analysis using modified United States Environmental Protection agency(USEPA) Method TO-15. At two locations (SV-122 and SV-127),samples were collected using six-liter canisters and submitted for analysis by USEPA Method TO-15 with selective ion monitoring. Minor deviations in the soil vapor monitoring plan provided in the Third Interim Report occurred during the Q1 and Q3 2021 sampling events, including: • Samples from location SV-126 could not be collected from the 10 feet bgs probe due to high vacuum resulting from limited airflow from lower permeability fine-grained soil at this depth. This is consistent with prior sampling events. • An additional sample was collected from location SV-124 from the 5 feet bgs probe during second quarter(Q2) 2021 to confirm or refute the anomalous analytical results received during the Q1 2021 sampling event at this location. • The soil vapor monitoring plan presented in the Third Interim Report indicated the second sampling event for 2021 was to be conducted in Q2 2021, however the second sampling event occurred in Q3 2021. Samples from locations SV-103 and SV-110 were collected during the first sampling event rather than the second as indicated in the soil vapor monitoring plan. 2RICH