902 N BROADWAY BASE FILE 2025-03-27

Page 7 canisters with <200 ml/minute flow controllers.These samples will be analyzed for gasoline range organics and volatile organic constituents (full scan) by EPA Method TO-15. Contemporaneous soil gas samples will also be collected adjacent to the building from the five existing vapor monitoring wells (SGW-1 through SGW-5) using one-liter evacuated Summa canisters with <200 ml/minute flow controllers and analyzed for gasoline range organics and gasoline-related constituents by EPA Method TO-15.7 In addition to the TO-15 analytical vapor testing, biogenic parameters, including methane, oxygen, and carbon dioxide, will also be field-measured at the five SGWs during sampling events as part of standard protocols.These samples will also be collected while the SVE system is in operation. Following Step 1 vapor sample collection,the SVE system will be shut down for a four-month period, during which time the Step 1 analytical data can be evaluated, and subsurface pressures will stabilize to non-SVE seasonal conditions. Cross-slab pressure conditions will continue to be monitored continuously using the DPT sensors. Step 1 analytical testing data will be available approximately two weeks following sample collection and before the initial (Step 2A) mobilization. If Step 1 analytical findings indicate sub- slab vapor concentrations exceeded MTCA vapor intrusion screening levels,then Step 3 indoor air sampling will be conducted immediately before the Step 2A subsurface sampling efforts (indoor air sampling will be conducted before sub-slab sampling to minimize the risk of biasing the indoor data through cross-contamination from exposure to sub-slab vapors). STEP 2 SUBSURFACE VAPOR SAMPLING(SVE SYSTEM OFF) ■ Step 2A(one month following SVE shutdown). For comparison to active SVE (Step 1) conditions during a similar seasonal period, EES will collect a second round of vapor samples from the sub- slab Vapor Pins and from the SGW well array following the one-month period of SVE system shutdown. The sampling and testing approach will duplicate Step 1. ■ Step 213 (four months following SVE shutdown). For comparison to Step 1 and Step 2A data and representing a different seasonal and building operational period, EES will collect a third round of sub-slab vapor samples from the Vapor Pins and from the SGW well array.The sampling and testing approach will duplicate Step 1. Following the completion of Step 213 sub-slab sampling after Month 4,the SVE system will be restarted while DPT cross-slab monitoring devices are operated for a minimum of one week following SVE restart, before being demobilized. STEP 3 CONTINGENCY INDOOR AIR SAMPLINGITIER 2 EVALUATION Ecology guidance indicates that "Tier 2" indoor air assessment is necessary when sub-slab vapor concentrations exceed published VI screening levels. EES will therefore conduct indoor air assessment only if triggered by either Step 1 (SVE operating) or Step 2 (SVE disabled) sub-slab vapor findings confirming actionable sub-slab vapor concentrations.At this time, only a single indoor air sampling Benzene,Toluene, Ethylbenzene,Total Xylenes, 1,2-Dichloroethane(EDC), 1,2-Dibromoethane(EDB), Methyl- Tert-Butyl-Ether(MTBE), Naphthalene, Gasoline Range Organics(GRO). EES ENVIRONMENTAL CONSULTING,INC. 2120-02 2024 SOW 20240408f