902 N BROADWAY BASE FILE 2025-03-27

Page 6 TASK 1A: BUILDING ASSESSMENT: TIER 1 SUB-SLAB VAPOR SAMPLING, WITH CONTINGENCY FOR TIER 2 INDOOR AIR ASSESSMENT To facilitate sub-slab vapor sample collection and related differential pressure measurements above and below the building's concrete floor slab, dedicated reusable low-profile sampling ports called "Vapor Pins" will be installed at each of the five sampling locations (Figure 2).The small, protected, re-usable Vapor Pins will be left in place following monitoring/sample collection for future use. Using the Vapor Pins and with the existing SVE system first remaining in operation and then deactivated, EES will deploy several Differential Pressure Transmitters (DPTs manufactured by OBAR, or similar)to measure relative atmospheric pressures simultaneously above and below the floor slab.' The DPTs will provide baseline data to evaluate cross-slab pressure dynamics and potential vapor intrusion mechanics over a range of climatic and building heating/cooling/ventilation (HVAC) conditions at select SSV locations.This instrumentation will contribute a strong basis for indicating if/when pressure conditions conducive to vapor intrusion are occurring and whether the SVE system may be producing detectable vacuum below the building's floor slab. While the DPT devices are continuously logging cross-slab building air pressures, sub-slab vapor samples will be collected initially with the existing SVE system in operation (Step 1) and then after the SVE system has been shut down for a period of one month (Step 2A) and four months (Step 213)to determine seasonal variability in accordance with Ecology guidance. ■ If the Step 1 analytical testing results indicate vapor concentrations exceed MTCA screening- levels while the SVE system is active,then vapor intrusion preconditions are adequately demonstrated, and indoor air assessment and sampling are required based on Ecology's "Tier 2 VIA" decision-making guidance. Indoor air sampling protocols are described as Step 3 under this work plan. ■ Alternatively, if initial Step 1 sub-slab vapor concentrations are below the MTCA screening levels, then for comparison Step 2 sub-slab sampling will be conducted approximately one month after temporary shutdown of the SVE system (Step 2A), and then again approximately three months later(Step 213) to establish a range of seasonally variable data. ■ Step 3 Contingency Indoor Air Sampling will not be conducted under this work plan if Step 1 and Step 2 sub-slab vapor concentrations are uniformly below the MTCA vapor intrusion screening levels. Regardless of vapor concentration results, differential pressures above and below the floor slab will be measured continuously throughout the observation period,which includes both active and deactivated SVE operations over the span of approximately four months.The supporting methodology and testing approach are described below. STEP 1 INITIAL SUBSURFACE VAPOR SAMPLING(SVE SYSTEM OPERATING) Following Vapor Pin installation and a restabilization period of approximately one week, initial sub-slab vapor samples will be collected while the SVE system remains in operation, using one-liter evacuated 'https://www.obarsystems.com/product/edg-wireless-cellular-gateway/, https://www.obarsystems.com/product/gbr-dpt-differential-pressure-transmitter/ EES ENVIRONMENTAL CONSULTING,INC. 2120-02 2024 SOW 20240408f