902 N BROADWAY BASE FILE 2025-03-27

Page 8 event is anticipated; additional/supplemental indoor air sampling needs, if identified, would be considered separately, and are not addressed under this plan. To accomplish this Step 3 indoor air assessment, EES will work directly with VaporSafe to obtain continuous real-time analytical data for measuring COls in air throughout the Property building interior, using a laboratory-grade portable gas chromatograph (GC) instrument.This approach will facilitate rapid development of the VI conceptual site model, with findings tied to building operating conditions in a matter of days and a single mobilization as opposed to many months and multiple mobilizations if collected using traditional indoor air sampling methods.The GC is equipped with a multiplexed port valve component to allow for sample collection from multiple locations in the building simultaneously. The discrete vapor samples are analyzed in minutes at the subject Property, and the test findings are delivered to a remote processing Cloud-based software platform, available to the project team quickly. Barometric pressure, differential pressure, and temperature are also simultaneously monitored and reported in this manner. In addition to addressing Ecology's VIA requirements,this indoor air assessment approach provides the following streamlined technical and schedule benefits: ■ Provides real-time analytical results for multiple indoor air sampling locations. This data will enable rapid determination of whether and in what locations indoor air quality achieves the standard MTCA Method B unrestricted-use indoor air exposure cleanup levels.$ ■ Enables evaluation of air conditions under variable building operating conditions (diurnal heating/cooling/ventilation, workplace activities and possible ambient COI sources, climatic/storm events, if changing over the multi-day sampling event, etc.). ■ Enables focused evaluation of possible subsurface vapor entry points (e.g. slab cracks, utility penetrations/conduits, and/or other sources of ambient/indoor COI contributions). ■ Allows for pairing analytical data with continuous differential pressure monitoring data to provide a basis for determining when and under what building/barometric pressure conditions that VI may be occurring. BUILDING OPERATIONS SURVEY Before mobilization of the GC unit, a building-operations survey will be conducted to document construction and operating conditions.The building survey will include research to evaluate factors that may influence indoor air quality and will provide an opportunity to present the building management and occupants with guidelines on how to prepare for the sampling and to help avoid common sources of background air contamination. Initial data collection tasks will include the following: ■ Research basic factors that may influence indoor air quality including (if possible): building foundation/construction characteristics and floor plan; observation for obvious cracks, seams, or other preferential indoor vapor migration pathways in foundation materials; initial determination of HVAC equipment in use at the building; identifying types and locations of 'https://ecology.wa.gov/Regulations-Permits/Guidance-technical-assistance/Contamination-clean-up- tools/CLARC/Data-tables EES ENVIRONMENTAL CONSULTING,INC. 2120-02 2024 SOW 20240408f