902 N BROADWAY BASE FILE 2025-03-27

Page 9 underground utilities; and (to a reasonable extent) compilation of an inventory of products in use in the building that have the potential to affect indoor air quality. ■ Develop a conceptual building model to include: a plan view drawing of the building and other key features; preliminary summary of the inferred airflow pathways for the building; and a narrative that provides explanations for key issues and assumptions made in depicting building conditions. The primary objective of the Step 3 indoor air assessment is to employ continuous monitoring of vapor concentrations and related surface and subsurface physical parameters to understand likely exposure risks throughout the Property building during this time period. By incorporating information gathered during the differential cross-slab monitoring(DPT) and building operations survey, the continuous monitoring VIA approach will seek to evaluate specific areas of the building during times when advective flux of subsurface vapors is most likely to be occurring, so that VI issues, if identified, can be evaluated as quickly as possible. ADDITIONAL SAMPLINGIANALYTICAL TESTING TO ACCOMPANY INDOOR AIR ASSESSMENT TASKS Note that if Step 3 indoor air assessment and sampling activities are conducted, several other sampling/analytical tasks will also be triggered. ■ Confirmatory discrete indoor(x5) and outdoor(x3) air samples will be collected using 6L summa canisters. Indoor air samples will be collected during real-time sampling conducted using<200 ml/minute flow controllers intended to generally replicate the continuous real-time sampling activities; outdoor air samples intended to represent ambient background conditions will use 6L summa canisters fitted with 8-hour flow controllers. Summa samples will be analyzed by Method TO-15 for COls as described under Tasks 1 and 2. Wind direction,wind speed, and outdoor barometric pressures will be logged using a portable weather station during this sampling event. ■ Sub-slab vapor samples representing disabled SVE conditions (Step 2) will be collected immediately after the completion of indoor air sampling. In this case,the mobile GC will be used to analyze sub-slab vapor samples collected from Vapor Pins, and the Step 2 sub-slab analytical approach will include duplicate sampling using summa canisters that will be shipped to a fixed laboratory for comparative, confirmatory TO-15 analysis (mimicking the Step 1 analytical approach). TASK 1113: UTILITY TRENCH VAPOR WELL INSTALLATION AND SAMPLING Subsurface vapor assessment in the utility areas along the eastern boundary of the Property will inform an understanding of potential petroleum vapor migration through underground utility corridors in this area. EES proposes to install three re-useable vapor monitoring wells(SGW-6 to SGW-8)within the sanitary sewer trench backfill, if possible, at the locations indicated on Figure 2. Subject to approval by the utility owner(City of Everett), vapor wells SGW-6 through SGW-8 will be constructed with two-inch PVC well screen and riser(with annular filter media and surface seal).The depths of SGW-6 through SGW-8 will be dependent on depth of the sanitary sewer trench, approximately 8-10 feet below ground surface (bgs).These wells will be carefully installed by a Washington-licensed well driller using air-knife and/or hand-auger drilling to minimize the risk of utility EES ENVIRONMENTAL CONSULTING,INC. 2120-02 2024 SOW 20240408f