902 N BROADWAY BASE FILE 2025-03-27

Page10 damage and to allow for direct visual observation of pipe/bedding materials, if possible, such that well screens are optimally established. After a minimum of 24-hours following installation, utility trench vapor well samples will be collected using six-liter evacuated Summa canisters with eight-hour flow controllers and analyzed for gasoline range organics and related constituents by EPA Method TO-15.9 These three-utility trench vapor monitoring wells will be re-sampled in September-October 2024 in accordance with Ecology guidance on seasonal variability.10 TASK 1C: SANITARY SEWER CLEANOUT VAPOR SAMPLING Sewer pipes often are not airtight and can be a route of vapor migration and source of vapor intrusion into buildings. EES proposes to collect vapor samples from the two sanitary sewer cleanouts associated with the lateral connections that serve the Property building. Sanitary cleanout vapor samples will be collected using six-liter evacuated canisters with eight-hour flow controllers and analyzed for gasoline range organics and related constituents by EPA Method TO-15.11 The sanitary cleanout locations are depicted as SC-1 and SC-2 on Figure 2.These two sanitary sewer cleanouts will be re-sampled in September-October 2024 in accordance with Ecology guidance on seasonal variability.12 TASK 2: VADOSE ZONE SOIL AND PERCHED ZONE CONTAMINANT DELINEATION The findings from the 2023 High Resolution Site Characterization (HRSC) investigation identified substantial residual gasoline at the Property in the vadose/perched water zone extending as deep as 38 feet bgs, with the greatest gasoline and benzene concentrations identified in the southeastern portion of the Site near HRSC borings MIP8 and MIP13, possibly extending into the North Broadway Avenue right-of-way further to the east (see Figure 3).13,14 Significant petroleum impacts were also observed in MIP10 to the northeast and MIP1 to the north, indicating that contamination may extend into the right- of-way associated with Tower Street. Other impacts identified in MIPS, along with elevated soil gas concentrations in this area and information relating to historic Shell infrastructure located in this southern portion of the Property, support at least limited additional delineation efforts in this area. The extent of petroleum contamination has not been delineated in these areas, and investigative data gap resolution is the focus of this supplemental RI work. Resolution of these data gaps is necessary to satisfy MTCA Site characterization criteria and to provide the basis for development of a permanent remedy. s Benzene,Toluene,Ethylbenzene,Total Xylenes, 1,2-Dichloroethane(EDC), 1,2-Dibromoethane(EDB), Methyl-Tert- Butyl-Ether(MTBE), Naphthalene,Gasoline Range Organics(GRO). 10 Guidance for Evaluating Vapor Intrusion in Washington State.Toxics Cleanup Program,Washington State Department of Ecology, March 2022. 11 Ibid. 12 Ibid. 13 Certain borings were advanced deeper than 25 feet bgs into the more permeable glacial outwash material below. " Technical Memorandum: Remedial Investigation Status Update (Q1-Q2 2023), EES Environmental Consulting, August 28,2023. EES ENVIRONMENTAL CONSULTING,INC. 2120-02 2024 SOW 20240408f