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Page 3 <br /> RECOMMENDED SUPPLEMENTAL RI TASKS <br /> TASK 1: VAPOR INTRUSION/MIGRATION ASSESSMENT <br /> Quarterly soil vapor analytical data collected in 2023 from monitoring points adjacent to the Property <br /> building (SGW-1, SGW-2, SGW-4, and SGW-5) exhibited gasoline vapor concentrations in exceedance of <br /> MTCA commercial building vapor intrusion screening criteria.3 These observations are consistent with <br /> historic facility operations and indicate the possibility that petroleum impacts extend under the existing <br /> Property building and trigger the regulatory process requiring preliminary(Tier 1)vapor intrusion <br /> assessment (VIA).4 The magnitude of vadose-zone soil contamination present along eastern portions of <br /> the Property, in close proximity to various underground utility features that may represent vapor <br /> migration/exposure pathways, also compels VIA at that portion of the Site. <br /> Actions under Task 1 are intended to help resolve the following fundamental data gaps related to vapor <br /> intrusion/migration, and provide a reasonable basis for final cleanup planning regarding VIA conditions: <br /> A. Evaluate potential vapor intrusion in areas (1) beneath the Property building, and (2)within or <br /> adjacent to the sanitary sewer trench backfill alignment that penetrates the building and <br /> extends along the eastern Property boundary. <br /> B. Assess whether releases in the footprint of the former Shell Auto Service Building, if present, <br /> may represent a source of mobile contamination. <br /> C. Determine whether the current SVE system is an effective control to limit vapor migration <br /> beneath the Property building from petroleum impacts located in the area of the former UST <br /> infrastructure. <br /> D. Evaluate seasonal conditions over the span of approximately 4-5 months to establish a <br /> reasonable basis for potential "temporal variability"that could affect possible VI characteristics. <br /> EES notes that monitoring results from December 2023 indicated much lower hydrocarbon vapor <br /> concentrations than previously measured during drier seasons.This recent data indicates likely seasonal <br /> variability of vapor-phase contaminant behavior in the shallow vadose zone, as might be predicted <br /> based on expectations in widely published guidance for temporal variability. While this current scope of <br /> work endeavors to characterize a reasonable range of potential vapor intrusion conditions at the <br /> Property building, Ecology may require additional future/seasonal assessment to refine the Conceptual <br /> Site Model. <br /> This work plan includes a contingency for supplemental "Tier 2" assessment to evaluate indoor air <br /> conditions if sub-slab contaminant vapor concentrations are found to exceed Ecology's published vapor <br /> 3 Technical Memorandum: Remedial Investigation Status Update(Q1-Q2 2023), EES Environmental Consulting, <br /> August 28,2023. <br /> 4 Guidance for Evaluating Vapor Intrusion in Washington State.Toxics Cleanup Program,Washington State <br /> Department of Ecology, March 2022. <br /> EES ENVIRONMENTAL CONSULTING,INC. 2120-02 2024 SOW 20240408f <br />