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Andrew Richle - 2 - 3 April 2024 Phase 1 (which is detailed in the Work Plan): • Collect depth discrete soil vapor samples to determine the extent of residual PCE in soil and soil vapor behind the 212 West Pine Street building and the intersection of the sewer lateral emanating from the building and the main sewer lateral; and • Install sub-slab soil vapor sample pins and collect collocated passive indoor air samples at 212 West Pine Street and Guild Cleaners Inc (Guild), located at 17 South Church Street; and • Install sub-slab soil vapor sample pins and collect samples at 210 West Pine Street. No paired indoor air sampling is proposed at this sub-slab sample point location. Phase 2 (not detailed in the Work Plan) - proposes to address comments in the 9 February 2024 Central Valley Water Board staff letter: • Conduct additional delineation to define the extent of residual PCE in soil vapor near the 18 South Pleasant Avenue residence; and • Evaluate remedial alternatives at, and adjacent to, the building at 212 West Pine Street. Phase 3 (not detailed in the Work Plan; however, inferred to follow same procedure as proposed for the newly installed vapor pins): • Conduct an additional round of sub-slab soil vapor sampling from the existing vapor pins installed beneath the 212 West Pine Street building. This sampling would be conducted in August/September 2024 time frame after completion of interim remedial measures at the 212 West Pine Street building. On 28 March 2024, Central Valley Water Board staff and CHT discussed the proposed work and phasing of the investigation. Based on the information provided in the Work Plan and the telephone conversation, Central Valley Water Board staff generally concur with the proposed phased assessment approach with the following exceptions and comments. 1. Central Valley Water Board staff concur that the proposed Phase 1 soil and soil vapor characterization for VOCs in the sub-surface immediately south of the 212 West Pine Street building needs to be completed as soon as practical. This work will provide additional information on the extent of residual PCE contamination potentially requiring targeted remediation as well as help to identify potential building mitigation needs. 2. The City presently proposes to install temporary soil vapor sampling probes at various depths in the area behind (south) of the building at 212 West Pine Street. Central Valley Water Board staff request that these sample points be installed as semi- permanent probes to allow repeated sampling. 3. Central Valley Water Board staff concur with the installation of a sub-slab soil vapor sample pin beneath both the buildings 210 and 212 West Pine Street, and beneath the building at 17 South Church Street. However, the City has only proposed to collect passive paired indoor air samples at the location of the new sub-slab soil vapor pin in 212 West Pine Street and 17 South Church Street. Central Valley Water Board staff also request that a paired indoor air sample be collected at the new sub-slab vapor pin location within 210 West Pine.