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Andrew Richle - 3 - 3 April 2024 4. As discussed during the 28 March 2024 telephone conversation with CHT, Central Valley Water Board staff request the City collect passive soil gas (PSG) samples near the residential dwelling at 18 South Pleasant Avenue during the Phase 1 assessment implementation period. This data will allow the City to better evaluate locations for the Phase II additional assessment work. By 30 April 2024, the City needs to submit a figure showing the proposed locations of the additional passive sample locations. 5. Also pursuant to the 28 March 2024 telephone conversation, the City may use passive indoor air samplers. The City should try to target the sampling during building business hours and peak building occupancy. 6. Central Valley Water Board staff concur with the proposed Phase 2 assessment. The City needs to submit a work plan prior to the implementation of the field activities. This work plan should include vertical soil vapor profiling sample locations near the 18 South Pleasant Avenue residence along with general procedures for vapor sample collection and analysis. 7. Central Valley Water Board staff concur with the proposed Phase 3 assessment that includes an additional round of sub-slab soil vapor sampling from the existing sub-slab vapor pins installed beneath the 212 West Pine Street building. However, Board staff additionally request that depending on results of soil vapor samples from the pins installed beneath 210 West Pine and 17 South Church Streets that additional round of sampling be conducted at these locations to evaluate the Fall 2024 seasonality. Additionally, paired indoor air sampling is requested at the sub-slab soil vapor pin locations sampled. 8. The Work Plan does not propose additional indoor air sampling attempts at 11 South Church Street or 18 South Pleasant Avenue. The 9 February 2024 Central Valley Water Board staff letter acknowledged the challenges of gaining Site access for indoor air sampling at private properties. Board staff may be able to assist in gaining an access agreement to sample inside these properties. Please let Board staff know if this assistance is requested. In summary, Central Valley Water Board staff concur with the Work Plan proposed phased approach for continued vapor and indoor air assessment of the CPSA with the noted exceptions: • Use of passive air samplers for indoor air assessment are acceptable and shall be implemented during hours of building occupation (typically business hours) and not extend for excessive periods of time when building ventilation may become stagnant. • Installation of PSG samplers near the private residence at 18 South Pleasant Avenue shall be conducted during Phase 1 implementation. By 30 April 2024, the City needs to provide a figure showing the proposed PSG locations. • Paired sampling of indoor air with co-located sub-slab vapor pin sample locations should be conducted during Fall 2024 to evaluate seasonality of vapor intrusion potential.