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Hess-Dubois Cleaners, Stockton - 2 - 12 July 2023 risk, the crawlspace sample should be located under the northeast corner of the house, nearest to the SG2 location. 3. While Central Valley Water Board staff have concurred with the proposed crawl space sampling, you may collect either a crawl space sample or an indoor air sample within the home, as warranted by conditions at the residence. If you opt to collect an indoor air sample, the 2023 Guidance recommends you do so in accordance with DTSC's October 2011 Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air. 4. AGI did not provide a reason for collecting three outdoor air samples. Collection of three outdoor ambient air samples is redundant and unnecessary. Only a single outdoor air sample is needed to evaluate ambient air concentrations. The location of the collected outdoor air sample should be between the crawl space sample and SG2 locations. The side yard north of the house would be ideal as it would limit potential theft/tampering of sampling equipment that could occur on the unprotected Site property. 5. AGI proposes not to perform a pre-sampling inspection of the residence for potential CVOC materials in the residence, but proposes the inspection procedures in the event indoor air sampling is proposed. The 2023 Guidance states that United State Environmental Protection Agency (USEPA) has concluded that either little attenuation occurs between a crawlspace and indoor air or that air exchange between the two spaces leads to approximate equilibrium in the concentrations, and the 2023 Guidance recommends an attenuation factor of 1 for crawl spaces. As such, the pre-sampling inspection should be performed prior to crawl space sampling. 6. While the proposed soil and soil gas sampling locations would assess the potential edges of the extent of CVOCs, it would not identify potential CVOC source areas at the Site, which could later be targeted by future remediation. Only a single historical Site soil sample has been analyzed for CVOCs: the sample collected beneath the former 350-gallon underground storage tank (UST). As the other former Site USTs likely contained both Stoddard solvent and chlorinated solvents, former leaking USTs likely released both petroleum hydrocarbons and CVOCs. All of the other three UST excavations contained high concentrations of petroleum hydrocarbons, and thus are likely to also contain CVOCs. A revised scope of work is needed to investigate potential CVOC source areas. Central Valley Water Board staff requests that you submit a revised work plan by 15 September 2023. Central Valley Water Board staff would like to meet to discuss the scope of work prior to submittal of the revised work plan. 7. Given the sensitive nature of the potential vapor intrusion to the adjacent residence, Central Valley Water Board staff requests that you complete the proposed crawl space and outdoor air sampling as follows: