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Hess-Dubois Cleaners, Stockton - 2 - 21 March 2023 <br /> hydrocarbon soil gas concentrations are below LTCP Petroleum Vapor Intrusion to <br /> Indoor Air residential and commercial thresholds, both on-Site and off-Site at the <br /> nearby school property. As such, Site conditions appear to meet all LTCP criteria. <br /> However, given the sensitive nature of the nearby school and adjacent residence to <br /> the south, Central Valley Water Board staff concurs with collecting an additional round <br /> of soil gas samples to confirm the results and evaluate potential seasonal variations. <br /> By 1 November 2023, please complete the confirmation sampling and submit an <br /> evaluation report. <br /> 2. Chlorinated volatile organic compounds (CVOCs) are present in soil, soil gas, and <br /> groundwater beneath the Site. Soil gas wells SV2 and SV3 contained <br /> tetrachloroethylene (PCE) and trichloroethylene (TCE) above the San Francisco Bay <br /> Water Board commercial subslab/soil gas vapor intrusion human health risk <br /> environmental screening levels (Soil Gas ESLs). SV2 is located in very close <br /> proximity to the adjacent residence to the south and contained PCE at a concentration <br /> two orders of magnitude above the residential Soil Gas ESL and TCE at a <br /> concentration 5 times the residential Soil Gas ESL. SV3 also contained cis-1,2- <br /> dichloroethene (cis-1,2-DCE) above the commercial Soil Gas ESL. Off-Site location <br /> SV4 was the only soil gas well which contained chloroform above the reporting limit, <br /> but was below the commercial Soil Gas ESL. The soil sample collected beneath the <br /> former 350-gallon UST in 2015 is the only Site soil sample analyzed for CVOCs. This <br /> sample contained PCE, at a concentration above reporting limits but below the San <br /> Francisco Bay Water Board commercial shallow soil direct exposure human health risk <br /> ESL. Site groundwater has historically contained PCE, TCE, cis-1,2-DCE, and vinyl <br /> chloride above water quality objectives. The highest historical and recent PCE <br /> concentrations were in MW-6 in the northeast (downgradient-most) corner of the Site, <br /> and the second highest were in MW-10 on the nearby school property. February 2023 <br /> soil gas results are summarized in Tables 1 and 2 and historical and recent CVOC <br /> groundwater results are summarized in Table 3, which are included at the end of this <br /> letter. <br /> Additional assessment is needed to delineated CVOCs in soil and soil gas. <br /> Additionally, the close proximity of high CVOC concentrations in soil gas to the <br /> adjacent residential property to the south, especially TCE, is of concern to Central <br /> Valley Water Board staff. By 20 May 2023, please submit a work plan for delineation <br /> of CVOCs in soil and soil gas and for indoor air sampling at the south-adjacent <br /> residence. <br /> 3. Please resume performing routine semi-annual groundwater monitoring at the Site <br /> during 2nd and 4'" quarters, and submit associated groundwater monitoring reports by <br /> 1 August and 1 February, respectively. Please perform the next groundwater <br /> monitoring event during 2nd quarter 2023, sampling all Site monitoring wells (MW-1 <br /> through MW-10, EW-1), with analysis to include CVOCs. Please submit a <br /> groundwater monitoring report by 1 August 2023. For the 2nd quarter 2023 sampling <br />