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Page 6 September 11,2023 Brusca File No. 260-004 Anderson Barngrover Ranch and chain-of-custody documentation are attached; laboratory quality control/quality assurance information is included in the laboratory reports. We also requested that Sunstar Laboratories, Inc. produce a revised laboratory report from the subsurface soil and groundwater testing performed in May 2023 (Borings TH5 through TH7) to include reporting for MTBE (as suggested by the CVRWQCB as a part of the LTCP evaluation). The revised laboratory report including MTBE results is attached. As shown, none of the soil or groundwater samples collected from the previous borings TH5 through TH7 contained MTBE at a concentration above the laboratory reporting limit, except that the groundwater sample collected from Boring TH5 contained MTBE at a very low concentration of 1.2 micrograms per liter(ug/L). RESULTS Soil Results As shown on Table I and the attached laboratory report,none of the soil samples from Boring TH8, TH9, or TH10 contained petroleum hydrocarbons or VOCs at concentrations above the laboratory reporting limits, except that the 70-foot-deep soil sample from TH9 contained low concentrations of total petroleum hydrocarbons (TPH) as diesel, sec-butylbenzene, and naphthalene (660 milligrams per kilogram[mg/kg],0.16 mg/kg,and 0.29 mg/kg),respectively, and the 60-foot-deep soil sample from TH10 contained a low concentration of TPH as diesel (650 mg/kg). As shown on Table I, none of these detected concentrations exceed commercial/industrial Environmental Screening Levels (ESLs) published by the San Francisco Bay Regional Water Quality Control Board(SFBRWQCB).9 Based on the soil analytical data from Borings TH8, TH9, and TH10, along with the soil data from previous borings at the site (summarized on Table I), the extent of soil impacts at site related to the removed UST appear to be reasonably well characterized. Groundwater Results A primary purpose of this investigation was to further define the northerly,westerly, and southerly limits of petroleum hydrocarbon impacts to groundwater attributable to the removed diesel UST at the site (the easterly extent of the plume was previously defined by non-detect conditions in Boring TH6 performed in May 2023). As shown on Table II and the attached laboratory report, the groundwater sample collected from Boring TH8 situated to the north of the former UST did not contain petroleum hydrocarbons or VOCs at concentrations above the laboratory reporting limits, except for a very low concentration of TPH as diesel (130 ug/L). The concentration of diesel detected in the groundwater sample from Boring TH8 is below the ESL value (MCL Priority) and is similar to the concentration of TPH as diesel that was previously detected in the sample collected from the nearby irrigation water well (identified as North Water Well) in May 2023. It is possible that operation of this water well has drawn a portion of the diesel groundwater plume to the north to some degree, and it is considered unlikely that groundwater impacts extend 9 San Francisco Regional Water Quality Control Board; "Environmental Screening Levels'; Commercial/Industrial, Direct Exposure Human Health;July 2019.