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Geosyntec° consultants necessary and that the Port would have to evaluate the potential for vapor migration to indoor air (CVRWQCB 2010). In response to the CVRWQCB conclusion to the 2008 investigation, ERS prepared a Draft Corrective Action Plan(CAP) (ERS 2010)to address residual petroleum hydrocarbon contamination in soil and groundwater at UST 817B by performing excavation,pumping, groundwater amendment with an oxygen-releasing compound(ORC), and soil treatment. CVRWQCB provided comments on the Draft CAP on June 22, 2010. No additional documentation of a revised or final CAP has been identified in the Port's records at this time. Documentation of the implementation of the CAP has also not been identified; however, James Durkin of DTSC verbally confirm that the CAP was implemented during a March 13, 2024 quarterly progress meeting. According to James Durkin of DTSC,petroleum hydrocarbon impacted soil was excavated and temporarily stockpiled nearby the Site to allow the soil to aerate, consistent with the CAP. Following aeration, the stockpile was sampled to document a reduction in soil concentrations and was then utilized to backfill the excavation. Review of historical aerials also confirms an extensive excavation occurred at the Site around 2010 (Geosyntec 2024), the estimated extents of which are shown on Figure 2. According to the CVRWQCB's letters to the Port dated September 26, 2017, February 16, 2018, and September 20, 2018 (CVRWQCB 2017, 2018a, 2018b), UST 817B met the general and media specific criteria for closure under the LTCP, except for concentrations of benzene in soil, which exceeded the LTCP media-specific criteria 3 for direct contact and volatilization to outdoor air(Geosyntec 2019). A data gap investigation in 2019 collected soil samples to provide additional analytical data necessary to evaluate closure of the UST. Analytical results indicated that TPH-g was detected in soil at a concentration of 1,100 milligrams per kilogram(mg/kg) in one boring (SS-817B-04A), which is above the LTCP media-specific criteria for vapor intrusion to indoor air(Geosyntec 2019). Based on this analytical result, the CVRWQCB requested that a soil vapor survey be conducted(Geosyntec 2019). Soil vapor samples were collected in June 2019, and the analytical results indicated the presence of elevated concentrations of TPH-g and BTEX. More notably, one of the soil vapor samples (SVP-03) detected benzene at a concentration of 2,300,000 micrograms per cubic meter(µg/m), which indicates the potential presence of light non-aqueous phase liquids (LNAPL). A limited soil and groundwater investigation was performed in 2022 based on the detections of TPH-g and BTEX in soil vapor to evaluate if LNAPL is present. Two borings were advanced within the former tank excavation and extended to the buried concrete pad to which the former USTs were secured. LNAPL was not encountered in the soil or accumulated groundwater within the boreholes. Elevated concentrations of TPH-g were detected in soil at the location of the former USTs up to a concentration of 1,900 mg/kg (detected in the soil sample collected from 2 to 4 feet bgs from boring SS-81713-06A). Elevated concentrations of TPH-g and benzene were also detected in the grab-groundwater sample collected from SS-81713-07A, at concentrations up to 15,000 micrograms per liter(µg/L) and 700 µg/L, respectively. Although LNAPL was not encountered, concentrations of benzene detected in a grab groundwater sample could equilibrate (per Henry's law)with soil vapor at a concentration as high as 3,800,000 µg/m3. Investigation Work Plan—UST 817B 3 December 2024