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Former San Joaquin Cogeneration - 4 - 17 December 2020 Lathrop, San Joaquin County were not detected in samples collected beneath 10 feet bgs. The highest PCB concentrations were generally detected in borings where waste was encountered beneath the former transformer area of the former power plant. • TPH-d was detected in three soil samples exceeding the SFRWB commercial ESL of 1,200 mg/kg with a high concentration of 2,400 mg/kg. • TPH-d was detected in two grab groundwater samples exceeding the SFRWB Tier 1 ESL (taste and odor threshold) of 100 µg/L at concentrations of 330 µg/L and 1,000 µg/L, however, the detection limit for nine grab groundwater samples collected in March 2019 was 500 µg/L, potentially masking the presence of concentrations of TPH-d below that level. • Dioxins/Furans were detected in all four waste samples collected, with two samples exceeding the SFRWB commercial ESL of 22 pg/g at TEQ concentrations of 64.5 pg/g and 96.6 pg/g. • Total lead was detected in two grab groundwater samples exceeding the MCL of 15 µg/L at concentrations of 17 µg/L and 370 µg/L. Regional Water Board Conclusions Central Valley Water Board staff generally concurs with the findings of the Report and has the following comments: • The waste identified beneath the southwest portion of the facility pavement has not been delineated. • Total lead exceeded the MCL of 15 µg/L in two grab groundwater samples. Dissolved lead concentrations need to be verified at locations were total lead in groundwater exceeded the MCL for lead. • Additional monitoring wells should be constructed to confirm groundwater gradient and to assess for the presence of PCBs, and dioxins/Furans downgradient of waste cells. • The downgradient extent of TPH-d in groundwater has not been defined. Groundwater samples should be collected downgradient of elevated concentrations of TPH-d identified in both soil and grab groundwater samples. • AGE had recommended excavation, removal and disposal of waste identified during site assessment activities, as documented in the 3 September 2019 Revised Site Assessment report. Remediation activities must follow the process provided in the 14 June 2019 letter from SJEHD providing the applicable Title 27 California Code of Regulations; Section 21810 clean closure process for the site which is defined as a pre-regulation disposal site. As outlined in the letter, the process includes site characterization, an Excavation and Materials Management Plan, and a final verification report documenting closure activities. • If other remedial alternatives, are being considered, such as capping and maintaining in place, a Remedial Action Plan needs to be submitted for review. The plan should include a conceptual model, remedial design, and a long-term