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Former San Joaquin Cogeneration - 4 - 29 December 2020 Lathrop, San Joaquin County 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 operation and maintenance plan. Due to the nature of the waste identified at the