CORRESPONDENCE_AGENCY CORRESPONDENCE

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Stockton Charter Way Commingled Plume - 2 - 18 April 2023 LTCP thresholds. Central Valley Water Board staff concurs with the proposed soil boring scope of work, as summarized in Table 1 below. Table 1 — Sum mar of Proposed Soil Borings Total Soil Sampling Boring Method Depth Depth Analysis (ft bgs) ft b s Hand Auger/ GRO, BTEX, MTBE, TBA, DIPE, HA-1, HA-2 Split Spoon 10 5, 10 ETBE, TAME, EDB, 1,2-DCA, naphthalene GRO= gasoline range organics; BTEX= benzene, toluene, ethylbenzene, and total xylenes; MTBE= methyl tert butyl ether; TBA= tert butyl alcohol; DIPE= di-isopropyl ether; TAME= tert amyl methyl ether; EDB= ethylene dibromide, 1,2-DCA= 1,2- dichloroethane; ft bgs= feet below ground surface By 21 July 2023, please complete the soil borings as discussed above and submit an assessment report. In this report, please provide an evaluation of shallow soil data against LTCP Direct Contact and Outdoor Air Exposure criteria. 2. In a February 2016 work plan, Stantec proposed several additional monitoring and remediation wells. Central Valley Water Board staff concurred, and all have been installed except for downgradient delineation wells SMW-34E, SMW-35D, and SMW-36D. Installation of these wells was delayed by off-Site access issues. Due to these issues, in July 2022 email correspondence, Stantec proposed revised locations for these wells, and Central Valley Water Board staff concurred. However, during the November 2022 groundwater sampling event, hydrocarbon concentrations in downgradient-most D- and E-zone monitoring wells SLMW-10D, SMW-35D, and SMW-35E were below laboratory reporting limits. As such, the plume appears adequately defined and installation of these wells is not necessary at this time. 3. In the Work Plan, Stantec proposes to remove 13 Site monitoring wells from the semi-annual and annual sampling schedule, and move them to an annual gauge and inspection only schedule. Central Valley Water Board staff concurs with the proposed changes to the sampling schedule with the exception of well SLMW-10C. This well (1) remains impacted, (2) contained in November 2022 the highest total petroleum hydrocarbons as gasoline (TPHg) concentration since 2016, and (3) is the downgradient-most C-zone monitoring well. Please continue to sample this well semi-annually as before. 4. Please continue to perform routine groundwater sampling events at the Site on a semi-annual schedule during 2nd and 4t" quarters. Moving forward, please implement the sampling schedule modifications as discussed above. The next groundwater sampling event should be performed during 2nd quarter 2023 with a monitoring report due 1 August 2023. Please also continue to submit reports of Site remediation activities on a quarterly basis. The report for 1 st quarter 2023 remediation activities is due by 1 May 2023.