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PHASE IV GROUNDWgTER INVESTIGATION RESULTS <br /> STOCKTON FORMERMANUFACTUREDGAS NRESUPLAW ITS <br /> • PNAS were detected in HydropunchTM samples collected from Zones Aand TU in MW-15 <br /> and Zone A from MW-14 during the drilling of those Zone C wells. These wells are <br /> located upgrad ate of the site,suggesting that there may be upgradient sources of PNAS <br /> in the groundwater. <br /> • Concentrations of PNAS have continued to decrease in Zones A, B,and C since 1992. <br /> Some of the lowest concentrations to date were detected during the most recent <br /> sampling event(March and April 1996).PNAs are known to sorb highly to soil and <br /> have very slow travel times within the vadose zone and when dissolved in <br /> groundwater. <br /> • Concentrations of VOCs and TPH have decreased over time in the Zone A monitoring <br /> wells located near former USTs. VOCs have not been detected in other Zone A <br /> monitoring wells. The extent of benzene has been defined as beneath Area I within the <br /> PG&E site in Zones A, B,and C. <br /> • As determined in the risk assessment, there is no current risk from COPCs in l <br /> groundwater as no complete exposure pathways exist. soior <br /> 5.0 Recommendations <br /> After agency approval of the Phase IV Groundwater Investigation report, the FS will be <br /> prepared to recommend a site-specific soil and groundwater remedy <br /> unit FS was submitted to the agencies on November 8, 1995. . A draft soil operable <br /> PG&E recommends that the following steps be taken with respect to groundwater at the <br /> Stockton site: <br /> • Finalize this Phase IV Groundwater Investigation report. <br /> • Continue to monitor PNAS, BTEX,and TPH in all Zone A, B,and C wells while the FS is <br /> being prepared and finalized. <br /> • Implement the remedy that will be recommended in the Final FS and described in the <br /> RAP. <br /> • Monitor groundwater concentrations of PNAS, BTEX, and TPH in Zone A, B, and C <br /> wells for at least 8 quarters to evaluate the effectiveness of the remedy. <br /> 6.0 Works Cited <br /> Bank of Stockton.Historic Photograph. Archive.April, 1996. <br /> CH2M HILL. Work Planfor Supplemental Remedial Investigation,Feasibility Study,and Remedial <br /> Action Plan at PG&E's Stockton Former MGP Site. October, 1991. <br /> CH2M HILL. Phase 11 Supplemental RI Work Plan Area II,PG&E Stockton Former MGP Site. <br /> April, 1992. <br /> CH2M HILL.Final Supplemental Remedial Investigation and Risk Assessment Report,PG&E <br /> Stockton Former MGP Site. August, 1993. <br /> SF0/G:\WaRoc\1107161x4\002.D0C <br /> 64 <br />