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` PHASE IV GROUNDWATER INVESTIGATION RESULTS <br /> STOCKTON FORMER MANUFACTURED GAS PLANT SITE <br /> • PNAs were detected in HydropuncWm samples collected from Zones A and B in MW-15 <br /> and Zone A from MW-14 during the drilling of those Zone C wells.These wells are <br /> located upgradient 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 soil or <br /> groundwater as no complete exposure pathways exist. <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.A draft soil operable <br /> unit FS was submitted to the agencies on November 8,1995. <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 /> • For purposes of preparing the FS,assume a nondetect(ND) line along the perimeter of <br /> Areas I and II of the site for Zone B groundwater. As new information regarding the <br /> extent of groundwater contamination in Zone B becomes available, the final FS will <br /> include this information. <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 /> • Investigate possible soil contamination in Area II. <br /> 6.0 Works Cited <br /> Bank of Stockton.Historic Photograph.Archive. April, 1996. <br /> CH2M HILL. Work Plan for Supplemental Remedial Investigation,Feasibility Study,and Remedial <br /> Action Plan at PG&E's Stockton Former MGP Site. October, 1991. <br /> SFO/G:\WPRCC\710716\P4\002.DOC 64 <br />