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Groundwater Well Installation Work Plan Sharpe Army Depot <br /> Lathrop, California <br /> Justification of Well Recommendations <br /> Increasing TCE concentrations at MW436C and PSC, and a TCE ACL exceedance in a new monitoring well <br /> installed in July 2021 (MW547C) indicate the C Zone TCE plume is expanding toward the southern <br /> installation border. Concentrations above the ACL were detected at MW436C for the first time in 3Q21 <br /> and at P5C in 3Q20, and a statistically-significant increasing trend has been observed at MW436C. <br /> Increasing concentrations at both wells show that TCE is migrating beyond the capture zones of EWC1, <br /> EWC2, and EWC3. Currently, only one monitoring well exists downgradient of the southern C Zone TCE <br /> plume (MW440C). TCE was not detected at MW440C in 3Q22 and had not been sampled since 2005. <br /> Additional monitoring wells east of MW440C along the southern edge of the facility are needed to monitor <br /> the southern C Zone TCE plume. <br /> 2.2.2 Central Area <br /> Three monitoring wells (MW551B, MW551C, and MW552C) are proposed to be added to the west of the <br /> Central Area,outside the Sharpe boundary.One well is proposed to be screened in the B Zone (MW551B), <br /> and two wells are proposed to be screened in the C Zone (MW551C and MW552C). These wells will fill <br /> data gaps south and west of the current plume boundary, aiding in defining the western plume boundary <br /> and monitoring plume migration. <br /> The proposed well locations and Central Area plumes are shown in Figures 6 and 8. Preliminary <br /> specifications for the proposed wells are in Table 1. Details of existing wells in the vicinity of proposed <br /> wells are in Table 2. Hydrogeologic cross-sections with the new groundwater wells in relation nearby wells <br /> and groundwater plumes are in Figures 3 through 5. <br /> Justification of Well Recommendations <br /> Two offsite plume boundary wells (MW532B and P7C) have shown statistically-significant increasing TCE <br /> trends (Ahtna, 2022a). In the B Zone,TCE concentrations at MW532B have been primarily above the ACL <br /> since 2009, but TCE has not been detected to the northwest at MW504B since 3Q07. In the C Zone, TCE <br /> concentrations at P7C have been above the ACL since 3Q17, but TCE has not been detected at <br /> downgradient wells P9C or P11C, indicating that the downgradient movement of the TCE plume is <br /> contained by the extraction well EWCC3R. Additional monitoring wells west of MW532B and P7C are <br /> needed to monitor TCE plume migration in the B Zone and C Zone west of the Central Area. <br /> 3.0 Field Activities <br /> This section describes the procedures, specifications, and requirements for the fieldwork. Field activities <br /> will conform to applicable regulatory requirements. Groundwater well installation activities will be <br /> completed following the Quality Assurance Project Plan (QAPP; Ahtna, 2022b), Accident Prevention Plan <br /> (APP; Ahtna, 2022c), the Site Waste Management Plan (Ahtna, 2022d), and applicable field standard <br /> operating procedures (SOP;Appendix C). <br /> Ahtna Environmental,Inc. 6 <br />