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Groundwater Well Installation Work Plan Sharpe Army Depot Lathrop, California fluctuated above and below the ACL since 2019 and a significant decreasing trend for TCE has been observed. The MW543 well cluster was installed offsite to evaluate South Balloon COC plume migration beyond the EWC4 capture zone. MW543C TCE concentrations were below the ACL between 3Q18 and 1Q21 but increased to 5.2 micrograms per liter (µg/L) in 3Q21, indicating plume migration. The next downgradient well cluster (MW530) is more than 1,600 ft from EWC4 (outside the interpreted capture zone; Figure 7). TCE was detected at MW530C at 1.2 ltg/L in 3Q21, which is the highest recorded TCE concentration at this well. West of the plume, PCE has not been detected above the ACL at MW538C and MW538CD since 2012. Though PCE concentrations at most of the wells within the plume show statistically-significant decreasing trends, the plume area has remained mostly stable since the 2007/2008 investigation (URS, 2009) and is projected to persist in the area beyond 2121 (Ahtna, 2022a). The four proposed extraction wells are needed to improve mass removal and containment of the C Zone TCE and PCE plumes. 2.1.2 Central Area One extraction well (EWCC7) is proposed on UPRR property, on the west side of the railroad tracks. This well is needed to limit migration of the TCE C Zone plume. The proposed well location and Central Area plumes are shown in Figure 6. Preliminary specifications for the proposed wells are in Table 1. Details of existing wells in the vicinity of proposed wells are in Table 2. Hydrogeologic cross-sections with the new groundwater wells in relation to nearby wells and groundwater plumes are in Figures 3 through 5. Justification of Well Recommendations Although the total lateral extent of the composite South Balloon-Central Area TCE plume footprint has decreased since 2009 (Ahtna, 2022a) and operation of EWCC6 (installed in 2019 adjacent to MW525C) is improving containment at the leading edge of the plume, the eastern side of the plume is projected to migrate northward over the next thirty years due to a gap in the extraction well network along the central- eastern boundary in the MW507C and MW511C area (Ahtna, 2022a). The proposed extraction well is needed to limit the migration of the TCE C Zone plume. 2.2 Monitoring Wells 2.2.1 South Balloon Two monitoring wells (MW549C and MW550C) are proposed on the southwestern edge of the Sharpe facility, screened in the C Zone. These wells are needed to fill a data gap in monitoring wells on the south side of the facility,west of MW440.These wells will aid in defining the southern plume boundary. Proposed well locations and South Balloon plumes are shown in Figure 6. Preliminary specifications for the proposed wells are in Table 1. Details of existing wells in the vicinity of proposed wells are in Table 2. Hydrogeologic cross-sections with the new groundwater wells in relation to nearby wells and groundwater plumes are in Figures 3 through 5. Ahtna Environmental,Inc. 5