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Groundwater Well Installation Work Plan Sharpe Army Depot Lathrop, California Ahtna Environmental, Inc. 4 2.0 Rationale The need for the proposed new groundwater extraction and monitoring wells is described in the SHAD- 007 & -008 Waste Characterization Report (Ahtna, 2019a) and the 2018 FFA Annual Progress Report (Ahtna, 2019b): • Although below the ACL, increasing TCE concentrations at MW505C are beyond the interpreted capture zone of existing extraction wells • Data gaps have been identified in the monitoring network for the South Balloon–Central Area plumes • Replacement of EWA1 is required to improve plume capture and limit plume migration in the South Balloon A plume capture zone analysis was performed to optimize the placement of EWNC7. The groundwater model update from the 2019 FFA Annual Progress Report was used for the analysis. The groundwater model simulated well locations based on flow conditions through September 2019 (Attachment 1; Ahtna, 2019c). Two well location scenarios were evaluated: • Scenario 1 — A new extraction well located in the offsite open area west of the railroad t racks adjacent to existing well EWNB6 • Scenario 2 — A new extraction well located on the same offsite property as Scenario 1, but located 300 ft north into the parking area south of Roth Road The Scenario 2 location was selected for EWNC7 based on the potential for optimal capture of TCE. Figure 5 shows the hydrogeologic cross-section with EWNC7 in relation to nearby wells and groundwater plumes. Figure 8 shows the optimized location of EWNC7, as well as the 3Q19 C Zone contaminant plumes in the area. As this well will be located on private property, final well placement will depend on landowner preference. Five new groundwater monitoring wells, three screened in the B Zone, and two in the C Zone will help define plume boundaries and monitor plume migration in the South Balloon–Central Area (Figures 9 and 10). Figures 3 and 4 shows the hydrogeologic cross-section with the five new groundwater monitoring wells in relation nearby wells and groundwater plumes. EWA1 is located at the head of the South Balloon TCE and PCE plumes, but was shut down in 2005 due to poor performance. TCE and PCE concentrations at EWA1 have fluctuated around their respective ACLs since 1Q96 and were slightly above their ACLs in 3Q19 (Figure 11; Ahtna, 2019c). Replacement of EWA1 is required to improve plume capture and limit plume migration in the South Balloon. Figure 3 shows the hydrogeologic cross-section with EWA1R in relation nearby wells and groundwater plumes. Figure 12 shows the location of replacement well EWA1R. Table 1 provides the preliminary specifications for the proposed wells. Table 2 provides details for existing wells in the vicinity of the proposed extraction and monitoring wells.