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Groundwater Well Installation Work Plan Sharpe Army Depot Lathrop, California are the dominant soil types beneath Sharpe, and those deposits are interbedded with sands. The deposit with the highest apparent continuity consists of a mix of clay and silt with thin silty sands in the Lower A Zone and the Upper B Zone (approximately 30 ft bgs to 70 ft bgs). This fine-grained deposit (also known as the A/B aquitard) slows groundwater and contaminants' horizontal and vertical migration. Sands are generally thicker and more abundant in the C and D Zones. Because the transmissivities of the sands in those zones allow groundwater to move at a higher velocity, agricultural and domestic supply wells are installed in those two zones and some cases, deeper zones in the area of Sharpe. Depths to groundwater measured at Sharpe ranged from approximately 8 to 31 ft bgs during 2022 groundwater monitoring. Hydrogeologic cross-sections for the well installation areas are shown in Figures 2 through 5. 2.0 Rationale The need for the five new groundwater extraction and five groundwater monitoring wells is described in the 2021 FFA Annual Report(Ahtna, 2022a)and summarized below in Sections 2.1 and 2.2. Extraction well locations are based on the evaluation of the updated groundwater flow model and particle tracking simulation (Appendix A). Preliminary specifications for proposed wells are in Table 1, and details of existing wells in the vicinity of proposed wells are in Table 2. Preliminary specifications for proposed wells (including total depth and screen interval)were based on (1)final specifications of existing wells in the vicinity and (2) known plume depths shown in Figures 2 through 5. Final well and screen depths will be determined in the field and will depend on lithology. Section 3.2.7 describes the discrete sampling methodology that will be used to determine the final location of screens in all wells. 2.1 Extraction Wells 2.1.1 South Balloon Four extraction wells (EWC6, EWC7, EWCB, and EWC9) are proposed west of the South Balloon—two on the Sharpe boundary and two on UPPR property. These wells are needed to improve mass removal and containment of the C Zone TCE and PCE plumes. 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. Justification of Well Recommendations CC14, PCE,and TCE plumes in the A, B,and C Zones have been detected on the western border of the South Balloon since 1996. EWC4(2000), EWB4(2011), EWA12(2021),and EWC5(2021)were installed to contain these plumes, but the western side of the C Zone PCE plume and the downgradient end of the C Zone TCE plume are interpreted to be outside of the EWC5 and EWC4 capture zones(Figure 7). MW497CD provides data to evaluate the downgradient stability of the plume and the EWC4 capture zone. Multiple COC detections greater than aquifer cleanup levels (ACLs) have been detected at MW497CD, but results have Ahtna Environmental,Inc. 4