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Groundwater Well Installation Work Plan Sharpe Army Depot Lathrop, California Ahtna Environmental, Inc. 3 gradients have been caused by pumping of on-depot and off-depot water supply wells. The B, C, and D Zones have hydraulic characteristics of leaky confined aquifers. Subsurface soils vary from clay and silt to coarse sand. Within each of the zones, deposits containing each of the soil types occur; however, the deposits vary in thickness vertically and horizontally across Sharpe. Silt and clay 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 the horizontal and vertical migration of groundwater and contaminants. 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 in some cases, deeper zones in the area of Sharpe. Depths to groundwater measured at Sharpe ranged from approximately 6 to 21 feet bgs during 2019 groundwater monitoring. Hydrogeologic cross-sections for the well installation areas are shown in Figures 2 through 5. 1.3 Previous Investigations Well cluster MW505 was installed in 1985 to monitor carbon tetrachloride, tetrachloroethene (PCE), and trichloroethene (TCE) plumes in the A, B, C, and D Zones and the C/D transition northwest of the North Balloon. VOC concentrations have consistently been below their respective aquifer cleanup levels in samples collected from the MW505 well cluster since the first sampling event (first quarter 1996 [1Q96]). However, at MW505C, TCE concentrations have been increasing slightly since 2005 (Figure 6). In 2018 and 2019, groundwater samples were collected in the MW505 area to delineate the extent of TCE contamination in the A, B, and C Zones and the A/B and B/C transition zones (Ahtna, 2019 a). TCE was detected at varying depths (Figure 7), ranging from 0.15 µg/L in the A/B Zone (65 ft bgs) at HP1708 to 1.8 µg/L in the C Zone location HP1705 (125 ft bgs), but did not exceed the Aquifer Cleanup Level (ACL; 5 µg/L). Two other VOCs were detected above their respective ACLs: chloroform in the A/B Zone at HP1701 and methylene chloride in the B/C Zone at HP1705. The presence of chloroform in groundwater at SHAD- 007 & -008 is likely due to non-depot-related irrigation activities. An expansion of the existing OU 1 remedy to include an additional C Zone groundwater extraction well in the MW505 area was recommended to capture increasing TCE concentrations and prevent possible plume migration (Ahtna, 2019a). Installation of five monitoring wells and the replacement of EWA1 were recommended in the 2018 FFA Annual Progress Report to fill data gaps identified in the monitoring network for the South Balloon–Central Area plumes and improve plume capture and limit plume migration (Ahtna, 2019b). EWA1 was shut down in 2005 due to poor performance and a replacement well is required.