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Groundwater Well Installation Work Plan Sharpe Army Depot Lathrop, California • MW551C (Table 1, Figure 12) o Blank casing from the ground surface to 130 ft bgs o Screened casing from 130 to 140 ft bgs • MW552C (Table 1, Figure 12) o Blank casing from ground surface to 125 ft bgs o Screened casing from 125 to 135 ft bgs Each well will be capped with a threaded stainless steel end cap.A five-foot sump will be installed at each extraction well in order to extend the lifespan of the wells. 3.2.1.2 Well Screen and Centralizers Well screens for extraction wells will be 10 ft of 8-inch diameter, continuous slot, wire-wrap, threaded, Type 304 stainless steel. Well screens for monitoring wells will be 10 ft of 4-inch diameter Schedule 40 PVC. Nominal screen apertures will be 0.020 inches for extraction wells and 0.010 inches for monitoring wells.The supplier will provide clean, wrapped screened casing as described above. Stainless steel centralizers will be placed at the top and bottom of the screen, and every 40 ft upward (as applicable), from the top of the screen. Centralizers will not be placed within the bentonite seal. Plumbness will be aided by the centralizers and by keeping tension on the well casing during construction. The plumb is checked by lowering a minimum 10-foot swab to the bottom of the well during the filter pack setting phase of construction. 3.2.1.3 Sounding Pipe The preliminary design for the extraction well includes a sounding pipe extending from the ground surface to 110 ft bgs for EWC6 and EWC7(Figure 9),from ground surface to 118 ft bgs for EWC8 and EWC9(Figure 10)and from the ground surface to 140 ft bgs for EWCC7(Figure 11).The sounding pipe will be constructed of a 2-inch diameter, Schedule 80 PVC. The pipe surface will be free of solvents, oils, grease, paint, and dirt and will be certified clean from the factory or steam-cleaned on the site before installation. Sections of pipe will be received from the supplier in polyethylene protective wrappers with sealed ends to limit exposure to potential contaminants.The protective wrapping and seals will be removed before installing the pipe. 3.2.1.4 Filter Pack and Transition Sand Filter pack material will be composed of clean, round, water-worn, siliceous sand, or equivalent, that is free of flat and elongated pieces, organic matter, and other foreign matter (#3 Monterey sand or equivalent). Before placement,the filter pack material will be stored in plastic-lined paper bags or plastic sheeting in a location protected from weather and contamination. Filter pack material will be placed in the annular space in the borehole using a tremie pipe and extend a minimum of 2 ft above the top of the well screen. Following filter pack installation, an approximately 1— foot thick bridge of fine transition sand (#30 mesh or equivalent) will be placed on top of the filter pack using a tremie pipe. Ahtna Environmental,Inc. 11