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Groundwater Well Installation Work Plan Sharpe Army Depot Lathrop, California Ahtna Environmental, Inc. 9 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 either plastic-lined paper bags or on 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 will extend a minimum of 2 ft above the top of the well screen. Following filter pack installation, an approximately 1- ft thick bridge of fine transition sand (#30 mesh or equivalent) will be placed on top of the filter pack using a tremie pipe. The filter pack will be placed as follows: 1. The annulus between the well screen and borehole will be probed with a weighted tape measure to check that the hole is open to a depth approximately equal to the bottom of the borehole. 2. The filter pack will be placed into the annulus using a tremie pipe. Selected filter pack size (#3, Monterey sand, or equivalent) will be compatible with well screen and formation material. Lift thickness will be confirmed with a weighted tape measure. During installation, the filter pack material will not be allowed to free fall more than 5 ft from the end of the tremie pipe. 3. The tremie pipe will be slowly retracted upward, allowing the filter pack material to settle out through the bottom of the tremie pipe. The tremie pipe will be retracted no more than 6 inches at a time. 4. The depth of the filter pack material will be continually measured to check that the filter material has settled. If a sand bridge develops during filter pack installation, the tremie pipe will be used to dislodge the bridged material, and then the above procedure will be repeated. 5. When the designed depth of the filter pack material has been reached, a swabbing tool will be used inside the well screen to induce pack settlement. Swabbing the well will be performed for approximately 20 to 30 minutes. This step will assure that the appropriate volume and thickness of the filter pack is installed in the well. Filter pack placement and swabbing will continue until no further settlement can be measured, and the top of the filter pack is at the design depth. Bentonite Seal Each extraction well and monitoring well will be constructed with a minimum 3-ft bentonite seal placed above the transition sand. The bentonite seal will consist of hydrated bentonite pellets or chips. The bentonite seal will be located in saturated conditions (i.e., below the water table). Bentonite pellets or chips will be placed by tremie pipe and flushed into place with potable water onto the top of the transition sand to prevent bridging of the pellets in the annular space. The pellets will be allowed to hydrate in accordance with the manufacturer’s recommendations prior to grout placement (minimum of 30 minutes, but longer if coated, time-release pellets are used). Cement Grout Cement grout will be placed to fill the annular space above the hydrated bentonite seal. Cement grout will consist of Type II Portland cement with 3 to 4 pounds of powdered non-beneficiated bentonite and seven gallons of water per 94-pound sack of cement. The cement grout will be mechanically mixed with a paddle-type mixer. No accelerating agents will be added to the grout. The grout will be placed through a