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Groundwater Well Installation Work Plan Sharpe Army Depot Lathrop, California 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.The filter pack size (#3, Monterey sand, or equivalent)will be compatible with the 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 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 ensure that the appropriate volume and thickness of the filter pack are 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. 3.2.1.5 Bentonite Seal Each extraction well and monitoring well will be constructed with a minimum 3-foot bentonite seal placed above the transition sand. The bentonite seal will consist of hydrated bentonite pellets or chips. The bentonite seal will be 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 following the manufacturer's recommendations before grout placement(minimum of 30 minutes, but longer if coated, time-release pellets are used). 3.2.1.6 Cement Grout The cement grout will be placed to fill the annular space above the hydrated bentonite seal.The 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.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 tremie pipe by a positive displacement method, such as pumping. The tremie pipe will extend from the surface to the bottom of the zone to be grouted. The tremie pipe will be positioned in the hole with the pipe bottom not greater than 5 ft above the bentonite seal and then directed to discharge to the sides of the borehole(rather than downward).The tremie pipe will be slowly raised as the grout is placed and the drill rod retracted; however, the discharge end of the pipe will be submerged in the grout at all times. Additional grout will be added to maintain the level of the grout at the ground surface as settlement occurs. Ahtna Environmental,Inc. 12