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� • 1 <br />r <br />The fourth proposed well will be located at the northeast (hydraulically <br />downgradient) corner of the site property, in a cluster with the water table <br />well proposed for that area. This well will be constructed with a 10 -foot <br />screened interval located near the bottom of the uppermost aquifer <br />(estimated at approximately 50 feet below ground surface). <br />be 2 <br />All of the proposed wells will constructed of -inch inside diameter, <br />flush -threaded schedule 40 polyvinyl chloride (PVC) casing. Due to the fine <br />' <br />grain size of the aquifer matrix encountered during previous site <br />investigations, 0.01 -inch slotted PVC will be used for the screened portion <br />' <br />of the well. Since the grain size analyses of soil samples collected from <br />within the aquifer matrix during previous site investigations showed that <br />' <br />greater than 90 percent of the material passes the No. 200 sieve, a design <br />filter pack for the aquifer is not feasible. Therefore, Lone Star #2/12 <br />sand has been chosen for the filter pack, because of its relatively fine <br />grain size (for a filter pack) and high uniformity coefficient (typically <br />greater than 2.0). Figure 3 shows the construction specifications for an <br />observation well typical for this investigation. <br />The borehole for each monitoring well will be advanced with 8 -inch <br />outside diameter hollow -stem augers. When the augers have been advanced to <br />the desired depth of the well, the PVC casing will be lowered down the <br />center of the augers and set to the desired depth. The annular space <br />' between the augers and the screened portion of the well casing will then be <br />backfilled with filter pack material. As the filter pack is placed, the <br />augers will be withdrawn from the hole so the sand flows out of the augers <br />and fills the annular space of the borehole. This process will be repeated <br />until the filter pack is placed to approximately 1 foot above the top of <br />the screen. A 1 -foot seal of bentonite pellets will then be placed <br />directly in the annular space above the filter pack. Approximately 3 <br />gallons of potable water will be added to allow swelling of the bentonite. <br />After sufficient time has passed for the bentonite to swell (at least 1 <br />hour), the remaining annular space will be backfilled to the ground surface <br />with a cement -bentonite grout. Representatives of the San Joaquin County <br />1 3542a/BAV102 13 <br />