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0971c*-4 B. If the Phase I results indicate that contamination has moved off site, then Phase II of the sampling plan will be modified to address that fact. The modification to the Phase II sampling plan would include advancement of a minimum of 4 borings inside the structure and the placement of at least four additional monitoring wells downgradient from the sources and further away than the Phase I monitoring wells. The approximate locations of these borings are shown in Figure 4. All additional off site wells will be secured with locking Christy boxes. The location of these wells will be determined by the analytical results from Phase I of the program. 3.0 MONITORING WFLL INSTALLATION In order to determine local groundwater gradients, tidal influences, and to collected water samples, a series of monitoring wells will be installed. At least one well will be placed in an upgradient location and the other monitoring well will be placed inside the building and in downgradient location outside. All wells located outside the building will be fitted with locking, metal Christy Boxes to ensure well integrity. All monitoring wells will be labeled and have their top of casing surveys for elevation. Surveyed monitoring wells will be tied to a local benchmark. We expect the groundwater gradient to be away from the levee toward the freeway. The materials used for the monitoring wells include: 2-inch I.D. , schedule 40 PVC piping (slotted and blank) , employing screw joints and friction-fit caps. The case slotting is 0.028 inches. The sand pack backfill will be washed, kiln dried, and sieved Lonestar No. 3 sand. The sand pack will be placed such that it will extend 1-foot above the top of the slotted casing interval. Above the sand pack will be an expansive bentonite seal which will maintain a minimum 1-foot thickness. The bentonite seal will be installed as a slurry. Above the bentonite seal, a concrete cap with a minimum thickness of two feet will be placed. A typical monitoring well using the above items and a locking Christy Box is shown in Figure 5.