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Pit AHASE I InDRCGEOLOGIC ASS1SSUFNT 7910 Lower Sacramento Road,Stockton,Califonila Delta Project No,40-88.147 Page 8 Monitoring well borings MW-1,MW-2,and MW-3 were advanced in the area surrounding the former tank excavation. Soil samples collected at a depth of 56.5 feet below grade in each of these borings showed concentrations of TPH below the detection limit of 0.1 ppm. .: 3,3 Monfiorfnn 9i!ells Tltres 4-inch-diameter monitoring wells (MW-1,MW-2, and MW-3) were installed on-site on August 11 and 13, 1989, and their locations are shown in Figure 2. The wells were constructed with Schedule 40 fPVC riser pipe; 0.010 slot size screen, #3 Monterey sand filter pack, and were sealed with bentonite i71pellets and neat cement grout. Construction details are presented in Appendix A. Each monitoring well was screened from 48 to 68 feet below grade and was developed after installation to remove fine-grained sediment. On August 14, 1989,monitoring wells MW-1through MWr3 were surveyed for horizontal and f vertical control. A temporary benchmark was used as the reference datum. This benchmark was Iocatrd at the south sauttwest corner of the sidewalk along the southeast side of the station building. An elevation of 100 feet was assumed for the temporary benchmark i E 3.4 Ground_ Water Depths to.ground water were measured and recorded in the monitoring wells on August 15, and i September 26, 1989 (Table 5). Depth to ground water ranged from approximately 57.5 to 59.44 feet. Calculations of We direction of"ground water.flow from water levels collected on August 13 and 77 September 26,1989,indicate thatfl*u towards tlts'northrast at a gradient of 0.001 and 0.002 feet/feet, respectively... A ground water contour map was generated from the water levels collected on — August 15,,1989,and is presented in Figure 6. TABLE 5 Ground Water Elevations Reference Elevation Depth to Ground Water Monitoring Well from Top of Casing Ground Water Elevation i LarateonDate feet -- _ fCell ffeet) t MW-1 81151P39 9(16/39 98.11 57.50 40.61 _ 98.11 58.59 39.52 MW-2 8115/89 � ---' - ---- - -- - ---- -.._.__99.09 .------------- — - - - 9/26189 : 99.095--------- — — 59.67 39.42 8115/89 98.93 .3 58.35 40.58 u s 1 9/26189 98.93 59.44 39.49 Note: An assumed elevation of 100 feet was used as the benchmark elevation.