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Workplan for Vertical Groundwater Investigation And Bench Test for In-Situ Oxidation 7835 Thornton Road, Stockton, CA Page 4 of 8 WORKPLAN FOR INVESTIGATION OF VERTICAL EXTENT OF GROUNDWATER CONTAMINATION Drilling,Well Construction and Soil Sampling One monitoring well will be installed to a depth of approximately 100 feet bgs to investigate whether dissolved gasoline constituents exist at this depth. The proposed location is presented on Figure 9. The location of the proposed deeper zone well was selected to obtain groundwater samples downgradient from the former USTs and dispensers. The proposed depth of the deeper zone monitoring well corresponds to the first sandy zone (-90-95 feet bgs) below the screened interval of well MW5 (40-70 feet bgs). In order to minimize the potential of cross-aquifer contamination from shallower water-bearing zones, the deeper zone well will be drilled using mud rotary equipment with overbalanced drilling mud. Construction will involve the setting of a conductor casing and the use of a grout shoe. Prior to drilling, permits will be obtained from EHD. The well will be installed by a C57 licensed drilling contractor using a mud rotary drill rig under the direct supervision of an experienced field geologist and a California Registered Geologist. A 10-inch diameter boring will be drilled using mud rotary equipment to a depth of approximately 80 feet bgs at which point a clay bed should be present. Soil samples will be collected between the depths of approximately 40 and 60 feet bgs for use in the in-situ oxidation bench test. Samples will be collected ahead of the drilling bit using a 94mm core sampler. Since the deep well will be installed in the vicinity of previously drilled deep boring DB 1, no soil samples will be collected for logging purposes. Drilling mud will be kept in a slightly overbalanced condition in order to minimize fluid entry into the well bore. 6.25-inch diameter welded steel conductor casing led by a grouting shoe will be set in the clay bed for grouting to seal the deep well through and below the primary water-bearing sand screened by well MW5. Three equally spaced centralizers will be installed on the casing to ensure adequate grouting. The conductor casing will be grouted through the shoe with Class H neat cement, which will be circulated through the annulus to the surface and allowed to cure for a period of 72 hours. The conductor casing will then be bailed free of water, the grout shoe will be drilled out and a 6- inch diameter boring will be advanced to a depth of approximately 100 feet bgs using mud rotary equipment. Drilling mud will be kept in an overbalanced condition to minimize fluid entry and to control the expected heaving sands. To complete the well, 2-inch diameter PVC casing and 0.020 slot pre-packed screen equipped with centralizers will be set from the bottom of the boring to grade. The screened interval will be 90 to 100 feet bgs. The remaining annulus will be completed with 2 feet of coated bentonite overlain by neat cement grout to the surface emplaced with a tremie system. At the surface the well will be fitted with a locking cap and water tight, traffic rated well box. A well construction diagram is shown on Figure 10. G:\GROUNDZEIKNOWLES.HAMIREPORTS\WKPLAN—deepwell—bench2.doc