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5 ATC Associates Inc. <br /> VATC 1117 Lone Palm Avenue, Suite B <br /> Modesto, California 95351 <br /> 209-579-2221 <br /> ASSOCIATES l e c . fax: 209-579-2225 <br /> constructed with a gravel pack extending from 50 to 120 feet bgs, thus creating a potential <br /> vertical conduit from 50 feet bgs to the terminal depth of the well screen at 120 feet bgs. The <br /> following information provides a detailed description of the drilling techniques, conductor <br /> casings, and monitoring well construction for the double cased monitoring well proposed in the <br /> workplan and addendum previously mentioned. <br /> Borehole <br /> The outer borehole will be drilled using mud rotary techniques to approximately 55-feet bgs with <br /> 17 .5 -inch diameter augers. Refer to Figures 1 and 2, which depict the construction <br /> specifications of the proposed double cased sentinel monitoring well. <br /> Outer Conductor Casing <br /> The outer conductor casing will be constructed of steel with a 12.754rich outer diameter (OD) <br /> and a 12.3904nch inner diameter (ID), The casing will be set two to three feet within a silt-clay a� <br /> unit (if encountered) at a depth of 50 feet below ground surface (bgs). Centralizers will be <br /> welded to the conductor casing to stabilize the casing within the borehole. Casing lengths will <br /> be approximately 20 feet and joined using steel slip collars welded on the outside of the casing. <br /> All welding will occur in the field. The casing ends will be flush-cut without threads or flanges. <br /> Ilv The outer conductor casing will be grouted in-place using a 4-5% bentonite/cement grout <br /> tremied from the bottom of hole within the borehole annulus. The cement grout will be allowed <br /> to set for a minimum of 24 hours before drilling the inner conductor casing borehole, <br /> 1 ' <br /> Inner Conductor Casing <br /> The inner conductor casing will be completed at approximately 98 feet bgs and set two to three <br /> feet within a silt-clay unit. The inner conductor casing will be constructed of steel with an <br /> 8 .625-inch OD and an 8.329-inch ID. Drilling will be completed using a 12 .25-inch diameter bit <br /> with mud rotary technique. Centralizers will be welded to the casing for stabilization within the - 0 <br /> inner casing wall. Casing lengths will be approximately 20 feet and joined using steel slip <br /> collars welded on the inside of the casing. The casing ends will be flush-cut without threads or <br /> flanges. The inner conductor casing will be grouted in-place with a 4-5 % bentonite/cement <br /> grout tremied from the bottom of hole within the conductor casing annulus. The cement grout <br /> will be allowed to set for a minimum of 24 hours before drilling the monitoring well borehole. <br /> • ' 1 z <br /> Monitoring Well <br /> The monitoring well will be constructed of 2.0-inch ID and 2.375 OD Schedule 40 PVC well <br /> material. Drilling will be completed with a 6 . 750-inch to 7.875-inch diameter mud rotary bit to <br /> approximately 110 feet bgs. Well construction will consist of ten feet of 0,020-inch slotted <br /> screen and 100 feet of PVC riser. Sand pack will be placed within the well annulus from the <br /> terminal depth to approximately three feet above the screen with approximately three feet of <br /> hydrated bentonite chips above the sand pack. The remaining annulus will be grouted in-place <br /> with a 4-5 % bentonite/cement grout slurry to the surface. <br /> The two bids received by ATC for the installation of the double cased sentinel well are $ 14,939 <br /> and $ 18,305 . ATC recognizes the proposed scope of work is expensive, but feels that <br /> 2 ATC Associates Inc. <br /> S:\Environmental\62514.0001\Pre-approval.doc <br />