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Ms. Rebecca E. Setliff Page 2 of 4 of two wells installed within a single borehole and screened in zones corresponding to the highest soil and groundwater contaminant levels identified in the recent deep soil borings. Ground Zero has no objection to constructing a well cluster in lieu of a nested well. We also agree to install a third well screened from 105 to 115 feet. The new nomenclature for the wells is MW15A (to be screened from 35-55 ft bgs), MW15B (to be screened from 70-80 ft bgs) and MW15C (to be screened from 105-115 ft bgs). Except for the modification of the screened intervals and construction in separate boreholes, wells MW 15A and MW 15B will be sampled and constructed as described in the Workplan. Since well MW15C will be screened in a potentially clean, or minimally contaminated, water- bearing zone, special precautions will be taken to ensure that the annular seal will be adequate to prevent cross-aquifer contamination. The proposed drilling and construction methodology is intended to minimize the possibility of inter-aquifer transfer of groundwater during drilling and after completion of the well. The boring will be drilled with mud rotary equipment and conductor casing will be set across the potentially contaminated upper aquifer zones. The well will be completed with a screened interval from approximately 105-115 feet below ground surface (bgs). A 12 1/4-inch diameter boring will be drilled with mud rotary equipment to a depth of approximately 90-95 feet bgs at which point a clay bed should be present. The boring will be continuously sampled from 90-95+ feet with a split spoon sampler in order to confirm the presence and depth of the clay layer that was previously logged in CPT borings CPT1 and CPT5. Drilling mud will be kept in a slightly overbalanced condition in order to minimize fluid entry into the wellbore. 6 5/8-inch diameter welded steel conductor casing led by a grouting shoe will be set in the clay bed for grouting. Three equally spaced centralizers will be installed on the casing to ensure a uniform thickness of annular seal and to prevent channeling. The conductor casing will be grouted through the shoe with Class II neat cement, which will be circulated through the annulus to the surface and allowed to cure for a period of approximately 72 hours. After the cement has cured the grout shoe will be drilled out and a 6-inch diameter boring will be advanced to a depth of approximately 115-120 feet bgs using mud rotary equipment. Drilling mud will be changed after drilling out the shoe and will be kept in an overbalanced condition to minimize fluid entry and to control possible heaving sands. To complete the well, 2-inch diameter PVC casing equipped with centralizers and 0.020 slot pre- packed screen will be set from the bottom of the boring to grade. The screened interval will be from approximately 105 to 115 feet bgs. The remaining annulus will be completed with 2 feet of bentonite overlain by neat cement grout to the surface emplaced with a tremmie system. At the surface the well will be fitted with a locking cap and water tight, traffic rated well box. A well construction diagram for MW15C is shown on Figure 3. Drill cuttings and drilling mud will be stored in lined rolloff bins pending disposal. Water generated from equipment decontamination, well development and sampling will be contained in labeled DOT approved drums. All waste will be stored on site pending laboratory results and the development of disposal options. Roekklosure\..,kpI.\zdd.d..d.c