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r, Taj Bahadori Page 2 of 9 December 23,2014 • The diameter of the bore hole is also larger, 10-in compared to the previous 6-in. The larger bore hole and larger well casing tends to make for a more plumb well installation. The increased plumbness of the well allows for a greater ease of installation of the sampling pump. Because of the significant depth of the monitoring wells (250 to 300 ft) at the Foothill site, well plumbness is an important factor in collecting a water sample. • The previous well design showed a single 20-foot(ft)long screen. The revised design shows a double screen. The upper screen is 15-ft long and the lower screen is 10-ft long. The two screens are separated by a 15-ft long blank section of well casing. This blank section allows for the placement of the inflatable packer to provide a hydraulic seal between the upper and lower screens. This blank interval also allows for the placement of a 5-ft thick bentonite seal in the bore hole to provide a hydraulic seal between the two screens. These two seals, the packer inside of the well, and the bentonite seal around the outside of the well, prevent groundwater movement within the aquifer (Mehrtren Formation),and between the two screened intervals. • The overall length of penetration into the aquifer by the double screened design is 45-ft,as compared to the previous 25-ft penetration of the single screened design. The extra screen and greater length of penetration is to allow this well to have a greater longevity of monitoring during this period of prolonged drought, over pumping of the aquifer,and groundwater level decline. All of the other well installation procedures between these two work plans are essentially unchanged. The primary intent of the double screened design to lower the potential cost of longer term groundwater monitoring at the Foothill site. Because the wells are significantly deep, it is cost effective to plan for eventually drop in groundwater levels by incorporating a deep screen in this well installation. The County has previously installed double screened wells at their Corral Hollow landfill to a greater depth for this same reason. The next sections of this document describe the geology of Foothill site and provide evidence that a continuous water bearing zone exists beneath the site to justify the double screen well installation. Site Geology The Foothill Sanitary Landfill is underlain by a continuous section of the Tertiary Age non-marine Mehrten Formation (Tm) as shown on the Geologic Map for the landfill area. During the Miocene to late Pliocene age of the Tertiary Period, volcanic material was ejected from several volcanic centers along the crest of the Sierra Nevada Mountains. Eventually,the MW-6 Design.docx