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each location was then drilled deeper and converted into new tri-level soil gas monitoring <br /> probes SG-2A, SG-3A, SG-6A, and SG-1 IA. Existing single-level soil gas probe SG-10 <br /> was not overdrilled due to being located adjacent to an advancing borrow pit. Tri-level <br /> soil gas probe SG-l0A was installed approximately 15 feet west of existing single-level <br /> probe SG-10. Existing probe SG-10 will be abandoned once the borrow pit excavation <br /> reaches the area. <br /> 3.5 Soil Gas Monitoring Well Installation <br /> Between April 19, 2016 and May 19, 2016, Yellow Jacket drilled and installed 5 tri-level <br /> soil gas monitoring (SG) probes using a sonic drill rig. Total depth of the wells ranged <br /> from 198 to 262 feet below ground surface (bgs). A CB&I geologist provided drilling <br /> oversight and logged the sonic borehole cores (see Appendix B for the boring logs and <br /> construction details). The location of the tri-level SG probes are shown on Figure 2. All <br /> tri-level SG probes were installed in the Mehrten Formation, which consists of sandstone <br /> interbedded with conglomerate, siltstone, claystone, and mudflows, all of andesitic and <br /> basaltic origin. In the upper portion of the Mehrten Formation, the sandstones, siltstones, <br /> and claystones have weathered to sandy, silty, and clayey material. <br /> Each new SG probe was constructed with three, 2-inch diameter schedule-80 polyvinyl <br /> chloride casings in each borehole. Each casing has a 20 foot-long, 0.020-inch slotted <br /> screen. Each borehole is 10 inches in diameter until the bottom of the middle casing, and <br /> continues at 8 inches in diameter from the bottom of the middle casing until the bottom <br /> of the borehole for the deepest casing. The filter pack used in each SG probe is 3/8-inch <br /> pea gravel. Each screen is isolated with five feet of hydrated bentonite chips placed one <br /> foot above and one foot below the screen. Hydrated bentonite was placed from one foot <br /> above the shallowest screen to 1.5 feet bags. Neat cement with 5% bentonite powder was <br /> placed from 1.5 feet bgs to ground surface. <br /> Drilling and Installation—SG-2A <br /> Drilling and installation of SG-2A occurred from April 28, 2016 through May 2, 2016. <br /> SG-2A replaced SG-2. The well monument, casing, and annular fill from SG-2 were <br /> removed during the drilling of SG-2A. A 10-inch diameter borehole was drilled to 76 <br /> feet bgs and a 8-inch diameter borehole was drilled from 76 feet bgs to 262 feet bgs. <br /> Continuous core samples were collected from 34 feet bgs (just below the depth of SG-2) <br /> to 262 feet bgs. Three casings were installed. Each casing includes a 20-foot screen. <br /> The screens were placed at 50 to 70 feet bgs, 140 to 160 feet bgs, and 217 to 237 feet bgs <br /> (see Appendix B). The well monument was installed the week of May 23, 2016. <br /> The planned total depth of the deep casing was 260 feet bgs; however, groundwater was <br /> detected at approximately 252 feet bgs. The bottom of the deep casing was set at 237 feet <br /> bgs to keep the bottom of the screen 15 feet above the static groundwater level. <br /> Drilling and Installation—SG-3A <br /> Drilling and installation of SG-3A occurred from May 4, 2016 through May 9, 2016. SG- <br /> 3A replaced SG-3. The well monument, casing, and annular fill from SG-3 were removed <br /> 2 <br />