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