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L � Pilot Study Workplan November 7,2014 Project#54.62583.0001 Page 3 grade vault below a concrete floor. The vault was rectangular and has dimensions of approximately 5-feet by 5-feet by 6-feet deep. The well head was mounted flush to the bottom of the vault. The construction details of the FW, as observed on the video log, indicate that it appeared to be screened in more than one water-bearing zone and was a potential conduit for migration of groundwater between aquifers. The total depth of the facility well borehole was observed to be 133 feet below top of casing. The bottom 25 feet of the borehole was open. Based on observations from the bottom 25 feet of the boring, the borehole diameter was estimated to be 18 to 24 inches. The well casing was constructed of steel pipe with a diameter of approximately 1 foot and extended to a depth of 108 feet from the top of casing. The well was vertically perforated beginning at a depth of 67 feet below top of casing and extended to the bottom of the casing. Diamond Foods indicated that there were no future plans to utilize the well, therefore, ATC recommended that the FW be properly destroyed. On October 13, 2004, a geologist from ATC supervised the advancement of two soil borings, SB10 and SB11, to collect soil samples for use in a bench scale test. Soil boring SB10 was advanced at an approximate 35 degree angle from vertical in a southerly direction to obtain soil samples from three depth intervals beneath the cooling unit, soil that was potentially impacted by hexavalent chromium. Assuming drilling angles and using trigonometric functions, the sampling intervals with respect to vertical were calculated to be approximately 13.6 to 17.3 feet bgs, 23.3 to 27.1 feet bgs, and 33.4 to 37.3 feet bgs, respectively. Soil boring SB11 was advanced vertically in the vicinity of MW-7 to an approximate depth of 39.5 feet bgs, to collect soil samples from beyond the known extent of hexavalent chromium impacted soil. SB11 is located hydraulically crossgradient with respect to Vault 8 and the cooling unit. Soil samples were collected from SB11 at vertical depth intervals of 15 to 19.5 feet bgs, 25 to 29.5 feet bgs, and 35 to 39.5 feet bgs. The soil samples collected from SB11 were visually inspected to ensure the lithology was similar to that of the samples collected from SB10. The purpose of soil sample collection from SB11 was to provide non-impacted control samples or samples representative of background concentrations for baseline characterization and bench scale testing. The concentrations of hexavalent chromium detected in the soil samples collected from beneath the chiller unit were low and were comparable to the concentrations detected in the soil samples collected from beyond the known extent of Hexavalent chromium impacted soil. It was concluded that it was possible that previously impacted soil beneath the cooling unit has been flushed over time by cooling water leaching into the subsurface, resulting in low concentrations of hexavalent chromium in the soil. Due to the production of elevated manganese concentrations, bench scale testing of zero valance nano-scale iron was recommended. This material was shown to reduce hexavalent chromium to trivalent chromium and may have less propensity to produce high manganese concentrations as a secondary affect to groundwater (ATC, 2005a). On August 1, 2005, the facility well was permanently abandoned. FW was destroyed by using a Mills Knife to perforate the entire length of the well casing. The perforating tool was removed from the well and a cement and sand mixture (10 sack) was pumped into the well under pressure using tremmie pipe that was placed at the bottom of the well and raised as the cement and sand mixture rose to the top of the casing. The vault was filled with the cement and sand mixture to within approximately one foot of the top of the vault (at grade). Approximately 13.5 cubic yards of the cement and sand mixture were pumped into the well and vault (ATC, 2005b). Groundwater monitoring and sampling has been conducted quarterly following the installation of each monitoring well in accordance with MRP R5-2004-0840. In August 2011, ATC requested a reduction in frequency and analyses. The CVRWQCB issued a revised MRP (No. R5-815-2011) where groundwater monitoring and sampling was reduced to a semi-annual basis and analyses for a number of constituents was eliminated. This sampling schedule has been followed until the present. The locations of monitoring wells and soil borings are shown on Figure 2. Historical groundwater monitoring results are summarized in Tables 1 and 2.