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Pilot Study Workplan May 25, 2018 Diamond Foods Inc. Page 2 Vaults 1, 5, and 6, installed between 1974 and 1976, were constructed of unlined concrete and historically contained propylene glycol. Vaults 1 and 5 had 1,200-gallon capacities and Vault 6 had a 500-gallon capacity. These vaults were located along the eastern side of the property adjacent to large walnut storage bins. Vaults 7 and 8 were installed in the 1950s and were located against the north wall of the processing building, and both were constructed of poured concrete with fiberglass and asphalt lining. Vault 7 had a capacity of 3,000 gallons and was used to store a sodium dichromate chill water solution. Vault 8 had a 5,000-gallon capacity and was used to store a chill water solution containing both sodium dichromate and calcium chloride. In addition, copper sulfate solution may have been stored in these two tanks at some point in their history (Weiss, 1988). All five vaults were removed from service and cleaned in June 1990 (Weiss, 1991). In May 1993, all five vaults were abandoned in place by filling them with a controlled density fill material. 2.3 PREVIOUS INVESTIGATIONS Several phases of subsurface soil and groundwater investigation have been conducted at the site since December 1988. Initially, several investigations were conducted to evaluate the integrity of the vaults (Weiss, 1989). Subsurface investigations commenced in January 1991 and included the installation of three groundwater monitoring wells (MW-1, MW-2, and MW-3) in the vicinity of the former vaults to assess the condition of first-encountered groundwater. Specifically, groundwater monitoring well MW-1 was installed adjacent to Vault 1, while groundwater monitoring wells MW-2 and MW-3 were installed adjacent to Vaults 6 and 8, respectively. At the time, groundwater was encountered at a depth of approximately 71 feet below ground surface (bgs) in each boring. The three wells were constructed with screened intervals extending from approximately 56 to 89 feet below ground surface (bgs). In January 1991, Weiss personnel collected groundwater samples from each of the three groundwater monitoring wells. Groundwater samples collected from monitoring wells MW-1 and MW-2 were analyzed for ethylene and propylene glycol. The groundwater sample collected from MW-3 (i.e., adjacent to Vault 8), was analyzed for total soluble chromium, sodium and calcium, hexavalent chromium, and chlorides. All constituents were detected, but were below the California Maximum Contaminant Level (MCL) or below the EPA secondary MCL (aesthetic) established at that time with the exception of total chromium (0.10 milligrams per liter [mg/L1) and hexavalent chromium (0.11 mg/L). It should be noted that neither an established MCL nor Action Level for hexavalent chromium had been established at that time. To validate the January 1991 chromium results in MW-3, Weiss collected a round of verification groundwater samples in August 1991. Water samples were collected from monitoring well MW-3 and from two of four municipal water supply wells located at the corners of the facility (WP-1 and WP-2). All three samples were analyzed for total and hexavalent chromium, and the results indicated both municipal wells were below method detection limits for total chromium and hexavalent chromium. However, total and hexavalent chromium were detected in the water samples collected from well MW- 3, both at a concentration of 0.7 mg/L. In October 1993, RESNA Industries Inc. (RESNA) was retained to evaluate the extent of total and hexavalent chromium in soil in the vicinity of former Vault 8. Two soil borings, SB1 and SB2, were advanced north of the former Vault 8. The laboratory analytical results of soil samples obtained during this investigation indicated that soil impacts were limited to the area occupied by former Vault 8 and encompassed an area of approximately 450 square feet. Based on the results of this investigation, ft was concluded that total chromium and hexavalent chromium in soil impacts extends from directly beneath the former vault to the water table (RESNA, 1994).