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�r EWE A Cft May 31, 2024 Central Valley Regional Water Quality Control Board Mr. Robert Fagerness, P.E. 11020 Sun Center Drive, #200 Rancho Cordova, California 95670 Subject: Workplan for Subsurface Investigation Diamond Foods, Inc.,1050 South Diamond Street Stockton, California 95205 Dear Mr. Fagerness: Atlas Technical (Atlas) has prepared this workplan on behalf of Diamond Foods, Inc, describing proposed subsurface investigation activities to be conducted at the site located the Diamond Foods Inc.facility located at 1050 South Diamond Street in Stockton, California (Figure 1). The purpose of the subsurface investigation is to evaluate the extent of dissolved chromium VI at the site. This work plan was prepared pursuant to a request from the Central Valley Regional Water Quality Control Board (CVRWQCB) in a letter dated April 29, 2024. The scope of work is described in detail in this letter report. BACKGROUND The Diamond Foods facility has been primarily operating as a walnut processing plant since the 1950s. Five underground concrete vaults were located north and east of the main processing building and formerly contained refrigerants used in the plant cooling systems. Several of these vaults are believed to be the source of the hexavalent chromium impacts at the site. 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. 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 1117 Lone Palm Avenue, Suite 201 B, Modesto, CA 95351 209.579.2221 1 oneatlas.com