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WEISS ASSOCIATES 12 BACKGROUND i The DWG walnut processing plant is located northeast of the intersection of Charter Way and South Diamond Street in Stockton, California (Figure 1) The plant has been in operation since the 1950s The plant historically operated five underground vaults to store Plant refrigeration fluids Vaults 1, 5 and 6 are concrete, were installed between 1974 and 1976,and historically contained propylene glycol However,according to DWG,ethylene glycol was also stored in each of these vaults on at least one occasion Vaults 1 and 5 have 1,200 gallon capacities and Vault 6 has a 500 gallon capacity These vaults are located along the eastern side of the property adjacent to large walnut storage bins (Figure 2) Vaults 7 and 8 ' are constructed of poured concrete, are lined with fiberglass and asphalt and contained a calcium chloride and sodium dichromate chill water solution Vaults 7 and 8 have 3,000 and 5,000 gallon capacities, respectively, and are located against the north wall of the processing building Additional vault construction details are presented in the WA July 20, 1988 Phase I investigation work plan All of the vaults were taken out of service and cleaned in June 1990 ' (Falcon Energy, 1990) The plant refrigeration fluids are now stored in aboveground tanks 13 SITE SETTING The site is located approximately 2 5 miles east of the San Joaquin River The he direction of surface drainage at the site is controlled by the site topography, which slopes to the south toward Mormon Slough The Mormon Slough flows to the west and eventually drains into the San Joaquin River Four municipal wells (WP-1 through WP-4 on Figure 2) located at the corners of the site, supply water to the plant and community when demand on other water sources is high Of the four wells, only WP-1 and WP-3 are currently used For the year July 1989 to June 1990, the total water production from these wells was less than five percent of their potential production (Cox, 1991) Well logs from the State of California Department of Water Resources indicate that the four municipal wells were installed in 1955 and 1956 to depths between 416 ft and 491 ft below ground surface (bgs) and are sealed to at least 50 ft bgsJThe well logs indicate that the wells penetrate clay between 95 ft and 135 ft depth Wells WP-1, WP-2, WP-3 and WP-4 have intermittent perforations between depths 260-370+ ft, 143-410 ft, 173-426 ft and 254-487 ft bgs, respectively D %ALL1331DWG1331R1MA1 WP Page 2