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WEISS ASSOCGITES 2 BACKGROUND The DWG plant is located northeast of the intersection of Charter Way and South Walnut Street in Stockton, California (Figure 1). The facility is a walnut processing plant, cooperatively owned by walnut growers in the Central Valley. DWG indicates that the plant has been in operation since the 1950s. The five underground concrete vaults arc located north and east of the main processing building and contain chemicals used in the plant refrigeration systems. The site setting and vault history are discussed below. 2.1 SITE SETTING The site is located about 2.5 miles east of the San Joaquin River. The direction of local surface drainage is controlled by the site topography, which indicates that surface water initially flows south to Mormon Slough and eventually flows west to the San Joaquin River. Four municipal wells located at the corners of the site (Figure 2) supply water to the plant and to the surrounding community. Dwayne Cox, of the City of Stockton Department of Municipal Utilities, indicates that the wells were installed in 1955 and 1956 to depths of 420 to 460 feet. They have annular seals to at least 50 feet below ground surface. The well at the southeast corner of the site is currently being abandoned. The remaining three wells arc pumped as needed, or at least once per month for 15 to 30 minutes at flow rates of 900 to 1,000 gallons per minute (gpm). The static water levels in these wells indicate that the depth to ground water beneath the site is probably about 50 to 60 ft. Since, under non-pumping conditions, the direction of ground water flow in the Stockton area generally coincides with the surface topography, the ground water flow direction beneath the site is probably to the west. W P 1F:\ALL\331D W G\331R1AP8.DOC:74-331-00:V 1.036 Page 3