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w(l;flf" Southwest, Inc. 3.0 SITE BACKGROUND On April 18, 2003 at 10:15 pm, the facility experienced a spill of approximately 5,500 gallons of ammonia hydroxide. CALAMCO blends one part anhydrous ammonia with three parts water to form ammonia hydroxide. The spill occurred during this process. The 3,000-ton ammonia hydroxide tank (aka aqua ammonia) over pressurized and ruptured at the top of the tank due to a pressure gauge malfunction. The tank farm secondary containment walls laterally contained the spilled product on the surface. Spilled ammonia hydroxide flowed to and accumulated at the southwest corner of the tank farm. No product entered the Port of Stockton storm drainage system or escaped from the tank farm. Clean up records indicate that nearly all of the estimated 5,500 gallons of spilled ammonia hydroxide was recovered. However, a shallow soil sample collected by the Central Valley Regional Water Quality Control Board (CVRWQCB) from the earthen floor of the tank farm near the spill location was analyzed and found to contain 1,700 mg/kg of ammonia as nitrogen (NH3-N) and to have a pH of 9.4. Refer to Figure 3 for a site map depicting the location of the ruptured tank,the flow path, and accumulation area. 4.0 SITE GEOLOGY AND HYDROGEOLOGY The site is located on the delta of the San Joaquin River, approximately 2 miles east of the confluence of the San Joaquin and Calaveras Rivers. The subsurface geology consists of a thick sequence of alluvial deposits of late Tertiary to Holocene Age, overlying sedimentary rocks of Cretaceous to Tertiary Age, which in turn overlie crystalline basement rocks (Bartow and Nilsen, 1990). Data from nearby studies within the Port of Stockton area indicate a soil profile to a depth of about 20 feet below ground surface (bgs) consisting of interbedded sands and clays, mostly under reduced conditions (Sierra-Pacific Groundwater Consultants [SPGC], 1994). Lithologic data at an adjacent Port of Stockton site indicate that, in general, the interval from roughly 13 to 23 feet bgs comprises mostly silt, clayey silt, and clay. Silty sand occurs below this interval, which grades downward into loose, dark gray to greenish-gray, poorly-graded sand. This dark gray sand has been observed in nearby soil borings from approximately 30 to 41 feet bgs,the total depth explored in those borings. 725.PRI.00 Page 2 December 4,2003