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The two known release points are downstream from the pad. Since . addition of a roof drain line in 1986 the secondary clarifier periodically becomes overloaded during periods of heavy rainfall. Contaminants would be washed out of the clarifier onto the adjacent ground surface. Further downstream, the washpad wastewater discharged to an unlined, open pit. Prior to exiting the pit through a discharge pipe, contaminants could percolate into the soil. 2.2 Site Characteristics The site is located in the San Joaquin valley just south of the city limits of Stockton in San Joaquin County. Topography at the site is flat. Surrounding land use is predominantly agricultural with some scattered industrial and residential. Figure 1 , Area Map, indicates the site location and surrounding landmarks . The site has been graded and modified to promote surface drainage. Catch basins collect rainwater for flow through subterranean pipes into the adjacent airport storm water system. Eventually, surface drainage is to Little John Creek, a water course running east to west approximately 2 miles north of the site. Little John Creek flows into Camp Slough and eventually to the San Joaquin River. in 1986 a new hanger roof drain was plumbed into the aircraft washpad drainage system. The location of the junction, just upstream from secondary clarifier, can result in overloading during heavy flow periods . Soil at the site is predominantly fine grained clayey smolt. Observations made in an open pit 100 yards east of the washpad revealed uniform soil conditions down to 15 feet. Based on ponded water in the pit and on the ground surface, soil permeability appears to be low. Hydrographxcally the site is in the San Joaquin Hydrologic Basin, San Joaquin River Hydrologic Unit. Depth to groundwater is not known immediately under the site however recent groundwater depth measurements have been made 4 miles north at a Caltrans maintenance facility (adjacent to the county fairgrounds) . Groundwater depth under the Caltrans site is 60 feet. Groundwater depth under another State facility (Northern California Youth Center) , 3 miles east of the Stockton AASF, was reported at 75 feet in 1987 . 2.3 Washpad Characteristics The Stockton AASF aircraft washpad is an essential component to facility operation. it is used to clean aircraft, engines and related support equipment. Figure 2 illustrates the AASF washpad and drainage system. 4