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of• 1 • 1 I February 2003 AGE-NC Project No. 98-0520 Page 2 of 21 ' The Modesto, Riverbank and Turlock Lake Formations and.overlying Recent alluvium are the principal sources of domestic ground water in the 13,500-square mile San Joaquin Valley Ground Water Basin (Basin 5-22). This,basin is drained primarily by the San Joaquin River. . 2.2. LOCAL GROUND WATER SETTING ' The nearest surface water features in the vicinity of the property are the San Joaquin River and Little 'John Creek,' located approximately 5 miles southwest and north, respectively, of the site. Ground water at the site is currently encountered at a depth of between 20 and 25 feet below surface grade ' (bsg), and-has a northwesterly flow direction. Presently, ground water occurs in an interval of ' predominantly clayey silt, info rma y referred to _as layer. 2 (Figures 3 and 4); this layer is approximately 15 feet thick, at between 20' and 35 feet below surface grade (bsg). Apparently discontinuous, 3- to 5-foot thick sand lenses occur sporadically inlayer 2. 2.3. WELL SURVEY ' A well survey was performed in October 1999 to identify municipal, domestic, industrial,irrigation or monitoring wells and septic tanks located within a 2,000-foot radius of the site (Table 1).,,The ' nearest ground water wells were two City of Manteca wells (No. 7 and No. 19) located approximately 2,300 feet northeast of the site and one commercial well located approximately 1,000 feet southeast of the site. In addition, ground water monitoring wells were observed approximately 300 feet northwest, 1,000 feet northwest and 1,500 feet southeast of the site.No other receptors, including septic tanks, were identified within the 2,000-foot radius. ' 2.4. UST REMOVAL ' AGE has been infor-med'that one 10,000-gallon regular gasoline UST, one 8,000-gal.lon unleaded gasoline UST and one 550-gallon waste oil UST were removed from the site in 1988. During UST removal activities, soil samples were collected and analyzed from beneath the ends of each UST. Total xylenes were detected at a concentration of 230 micrograms per kilogram' (ug/kg) in a soil sample collected from the base of the gasoline UST Tank No. 2 excavation at a depth of 15 feet below surface grade (bsg). Total petroleum hydrocarbons quantified as motor oil (TPH-mo) was ' detected at a concentration of 97 milligrams per kilograms (mg/kg) in a soil sample collected at a depth of 12 feet bsg from the base of the waste ail UST (Tank No. 3) excavation. However, laboratory detection limits were elevated and may not have indicated low concentrations of ' petroleum hydrocarbon compounds above'standard reporting levels. According to an EHD inspection report, soil discoloration and petroleum hydrocarbon odors were noted beneath USTs ' Tank No. 1 and Tank No.,2. Advanced GeoEnvi ro n mental,Inc.