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15 June 2009 AGE-NC Project No.05-1306 Page 2 of 33 { fauna,while the upper portion(upper/lower delta plain)is braided river channel deposits with back too swamps and lucustrine delta fill and marshes or freshwater lakes.The lower portion flanks the upper with margin-basins and bay-fill(crevasse splays)marshes and swamps;further,the bay-fill deposits may form thin sand wedges stacked one top of the other and separated by fine grained interdistributary-bay and marsh deposits.The general fauna of the marsh deposits was plant material, weeds, grasses, high biotic activity and organic debris which backfilled the distributary channels, which were abandoned due to channel shifting. The organic debris and general fauna was again covered by the promigrating delta sequence and decomposed into organic hydrocarbons and organic gases by kemosynthesis or bioaccumulations. r� The site conditions depicted delta deposits and high or noticeable hydrogen sulfide odors in samples collected from deep soil borings correlate with these types of delta deposits. The naturally occurring organic-hydrocarbon gas in this region is well documented and methane is common in western Stockton ground water and the Delta ground water basin. %W Based on General Soil Map from the San Joaquin County Soil Survey,published by the United States Department of Agriculture Soil Conservation Service in 1992, the site area is within the Guard- Devries-Rioblancho (GDR) association. The GDR soils are located within basin rims along the i— eastern edge of the Sacramento-San Joaquin Delta area,and generally consist of poorly drained,fine to coarse textured soils. The GDR soils generally formed in alluvium from mixed rock sources. The Modesto,Riverbank,Turlock Lake Formations and overlying recent alluvium are the principal source 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. The nearest surface water is #` the Stockton deepwater channel, located approximately 650 feet north of the site. `= 2.2. UNDERGROUND STORAGE TANK USE AND REMOVAL In January 1988, one 10,000-gallon diesel fuel UST was removed from the site, under the supervision of the San Joaquin County Environmental Health Department(EHD).The former UST, which reportedly contained gasoline,was removed from the southern portion of the site (Figure 2). L„ Historical repairs and release records are not readily available. The tank excavation extended to a depth of approximately 14 feet below ground surface (bgs). Analytical testing indicated detectable concentrations ofpetroleum hydrocarbon constituents in samples collected beneath the formerUST. ;., Total extractable petroleum hydrocarbons(TEPH)were reported at concentrations of 160 milligrams per kilogram (mg/kg) and 24 mg/kg. Volatile organic compounds were not detected in the soil samples collected. Advanced GeoEnvironmental,Inc. k,.