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i 12 October 2000 AGE-NC Project No 99-0637 rPage 2 of 7 I2 2 UNDERGROUND STORAGE TANK REMOVAL One 10,000-gallon diesel UST (TK1), one 10,000-gallon leaded gasoline UST (TK2) and one 10,000-gallon unleaded gasoline UST(TK3)were removed from the site on 05 August 1999 The USTs were removed by California Hazardous Services of Santa Ana, California, under the supervision of the San Joaquin County Public Health Services - Environmental Health Division (PHS-EHD) Excavated soil was stockpiled on-site, adjacent to the excavation Two soil samples were collected from beneath each UST and eight samples were collected from the soil stockpile Methyl-tertiary butyl ether (MTBE) was detected in three of the six soil samples collected beneath the USTs at concentrations as high as 0 32 milligrams per kilogram (mg/kg) I Petroleum hydrocarbons and other fuel oxygenates were not detected in samples collected beneath the USTs Total petroleum hydrocarbons quantified as gasoline (TPH-g) and diesel (TPH-d) were detected in stockpile samples at concentrations as high as 4 8 and 40 mg/kg, respectively ASTs and I a concrete pad have subsequently been installed over the former UST pit location Further assessment was subsequently required by the PHS-EHD The former location of USTs, sample locations and current location of ASTs are illustrated on Figure 2 Analytical results for soil samples are summarized on Table 1 23 REGIONAL GEOLOGIC SETTING The site is located within the northern San Joaquin Valley which comprises part of the Great Valley geomorphic province of California The San Joaquin Valley is formed by the Great Valley geosyncline, which is a large, elongate, northwest-trending asymmetrical structural trough (basin) It is bordered by the Coast Ranges to the west, the Klamath Mountains and Cascade Range to the Inorth, and the Sierra Nevada to the east. Thus trough has been filled with sediments derived from both marine and continental sources Thickness of the sedimentary fill ranges from thin veneers along the valley edges to more than 20,000 feet in the south central portion of the valley The sedimentary formations range in age with the older deposits being primarily marine in origin and the younger deposits being primarily continental Contmental-derived sediments were primarily deposited in lacustrine, fluvial,and alluvial environments with sediment sources being the mountain ranges surrounding the valley(Ohmsted and Davis, 1961) The site is located on unconsolidated and semi-consolidated alluvium, lake, playa and terrace deposits of Quaternary age (California Division of Mines and Geology, 1977) 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 (Basun 5-22) Advanced GeoEnvironmental,Inc I