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2.0 BACKGROUND The area surrounding the Stockton CARNG facility has been a mix of airport and agricultural use for over 40 years. The AASF has been in existence since the early 1950s as an aircraft fueling station and was expanded to its present size of approximately 25 acres in 1974. The Armory, CSMS and OMS appear from aerial photographs to have been constructed in 1963. 2.1 AASF The fuel storage for the AASF formerly used three 5,000-gallon steel USTs which were installed in the early 1950s. These USTs were removed in 1990. The former USTs, shown on Figure 3. rested on a six-inch thick concrete slab. The slab still remains in place at a depth estimated to be 11 feet bgs. The fuel dispensers were originally located on the north side of the USTs. During the 1974 expansion/modernization, the dispensers were relocated 30 feet to the east. Aviation gasoline was reportedly stored in the former USTs until the early 1960s when turbine powered aircraft began replacing older piston-engine powered airplanes. From approximately 1962 until 1989, JP-4 was stored in the former USTs. The three 5,000-gallon USTs were taken out of operation in 1989 and were removed in January 1990 (Wallace-Kuhl, 1993). Five soil samples were collected from beneath the tanks at the time of removal. Analytical results reported no detectable concentrations of fuel constituents from samples 2, 3, and 5. Analytical results of soil samples 1 and 4 detected up to 3,500 milligrams per kilogram(mg/kg) total petroleum hydrocarbons (TPH) as diesel fuel (TPH-d) and 5,100 mg/kg TPH as jet fuel (TPH j). An Unauthorized Release Form (No. 90-Ul 1) was completed and submitted to the San Joaquin County Public Health Services, Environmental Health Department (PHS-EHD) on January 31, 1990. Following the UST removal, several investigations were conducted to assess the limits of the petroleum-impacted soil. The results of the investigations characterized the majority of the contamination as limited to the soils adjacent to the former excavation. Three groundwater monitoring wells were installed (as shown on Figure 3) at the AASF in 1993 to assess the condition of groundwater beneath the former USTs, and have been monitored six times to date. TPH was last detected in the wells in 1996. Detections of the aromatic hydrocarbons: benzene, toluene, ethylbenzene and total xylene isomers (BTEX); and the fuel oxygenate methyl tertiary butyl ether(MTBE)have been infrequent, with maximum concentrations of benzene (14 micrograms per liter [,ug[L]) and MTBE (1.49µg/L) detected in 1996. During the last monitoring event in March 2000, benzene and total xylene concentrations of 1.1 y- and 1.7 ,ug/L, respectively, were detected in well MW-3; MTBE and petroleum hydrocarbons were not detected. The analytical results for this and historical sampling events are presented in Table 1. Six monitoring events over a period of seven years suggest that groundwater has not been actionably impacted by the release of jet fuel from the former USTs, and that detections of trace concentrations of the constituents of concern are ephemeral. 3 3027-03/104700.4766.230/JUL23'03