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. %✓ w Site Background Information: Yamada Brothers AGE-NC Project No. 98-0452 Page 2 of 3 SITE ASSESSMENT ACTIVITIES In October 1998, five soil probe borings were advanced at the site for collection of soil and grab ground water samples. TPH-d was detected in one soil sample at a concentration of 10 milligrams per kilogram(mg/kg)and in five grab ground water samples at concentrations as high as 2,700µg/1. Methyl tertiary-butyl ether (MTBE)was detected in four of the five grab ground water samples at concentrations as high as 25 µg/1. No other analytes of concern were detected in the soil and grab ground water samples collected during the October 1998 investigation. In November 1999,three ground water monitoring wells were installed at the site.With approval by the San Joaquin County Environmental Health Department, soil samples and grab ground water samples collected from the borings were not analyzed due to the lack of field indications of petroleum hydrocarbons. Due to MTBE detections in ground water samples from wells MW-1 through MW-3,AGE personnel made a site visit on 13 April 2001 to identify a potential source for the MTBE contamination; no apparent above-ground source was located. In October 2001, four soil probe borings were advanced at the site for collection of grab ground water samples.MTBE was detected in one grab ground water sample at a concentration of 2.7 µg/1. The lateral extent of MTBE was defined to the north by this investigation. In April 2004,four ground water monitoring wells were installed at the site to total depths of 25 feet bsg. MTBE was detected in one soil sample at a concentration of 0.060 mg/kg (MW5-10). Hydrocarbon-like odors were detected in the soil collected from MW-4 at 10, 15 and 20 feet bsg,but there were no detections in the laboratory results. These odors may have been due to hydrocarbon- impacted ground water in the saturated soil. The lateral extent of MTBE was defined to the south and redefined to the north by this investigation,but remains undefined to the east and west. QUARTERLY GROUND WATER MONITORING Quarterly monitoring procedures were instigated with the installation of monitoring wells MW-1 through MW-3 in November 1999.MTBE has been detected in samples collected from monitoring wells MW-1,MW-2 and MW-3,generally at low concentrations,and once from the active domestic well at the site. Tertiary amyl methyl ether(TAME)has been detected once in MW-2 and MW-3 at low concentrations. TPH-g(gasoline)has been detected since June 2003 in samples from MW-2 at or close to the concentration of MTBE detected, indicating a correlation. No other analytes of concern have been detected in ground water samples from MW-1 through MW-3 since monitoring began in November 1999. Advanced GeoEnvironmental,Inc.