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09 February 2004 AGE-NC Project No. 98-0452 Page 2 of 8 2.2. UNDERGROUND STORAGE TANK REMOVAL It is AGE's understanding that two USTs were removed from the site in July 1991. Ground water was encountered at a depth of approximately 7 feet below surface grade (bsg) during the UST removal. Following removal of the UST,total petroleum hydrocarbons quantified as diesel(TPH-d) were detected in a grab water sample collected from the excavation at a concentration of 210 micrograms per liter(µg/L); analytical results of soil samples collected were not available. The San,Joaquin County Environmental Health Department(EHD) subsequently required investigation to determine the extent of impacted soil and ground water. 2.3. PREVIOUS SITE ASSESSMENTS 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 concentations as high as 2,700 µg/L. Methyl tertiary-butyl ether (MTBE) was detected in four of the five grab ground water samples at concentrations as high as 25 µg/L. 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. Since initiation of ground water monitoring in November 1999, low concentrations of MTBE have been detected in samples collected from the three ground water monitoring wells and a domestic well at the site; no other analytes of concern have been detected. On 13 April 2001 AGE personnel made a site visit 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/L. The lateral extent of MTBE was defined to the north, but has not been defined to the east, west or south. 3.0. SCOPE OF WORK AGE proposes to install three shallow-screened ground water monitoring wells to delineate and monitor the lateral extent of impacted ground water. The scope of work will consist of the following tasks: Advanced GcoEnvironlnental,Inc.