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Geologicaf Tecfm' ics Znc. 274I r*wXxd %""W'Cww*95351 (209f 538�424/�QC#538-5852 L Additional Site Characterization Work Plan Monitoring Well Installation Morita Brothers 814 E. Charter Way Stockton,CA Project No. 724.2 July 7, 2000 1.0 INTRODUCTION LIn December 1991, four underground storage tanks (USTs) were removed from the site (Figure 1). Gasoline range petroleum hydrocarbons were detected in shallow soils beneath the tanks and seven soil borings were installed in March 1992 to further define the extent of the contamination. The borings revealed low to moderate concentrations of benzene,toluene, ethylbenzene and xylene (BTEX), and gasoline range petroleum hydrocarbons (TPH- Gasoline) in soils up to 50 feet below grade surface (bgs). In March 1993, shallow soils from beneath the former USTs and product piping areas were excavated for disposal. The resulting excavations were advanced to depths of 20-28 feet bgs, which generated approximately 750 cubic yards of soil for disposal. L Three monitoring wells (MW-1 through MW-3) were installed in September 1992, to determine if groundwater had been impacted by the release. Groundwater samples obtained from the wells confirmed that BTEX and TPH-G were present and the contaminant concentrations exhibited minor fluctuations during 18 monitoring events conducted between 1992 and 1999. Two additional monitoring wells (MW-4 and MW-5) were installed in September 1999, and incorporated into the monitoring schedule. Samples obtained in L November 1999 and March 2000, from well MW-4 contained elevated levels of benzene and TPH-G for the two events. LIn their May 3, 2000, letter the San Joaquin County Public Health Services, Environmental Health Division (PHS/EHD) directed Mr. Frank Morita to submit a work plan to further L L