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C406glUt reCk *S I"* L1101 70'Street Modesto,California 95354 (209)522-4119/Fax#5224227 Additional Site Characterization Work Plan LMonitoring Well Installation ` Morita Brothers ` 814 E. Charter Way Stockton, CA Project No. 724.2 June 27, 2002 L1.0 INTRODUCTION In December 1991, four underground storage tanks (USTs) were removed from the site L (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 t soils from beneath the former USTs and product piping areas were excavated for disposal. L The resulting excavations were advanced to depths of 20-28 feet bgs, which generated approximately 750 cubic yards of soil for disposal. Three monitoring wells (MW-1 through MW-3) were installed in September 1992 to determine if groundwater had been impacted by the release—see Figure 2 for well locations. LGroundwater 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 November 1999 and March 2000, from well MW-4 Lcontained elevated levels of benzene and TPH-G for the two events. On November 20—22, 2000, five additional groundwater monitoring wells were installed to investigate the lateral and vertical extent of gasoline-range petroleum hydrocarbons in the L