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Additional Site Assessment and Second Quarter 1999 Monitoring and Sampling Report 1 7-Eleven Inc Store#19976, 1399 N Main St Manteca California July 15, 1999 10 INTRODUCTION This report is submitted by IT Corporation to summarize the results of site assessment conducted at the 7-Eleven Inc , Store No 19976, located at 1399 N Main Street, in Manteca, California (Figures 1 through 3) The scope of work for the site assessment included the collection of background information,the drilling of five soil bonngs, installations of groundwater monitoring wells in four of the ' borings, collection of sod and groundwater samples, and preparation of this report summarizing the methods and results of the work performed The work was designed to fulfill the requirements of the San Joaquin County Public Health Services (SJCPHS) in their letter dated April 12, 1999 The work scope described in this report was completed at the site between May 11 and May 17, 1999 P Y Y ' Quarterly groundwater mondonng and sampling was completed on June 4, 1999 All work was performed in accordance with IT Corporation's Work Plan forAdditional Subsurface Investigation, dated January 29, 1999, and the Addendum to the Work Plan forAdditional Subsurface Investigation, dated ' March 4, 1999,which was approved by the SJCPHS 20 BACKGROUND Underground storage tank replacement and associated soil sampling activities were conducted in 1996 On September 19, 1996, Fluor Daniel GTI collected a total of five sod samples from the tank pd area (Figure 2) One sample was collected from each end of the three USTs at a depth of 15 feet below ground surface (BGS),with the exception of the north end of the central tank that was inaccessible due to the location of the sod stockpile The samples were analyzed for benzene,toluene, ethylbenzene, and ' total xylenes(BTEX), methyl-tert-butyl-ether(MTBE),total petroleum hydrocarbons as gasoline (TPH-G), following EPA method 8020/modified 8015, and total lead by EPA method 6010 If MTBE was detected ' by EPA method 8020, an additional analysis by EPA method 8260 was used to confirm its presence Table 1 summarizes the sod analytical results of the soil samples collected beneath the tanks on September 19, 1996 On September 19 through 24, 1996,the tank pit was extended approximately 20 feet to the south and excavated to a depth of 13 feet BGS to accommodate the installation of three new 10,000-gallon LISTS (Figure 2) Sods encountered during the tank removal included fine sand to silty sand to a depth of approximately 10 feet BGS followed by silt, silty clay, and clayey silt with some interbedded fine sand to a depth of 15 feet BGS Soil encountered at 15 feet BGS appeared very moist 19976g2 sar