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1 Fourth Quarter 2002 Monitoring and Sampling Report 3 7-Eleven, Inc Store No 17334,4501 North Pershing Avenue,Stockton,CA January 13 2003 On April 3 and 4, 2002, soil boring S13-1 was advanced to a depth of 50 feet bg where a grab groundwater sample was collected using a Hydropunch'14 (IT Corporation, 2002), to evaluate the vertical extent of dissolved constituents at the site per the SJCPH's request Dissolved benzene, TPH-G, and MTBE were detected in groundwater samples from SB-1 @ 50' bg, at concentrations of 28 µg/L, 7,100 µg/L, and 67 pg/L, ' respectively Two double-nested soil vapor extraction (SVE)/air sparge(AS)wells (SPV-1 and SPV-2)were also installed to 30 feet bg for potential remediation activities Soil samples were collected at total depth of drilling in each well The deep well segments are screened from 25 to 30 feet bg The shallow well segments ' are screened from 12 to 22 feet bg TPH-G was detected in the soil sample from SPV-1 at 30 feet bg, and MTBE was not detected in soil samples from either boring ' Additional offs iteldowngradient plume delineation is planned for 2003, pending completion of access agreement negotiations On May 16, 2002, an SVE test was conducted to evaluate the radius of vapor extraction influence and vadose-zone volatile organic vapor generation potential using wells SPV-1, SPV-2, and MW-5A Results of ' the SVE test indicated that extracted vapor levels ranged up to 300 parts per million by volume (ppm[v]) and a radius of influence between 30 to 35 feet Five SVE samples were collected in Tedlar bags and submitted for laboratory analysis Extracted benzene, TPH-G, and MTBE concentrations ranged up to 18 ug/L, 2,300 ug/L, and 140 ug/L, respectively �• Geology/Hydrogeology The relatively level site is located at an elevation of approximately 8 feet above mean sea level The site is underlain by Quaternary alluvial deposits assigned to the Modesto Formation (Fluor Daniel GTI, Additional Site Assessment Report, January 15, 1998) Materials encountered during previous drilling at the site generally consisted of clayey silt and sandy silt to approximately 24 feet bg, underlain by interbedded clayey ' silt and silty fine sand to maximum exploratory drilling depths of between 35 and 40 feet bg ' Depth to groundwater in Exxon well MW-9 has ranged from 30 feet bg in 1990 to 20 feet bg in 1996 During the current sampling event, equilibrated depth to groundwater in sampled wells at the site ranged between 19 30 and 18 14 feet below top of casing (TQC) Seasonal water table fluctuations average 2 to 3 feet 1 Historically, groundwater flow direction at the subject site and adjacent facilities has been to the northeast Sensitive Receptors Previous reports on sensitive receptors indicate that there are no public supply wells within 4,000 feet downgradient of the site The nearest crossgradient/upgradient water supply well is of unreported status and lies about 1,500 feet away Groundwater Monitoring Data . Before monitoring, the well caps were opened to allow the water in each well to stabilize to atmospheric ' pressure The DTW was measured to the nearest 0 01 foot from the top of the well casing using an interface probe 11334Q24 QMR DOC