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<br /> SITE CONCEPTUAL MODEL AND REMEDIAL ACTION PLAN 2
<br /> 7-Eleven Store No 14117, 2725 Country Club Blvd , Stockton,CA June 20, 2002
<br /> 1 2 Dissolved Data Summary
<br /> During the 1998 UST removal, grab samples collected from groundwater ponded in the base of the tank
<br /> pit contained up to 5,000 micrograms per liter(ug/L) benzene, 140,000 ug/L TPH-G, 68,000 ug/L MTBE,
<br /> and 23,000 ug/L TAME When the new tank pit(Figure 2)was excavated, a grab groundwater sample
<br /> ' was collected, analyzed, and found to contain 300 ug/L benzene, 3,100 ug/L TPH-G, and 27 ug/L MTBE
<br /> As the new USTs were being installed, conductor casing was placed in three locations between the two
<br /> ' new 10,000-gallon underground storage tanks, for use as groundwater monitoring well installation guides
<br /> The casing is 10-inch diameter unperforated PVC extending to 12 feet bg
<br /> ' Four groundwater monitoring wells were installed in May 1999 (IT Corporation, 1999) to depths of up to
<br /> 30 feet bg Well MW-1 is constructed to 30 feet bg, perforated from 10 feet bg to total depth Well MW-2
<br /> ' is installed in the central conductor casing in the new UST area, and is constructed to 25 feet bg,
<br /> screened from 12 feet bg to total depth The total depth of well MW-3 is 25 feet bg, and it is screened to
<br /> 10 feet bg Well MW-4 is constructed to 25 feet bg, and is screened from 10 feet bg to total depth
<br /> ' On October 30, 2001, IT Corporation installed groundwater monitoring wells MW-5, MW-6, and MW-7
<br /> (Figure 2) (IT Corporation, 2001), which are screened from 5 to 20 feet bg All well locations are shown
<br /> on Figure 2
<br /> ' Groundwater monitoring well data indicate the highest oxygenate concentrations have usually been found
<br /> in well MW-2, installed in the new tank pit area through one of the conductor casings As of the second
<br /> quarter 2002, dissolved benzene, TPH-G, MTBE, and TBA concentrations ranged up to 620 micrograms
<br /> per liter (ug/L), 14,000 ug/L , 910 ug/L, and 67,000 ug/L, respectively Highest MTBE levels were reported
<br /> in well MW-4, located crossgradient next to the existing UST area, whereas highest benzene
<br /> concentrations were detected in well MW-3, located crossgradient to the tank area and pump dispenser
<br /> ' island Highest TBA levels were detected in well MW-2, located in the middle of the new UST pit area
<br /> ' The benzene, MTBE, TPH-G, and TBA plumes are currently delineated within 5 ug/L to the south and
<br /> west of the UST areas at the site
<br /> ' 1 3 Source Area Evaluation/UST Tightness
<br /> The USTs at the site were replaced in July 1998 (Fluor Daniel GTI, 1998) In May of this year, the
<br /> ' SJCPHS contacted 7-Eleven to ascertain if a leak had been recorded, based on quarterly monitoring data
<br /> indicating increasing levels of TBA at the site
<br /> ' As of the date of this report, 7-Eleven, Inc (7-Eleven) has indicated that no leaks or tightness test failures
<br /> have been recorded for this location
<br /> ' 14117Scm2 doc
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