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1 Fourth Quarter 2002 Monitoring and Sampling Report 2 7-Eleven, Inc Store No 17334,4501 North Pershing Avenue,Stockton,CA January 13, 2003 ' Groundwater monitoring and sampling has been ongoing at the Exxon site since 1987, and conducted at the Cardlock and Shell sites since 1992 In 1988 Exxon installed well MW-9 to 30 feet below grade (bg), approximately 10 feet east and 10 feet south of the 7-Eleven UST area Depth to groundwater in well MW-9 ' has ranged from 30 feet bg in 1990 to 20 feet bg in April 1996 On February 20, 1997, Fluor Daniel GTI supervised the advancement of four angled and three vertical ' geoprobe borings (GP-1 through GP-7) at the site (Fluor Daniel GTI, 1997) Soil samples were collected from the borings at depths of 15 to 22 feet bg Six borings (GP-1 through GP-6)were drilled beneath the three ' 10,000-gallon USTs at the site as required by the SJCPHS prior to tank lining permit approval In addition, grab groundwater samples were collected from borings GP-3, GP-4, and GP-7 Benzene was detected in soil sample GP-4 @ 17 feet at a concentration of 0 0095 milligrams per kilogram (mg/kg) TPH-G was not ' detected in any of the soil samples collected The highest MTBE concentration was reported in sample GP-1 @ 15 feet at 0 91 mg/kg (EPA Method 8260) The highest dissolved benzene, TPH-G, and MTBE concentrations in groundwater were reported in sample GP-4 at 9,100 micrograms per liter(µg/L), 150,000 pg1L, and 84,000 µg/L, respectively ' Based on these data, the SJCPHS required additional assessment of soil and groundwater conditions beneath the site From September 23 through November 26, 1997, Fluor Daniel GTI supervised the drilling of five groundwater monitoring wells (MW-1, MW-1A, MW-2, MW-3, and MW-4) to depths ranging between 35 and 40 feet bg (Figure 2) Soil samples were collected during drilling at 5-foot-depth intervals from 5 feet bg to total depth In addition, soil from wells MW-1 and MW-2 was sampled continuously from 15 feet bg to total depth A total of twenty-three soil samples collected from the soil borings were submitted for laboratory ' analysis The groundwater monitoring wells are constructed of 2-Inch-diameter Schedule 40 poly-vinyl- chloride (PVC) casing with flush threads and 0 020-inch-slot well screen Wells MW-1, MW-2, MW-3, and MW-4 are screened from 13 to 33 feet bg, and well MW-1A is screened from 38 to 40 feet bg ' On August 14 1998 monitoring wells MW-1 and MWAA were abandoned, as they were located adjacent to ' the USTs that were removed on August 25, 1998 On August 25, 1998, approximately 200 cubic yards of soil were excavated from the new UST location to accommodate the installation of two double-wall steel 10,000- gallon USTs Samples were collected at depths of up to 14 feet bg below/adjacent to the former locations of ' the tanks/islands and lines (Fluor Daniel GTI, 1998) On December 17, 1998, double-nested groundwater monitoring well MW-5 (replacing abandoned well MW-11) ' was installed to 40 feet bg (Figure 2) Each well segment is constructed of two 2-inch diameter Schedule 40 PVC casings with flush threads and 0 020-inch-slot well screen The deep segment (MW-5B) is screened from 38 to 40 feet bg The shallow segment (MW-5A) is screened from 13 to 33 feet bg i i ' 173341124 QMR 110C