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Work Plan For Additional Groundwater Assessment 2 Ranch Market 23569 South Santa Fe Road Riverbank Cal�forn�a May 10,2001 • Analyses of groundwater samples for total petroleum hydrocarbons as gasoline (TPHg), benzene, toluene, ethylbenzene, xylenes (BTEX), fuel oxygenates methyl-tert-butyl ether(MtBE), dusopropyl ether(DTPE), ethyl-tert-butyl ether (ETBE), tert-amyl-methyl ether(TAME), tert-butanol (TBA), and fuel additives 1,2-Dichloroethane (1,2-DCA) and 1,2-Dibromoethane (EDB) using EPA method 8260 BACKGROUND The Site is located at the southeastern intersection of South Santa Fe Road and Henry Road in Riverbank, California (Figure 1) Facilities onsite consist of a single story brick building with a septic tank and leach field The remainder of the Site is covered with asphalt paving Currently the Site is being used as a mini-market without gasoline service Underground Storage Tank Removal On May 20, 1991, three underground storage tanks (USTs) were removed from a common tank pit(Figure 2) Strong hydrocarbon odors and discolored soil was observed by SJCEHD in the • tank pit following removal of the tanks. Tank pit floor soil samples collected at depths of 9 to 11 feet below ground surface (bgs) contained reportable concentrations of gasoline range petroleum hydrocarbons (Attachment 2,Table 1) Soil and Groundwater Assessment In January 1998 soil boring B-1 was advanced into the former tank pit to a depth of 30 feet bgs Analyses of soil samples collected from soil boring B-1 at 20, 25, and 30 feet bgs showed a decline in total petroleum hydrocarbon as gasoline (TPHg) concentrations from 4,140 milligrams per kilograms (mg/kg) at 20 feet bgs to 6 mg/kg at 30 feet bgs (Table 1) A grab water sample was also collected from boring B-1 that contained elevated concentrations of benzene, toluene, ethylbenzene, xylenes (BTEX), and TPHg (Table 2) In November 1998, Arrowhead Consultants oversaw the installation of four groundwater monitoring wells at the site MW-1 in the former tank pit and MW-2, MW-3, and MW4 centered around the former tank pit (Figure 2 and Table 2) Soil samples collected during monitoring well installation contained reportable concentrations of BTEX and TPHg (Table 1) The first monitoring and sampling event conducted in December 1998 revealed all four wells contained reportable concentrations of dissolved BTEX,TPHg, and methyl-tert-butyl ether (MtBE) (Table 3) No reportable concentrations of tertiary butanol (TBA), MtBE, chisopropyl ether(RIPE), ethyl-tert-butyl ether (EtBE), and tert-amyl-methyl ether(TAME) were detected in the water samples collected in December 1998 File Ranch Market\WorkPlan\Rh-CPT Workplan doc