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In July 1995, Thrifty Oil Company installed three off-site groundwater monitoring wells (MW-5, MW-6,
<br /> and MW-9) to approximately 60 ft. bgs. Petroleum hydrocarbons were not detected in the soil samples
<br /> collected from MW-5, MW-6, and MW-9. Dissolved-phase hydrocarbons were detected in groundwater
<br /> at maximum concentrations of 1,000 µg/L TPH (MW-9), 3.4 µg/L benzene (MW-9), 34 µg/L ethylbenzene
<br /> (MW-9), and 2.0 µg/L xylenes (MW-9) (Thrifty, 1995).
<br /> In July 1995, JDR Environmental (JDR) properly destroyed the three vadose zone wells (VWA, VWB, and
<br /> VWC) and three groundwater monitoring wells (MW-1, MW-2, and MW-8) as required by the SJCEHD
<br /> prior to station remodeling activities (JDR, 1995).
<br /> In July 1995, five single-walled, steel USTs were removed from the site and disposed of by Erickson of
<br /> Richmond, California. Approximately 1,500 cubic yards of soil were removed from the UST excavation
<br /> and transported to Forward Incorporated,of Stockton, California,for disposal.Ten soil samples(Tank Pit-
<br /> 1 through Tank Pit-9 and Tank-E-9')were collected from beneath the five USTs after excavation,and seven
<br /> soil samples (PL-1 through PL-7) were collected at a depth of 2 to 4 feet from beneath the product lines.
<br /> TPH and benzene were detected in soil samples collected from beneath the USTs at concentrations up to
<br /> 12,000 mg/kg TPH and 33 mg/kg benzene. Soil was excavated to approximately 15 feet bgs(Alisto, 1995).
<br /> In May 1996, AquaGeo Environmental Consulting (AquaGeo) installed six wells (VW-2, VW-4, VW-5, VW-
<br /> 6, MW-10/VW-1 and MW11/VW-3) to depths ranging from 16.5 to 58.5 feet bgs. Monitoring wells MW-
<br /> 10/VW-1 and MW-11/VW-3 were installed as nested wells. Well VW-2 was completed as a dual-
<br /> completion vadose zone monitoring well. Wells VW-4, VW-5, and VW-6 were completed as triple-
<br /> completion (A,B,C)vadose zone monitoring wells. Groundwater was encountered during drilling activities
<br /> at depths of approximately 44 to 50 feet bgs. TPH, benzene, and methyl tertiary butyl ether(MTBE)were
<br /> detected at maximum concentrations of 6,600 mg/kg (VW-6-25), 69 mg/kg (MW-11-40), and 670 mg/kg
<br /> (VW-6-40), respectively. Groundwater samples collected from wells MW-10 and MW-11 during the third
<br /> and fourth quarters of 1996 indicated TPH, benzene, and MTBE were detected at maximum
<br /> concentrations of 370,000 µg/L (MW-11), 33,000 µg/L (MW-11), and 880 µg/L (MW-11), respectively
<br /> (AquaGeo, 1997).
<br /> In March 1999, The Park Corporation (Park) installed one groundwater monitoring well (MW-12) to a
<br /> depth of 73 feet bgs. Groundwater was encountered at a depth of approximately 33 feet bgs. Laboratory
<br /> analysis of soil samples detected TPH concentrations from 1.4 mg/kg to 6,400 mg/kg, benzene
<br /> concentrations from 0.26 mg/kg to 100 mg/kg, and MTBE concentrations up to 35 mg/kg (EPA Method
<br /> 8020).The presence of MTBE in the soil samples was confirmed at concentrations ranging from below the
<br /> laboratory reporting limit to 1.1 mg/kg (EPA Method 8260). The highest concentrations of aromatic
<br /> volatiles in soil samples were detected in the 50 foot bgs sample. Laboratory analysis of groundwater
<br /> samples detected 21,000 µg/L TPH, 3,200 µg/L benzene, and 150 µg/L MTBE (EPA Method 8020). The
<br /> presence of MTBE was confirmed at a concentration of 120 µg/L (EPA Method 8260). In the surrounding
<br /> groundwater monitoring wells,TPH was detected at concentrations up to 760 µg/L, benzene was detected
<br /> at concentrations up to 2.1 µg/L, and MTBE was detected at concentrations up to 540 µg/L(EPA Method
<br /> 8020). The presence of MTBE was confirmed at concentrations up to 490 µg/L (EPA Method 8260).
<br /> Monitoring well MW-7 was redeveloped during assessment activities to enhance the well's productivity.
<br /> A 12-hour groundwater extraction test was also performed on monitoring well MW-11 at a maximum
<br /> sustainable flow rate of four gallons per minute (gpm) (Park, 1999).
<br /> In April 2004,Terra Vac observed the advancement of six soil borings (61 through B3 and B5 through 137)
<br /> in the vicinity of the USTs and dispenser islands. Terra Vac performed this work during due diligence site
<br /> assessment. Borings were advanced to depths of 10 to 25 feet bgs, and groundwater was not
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