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GeoHydrologic Consultants, Inc
<br /> TPHg was detected on all grab groundwater samples except CPT-2-94-98, at
<br /> concentration ranging between 104 pg/L and 15,200 pglL Benzene was detected at a
<br /> maximum concentration of 156 pg/L (CPT-4-65-70) MTBE was detected in one
<br /> groundwater sample (CPT3-65-69) at a concentration of 3 7 pg/L
<br /> The locations of wells and borings are included in Figure 2 and geologic cross sections
<br /> are presented in Figures 3A, 313, and 3C Historical soil laboratory analytical results are
<br /> presented in Table 1 and groundwater results are presented in Tables 2A and 213
<br /> Distributions of TPH, benzene, and MTBE in soil are presented in Figures 4A, 4B, 4C,
<br /> 41), 4E, and 4F, respectively Distributions of TPH, benzene, and MTBE in groundwater
<br />' are presented in Figures 5A, 513, and 5C, respectively
<br /> 3,3 Previous Remedial Activities
<br /> A vapor extraction pilot test was conducted in June 1995 by Alton Geosciences and
<br /> reported in their"Vapor Extraction Test Report", dated August 14, 1995 Thrifty
<br /> prepared a"Corrective Action Plan, Thrifty Service Station#171, 1250 North Wilson
<br /> Way, Stockton, California", dated September 26, 1995 in which they proposed to
<br /> remediate the Site with vapor extraction and groundwater extraction
<br /> Vapor extraction has been performed at the Site since July 1, 1999, using a Baker
<br /> Furnace Thermal Oxidizer Unit connected to wells MW-4, MW-5, TB-3, TB-4, TB-6,
<br /> TB-7, TB-9, TB-11, and TB-12 Vapor samples were collected bi-weekly during the first
<br /> lie month of operation and monthly thereafter Approximately 140,299 pounds of TPH-g
<br /> have been removed and destroyed by the vapor extraction unit as of August 16, 2004
<br /> during the 29,975 hours of operation
<br /> A 24-hour constant rate aquifer test was performed on we]I MW-5 on March 16 and 17,
<br /> 2002 Monitoring wells MW-2, MW-3R, MW-4, MW-6, TB-1, TB-7, and TB-10 were
<br /> used as observation wells during the pumping test Monitoring well MW-5 was also
<br /> gauged for recovery following completion of the pumping test The results of the aquifer
<br />' test are reported in GHCs "Aquifer Testing Report and Remedial Action Plan Report",
<br /> dated July 11, 2002 GHC recommended soil vapor extraction with pump and treat to
<br /> enhance the remediation of the Site Air sparging was not considered technically feasible
<br />' due to the relatively confined nature of the saturated zone (more permeable soil layers at
<br /> depths ranging from 65 to 70 feet bgs confined by fine grained soil above and below)
<br />' 4.0 SUPPLEMENTAL HYDROGEOLOGIC INVESTIGATION
<br /> 1 GHC installed groundwater extraction wells EW-5D and EW-5S on February 22 and 23,
<br /> 2004, respectively (Figure 2) These wells were installed following the SJCEHD
<br /> requirements stated in the letter to Thrifty dated December 7, 2004 Wells EW-5D and
<br />' EW-5S were installed to facilitate pilot testing to determine the effectiveness of multi-
<br /> phase extraction as a remedial alternative Monitoring well MW-5 was abandoned and a
<br /> vapor extraction well (VW-5) was installed in the same borehole on March 25, 2005 as
<br /> 1 'roc 17i Pilot Study Page 5
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