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E C 0 R Additional Site Assessment and Site Conceptual Model 7-11 Store#14117 September 30, 2005 Page 6 complex The offsite borings did not contain detectable concentrations of petroleum hydrocarbons The onsite borings contained benzene, total petroleum hydrocarbons as gasoline (TPHg), and methyl-tert-butyl ether (MtBE) in soil at maximum concentrations of 12 5 milligrams per kilogram (mg/kg) (GP-17@20'), 42,900 mg/kg (GP-15@15'), and 234 mg/kg (GP-14@10'), respectively (Table 3) On May 13 and 14, 1999, four groundwater monitoring wells (MW-1 through MW-4) were Installed to 25 and 30 feet below ground surface (bgs) (Table 2) Benzene, TPHg, and MtBE in soil were reported at maximum concentrations of 2 89 mg/kg (MW-3-15'), 429 mg/kg (MW-3-15'), and 0 119 mg/kg (MW-4-20'), respectively (Table 3) On October 30, 2001, three groundwater monitoring wells (MW-5, MW-6, and MW-7) were installed to 20 feet bgs (Table 2) TPHg, benzene, toluene, ethylbenzene, xylenes (BTEX), and MtBE in soil were not detected above laboratory reporting limits, except for 0 006 mg/kg total xylenes (MW-7@20') (Table 3) Selected halocarbons also present in the soil samples Indicated possible dry cleaner plume contributions (IT Corporation, 1999) On August 14, 2003, deep groundwater monitoring well (MW-8) and one soil boring (SB-1) were advanced onsite SECOR summarized this assessment In the February 27, 2004 Additional Site Assessment Report and Fate and Transport Model, which also presented calculations of the lateral migration of dissolved MtBE in groundwater Per the request of the SJCEHD, groundwater monitoring and sampling Is coordinated with a former Exxon site (currently a USA Gasoline station) Immediately east of the 7-Eleven site On July 22, 2004 , SECOR submitted a proposal for a Field Test Program for Evaluation of Oxidation Capacity of Groundwater to the RWQCB The RWQCB conditionally approved the proposal In a letter dated October 4, 2004 On July 27, 2004, SECOR submitted a Work Plan for Additional Offsite Assessment to the SJCEHD The work plan was approved by the SJCEHD in a letter dated August 23, 2004 The work plan proposed the installation of two offsite groundwater monitoring wells (MW-9 and MW-10), advancing three offsite soil borings (GP-1 through GP-3), two offsite CPT borings (CPT-2 and CPT-3) and one onsite CPT boring (CPT-1) 4 0 SOIL AND GROUNDWATER ASSESSMENT On April 26-28, 2005, SECOR supervised the advancement of two shallow geobrobe borings (GP-1 and GP-2), three deep Cone Penetration Test (CPT) borings (CPT-1, CPT-2 and CPT-3) and one oxygen injection well (1-1) (Figure 2 and Table 2) Data obtained from the installation of oxygen injection well 1-1 will be included In a separate report submitted to 117-Elevenl7-11 stares1141171Reports114117 SCM and Additional Assessment doe Page 6