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S E C ® R Additional Site Assessment and Site Conceptual Model 7-11 Store#14117 September 30, 2005 Page 9 Groundwater Petroleum Hydrocarbon Concentrations During the most recent quarterly sampling event, onsite groundwater contained concentrations of TPHg up to 5,100 micrograms per liter (pg/I_ ), MtBE up to 150 pg/L , and TBA up to 2,000 pg/L (Table 4) Groundwater samples collected from shallow borings GP-1 and GP-2 did not contain MtBE, TBA or TPHg at concentrations above the laboratory reporting limits MtBE was detected to the deep CPT borings at concentrations from of 1 3 pg/L (CPT-1-70') to 4 0 pg/L (CPT-3- 70') TBA was detected above the laboratory reporting limits in CPT-1 and ranged from 47 pg/L (CPT-1-70') to 86 pg/L (CPT-1-110') TPHg was detected in CPT-1 at concentrations of 1,000 pg/L (CPT-1-92') to 21,000 pg/L (CPT-1-110') (Table 4) The certified laboratory analytical report and chain-of-custody documentation are presented as Attachment F Iso-concentration maps were generated for MtBE, TBA and TPHg dissolved in groundwater per request by the SJCEHD and are included as Figures 7 through Figure 9 Data for the wells located at the USA Gas station adjacent to the 7-11 site was obtained from the State Geotracker website and are included as Attachment G . Based on the data described above, the vertical limits of hydrocarbon contamination in deep portions of groundwater onsite have not been defined However, the lateral extent of TPHg and TBA In deep portions of groundwater have been defined to the northeast and east of the site With the exception of 4 0 pg/L of MtBE at 70 feet bgs in CPT-3, the lateral extent of MtBE in the deep portions of groundwater have also been defined to the northeast and east The lateral extent of shallow contamination in groundwater has been defined to the north, west and southwest of the site Definition of the southern extent of dissolved TPHg, MtBE and TBA is pending installation of well MW-10 on the south side of Country Club Boulevard 4.8 Estimated Mass of Petroleum Hydrocarbons Remaining In Groundwater Not enough data are available to accurately calculate the estimated mass of petroleum hydrocarbons remaining in groundwater 5.0 SOURCES OF IMPACTS The source of impacts is currently assumed to be the dispensers and fuel lines 6.0 POTENTIAL GROUNDWATER RECEPTORS A review of a previous well search conducted by Flour Daniel GTI (Flour Daniel GTI, Onsite and Offsite Assessment Report, November 30, 1998) Indicates that there are 11 water supply wells located In various directions within a 0 5 mile radius of the site 117-Elevenl7-11 Slores1141171Reportsli4117 SCM and Additional Assessment doc Page 9