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EUM. <br /> • Mr James L Barton <br />' July 27, 2005 <br /> Page 3 <br />' petroleum hydrocarbons as gasoline (TPHg) by EPA Method 8015 (Modified) and for volatile <br /> organic compounds (VOCs) by EPA Method 8260B Recent analytical results from groundwater <br /> sampling are presented in Table 1 Cumulative analytical results from groundwater sampling <br /> are presented in Table 3 Maps depicting dissolved concentrations of TPHg and benzene in <br /> groundwater are included as Figures 7 through 14 The system layout is shown on Figure 15 <br />' Laboratory analytical results with chain-of-custody documentation are included as <br /> Attachment D <br />' Site Concel2tual Model Update <br />' In a letter dated February 8, 2002 the RWQCB (now the Central Valley Water Board, CVWB) <br /> requested a site conceptual model (SCM) for this site The letter stated that, at a minimum, the <br /> SCM must include two hydrostratigraphic cross-sections, one across the former Unocal site and <br />' one across the former Wards site A cross-section location map and cross-sections A-A', B-B', <br /> and C-C'are included as Attachment E The cross section maps are updated with the second <br />' quarter 2005 analytical results for TPHg, benzene, and methyl tertiary butyl ether (MTBE) <br /> According to ASTM document E1689-95 there are six basic activities associated with <br />' developing a SCM <br /> 1 identification of potential contaminants, <br />' 2 identification and characterization of the source(s) of contaminants, <br /> 3 delineation of potential migration pathways through environmental media such as <br /> groundwater, surface water, soils, sediment, biota, and air, <br />' 4 establishment of background areas for each contaminated medium, <br /> 5 identification and characterization of potential environmental receptors (human and <br /> ecological), and <br />' 6 a determination of the limits of the study area or system boundaries <br /> Contaminants and Nature and Extent of Sources <br /> The primary contaminants of concern at the site are petroleum hydrocarbons, particularly TPHg <br /> and benzene According to a report titled "Supplemental Investigation Report, Problem <br /> Assessment Report, and Work Plan" by Environmental Audit Incorporated (EAI) dated <br /> October 13, 2000, seven gasoline and oil underground storage tanks (USTs) were excavated <br /> and removed from the Wards site between June 22, 1986 and June 26, 1988 Additionally, the <br /> report states that the Unocal site had all of the USTs and associated piping removed in the first <br /> Celebrating 35 Years of Excellence In Environmental Services <br />