ARCHIVED REPORTS_QUARTERLY GROUNDWATER MONITORING AND REMEDIATION STATUS REPORT AND UPDATED SITE CONCEPTUAL MODEL 2009

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Mr. James L. Barton April 8, 2009 ' Page 4 ' 0.5 µg/L, respectively. The concentrations show that the recently upgraded remediation system is addressing dissolved concentrations in this area. • MW-25D— Except for seasonal fluctuations, the concentration of TPHg in groundwater has a ' slight decreasing trend. Benzene concentrations have decreased significantly since 2002 and have been non-detect or slightly above the detection limit since 2006. TPHg was detected at 270 pg/L in January 2009, while benzene was detected at a laboratory estimated concentration of ' 0.8 pg/L. The slight increases are consistent with those observed historically, and the overall dissolved concentrations are an order of magnitude lower than the highest concentrations observed for this monitoring well. Due to previous concerns of cross-contamination during past sampling events, G-R submitted equipment blank rinsate samples collected from pumps used to sample monitoring wells MW-2, MW-3, and MW-8D following sampling and decontamination. These samples were listed as EB-2, EB-3, and EB-8D on the ' COC and analyzed for the same constituents as the groundwater monitoring well samples. All rinsate blank samples were non-detect for these constituents during the first quarter 2009 sampling event. To further address cross-contamination concerns, AECOM also verified that samples were taken from ' monitoring wells in a clean-to-dirty order based on historical concentrations in these wells. Site Conceptual Model Update ' In a letter dated February 8, 2002, the CVWB requested that Site Conceptual Model (SCM) updates be included in quarterly monitoring reports for the Site and stated that, at a minimum, the SCM must include two hydrostratigraphic cross-sections: one across the former Unocal site and one across the former Wards site. A cross-section location map, and four geologic cross-sections termed A-A', B-B', C-C'and D-D' are included in Attachment D as Figures D-1 through D-5. Cross-sections are updated with the first quarter ' 2009 groundwater monitoring data and analytical results for TPHg, benzene, and methyl tertiary butyl ether(MTBE). Figure D-5 of Attachment D uses data from air sparge and vapor extraction wells. Cross- sections also include soil analytical data collected at the time of well installation. ' Remaining Contaminant Mass ' In a letter dated December 5, 2008, the CVWB requested "An evaluation of the effectiveness of cleanup activities and an estimate of pollutants treated versus pollutants remaining (may be submitted quarterly as an ancillary report)". ' The remaining mass for TPHg and benzene in the saturated zone (both groundwater and saturated soil) was estimated based on first quarter 2009 groundwater monitoring results. Estimates were calculated for ' the aerial extent of the contamination in the shallow hydrostratigraphic A/B unit using the iso-concentration maps presented as Figures 3 and 4. Estimates for Units C and D were not calculated because neither TPHg nor benzene was detected in the first quarter 2009 in Unit C or Unit D monitoring wells. The remaining estimated hydrocarbon and benzene mass is shown in the table below. ' AECOM Environment AECOM