ARCHIVED REPORTS_SEMI ANNUAL GROUNDWATER MONITORING OCTOBER 2010

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AXOM Mr. James L. L. Barton Regional Water Quality Control Board—Central Valley October 27, 2010 Page 4 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 E as Figures D-1 through D-5. Cross-sections have been updated with the groundwater monitoring data and analytical results for TPHg, benzene, and methyl tertiary butyl ether(MTBE)from the third quarter semi-annual event of 2010. Figure D-5 of Attachment E 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)". Since the fourth quarter 2008, AECOM has provided these mass estimates to the CVW B to comply with the directive, however, AECOM cautions reliance on these estimates. The calculations are based solely on the areal extent of impacts depicted on an iso-concentration map (which is an interpretation of the approximate extent of impacts in a given hydrostratigraphic unit), and assumptions on the consistency of the vertical extent of impacts across the hydrostratigraphic unit (which could either underestimate or overestimate the mass at a given time). Spatial and temporal variations in the plume configuration, due to seasonal concentration fluctuations, can also provide misleading mass estimates. For third quarter 2010, the remaining mass was calculated in the same manner as the previous quarters. Estimates were calculated for the areal extent of the TPHg and benzene impacts in the shallow hydrostratigraphic A/B unit using the iso-concentration maps presented as Figures 6 and 7. Estimates for units C and D were not calculated because neither TPHg nor benzene was detected in the third quarter 2010 in monitoring wells screened in those hydrostratigraphic units. The estimated hydrocarbon mass remaining is shown in the table below and is compared to the estimated hydrocarbon mass calculated for the fourth quarter 2008 and the first quarter 2010. Since fourth quarter 2008, the estimated remaining hydrocarbon mass of TPHg and benzene has been reduced by 88.9% and 99.3°/x, respectively. 4Q08 4Q08 1Q10 1Q10 3010 3Q10 TPHg Benzene TPHg Benzene TPHg Benzene Mass Mass Mass Mass Mass Mass (pounds) (pounds) (pounds) (pounds) (pounds) (pounds) Groundwater Mass 102 1.5 20.5 0.03 2.3 0.01 Saturated Soil Mass 1,668 4.0 335.6 0.08 38.2 0.02 Total 1,770 5.5 356.1 0.11 40.5 0.03 The spreadsheet used for calculating the remaining mass, as well as the assumptions used in the estimation, is provided as Attachment F.