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' LPH, and had the highest concentrations of TPHg (220,000 ug/L) and benzene (2,200 ug/L), and minor concentrations of MtBE in well MW-9, concentrations of petroleum hydrocarbons and ' MtBE have generally decreased to non-detectable levels in other site wells TPHg and BTEX concentrations in well MW-1 have significantly declined over time (Figure 7) Residual concentrations of benzene, toluene, and ethylbenzene that have been detected in well MW-1 ' over recent quarters are below the Tier 11 SSTLs that were applied during the RBCA assessment (Appendix D) A groundwater chemical concentration map depicting concentrations from the most recent quarterly sampling event conducted in October 2004 is ' included as Figure 8 Analysis for the presence of MtBE was first performed on groundwater samples from site wells in October 1997 Historical results through October 2004 indicate that low levels of MtBE have ' been detected in wells MW-1 through MW-7, MW-9, and MW-10 With the exception of some detections of MtBE reported in well MW-4 between January 1998 and November 1999 up to a historical site maximum concentration of 41 2 ug/L, MtBE concentrations in the aforementioned ' wells have historically been reported at levels below the California Department of Health Services (DHS) maximum contaminant level (MCL) of 13 ug/L Historical results for fuel oxygenates other than MtBE (TBA, DIPE, EtBE, TAME) compiled since July 1999 indicate that t these constituents have not been detected in the site wells Historical groundwater analytical - results for fuel oxygenates are presented in Table 5 ' Overall, active remediation and natural attenuation processes at the site have resulted in the reduction of the dissolved plume beneath the site, which is limited in its ext'bnt to the vicinity of well MW-1 The lateral extent of the dissolved plume is defined across the site The vertical extent of dissolved petroleum hydrocarbons is defined outside the source area, with the exception of in the area of well MW-1, which has not been defined to non-detectable levels of TPHg and MtBE beyond the completed depth of the well (101 5 feet bgs) Residual concentrations of benzene, toluene, and ethylbenzene that have been detected in well MW-1 ' over recent quarters are below the Tier II SSTLs that were applied during the RBCA assessment (Appendix D) A schematic illustration of the historical pathway of the dissolved plume beneath the site is presented as Figure 9 ' 73 Estimated Residual Mass According to estimated mass calculations, approximately 24,335 pounds (3,479 gallons) of residual TPHg were present in soil, and approximately 37 84 pounds (5 41) gallons of TPHg were present in groundwater before the implementation of remedial action at the site (SECOR, September 8, 1998) Historical pre-remediation sod analytical results in the area of the former USTs and in the vicinity ' of well MWA were used to estimate the area of residual petroleum hydrocarbon impacts remaining in soil beneath the site after remediation Based on the results obtained during the November 2004 post-remediation soil confirmation sampling event, an estimated 789 7 pounds of TPHg remain beneath the site in the area of the former USTs, and an estimated 148 0 pounds of TPHg are present in soil in the vicinity of well MWA Historical groundwater data collected from well MWA during the operation of the SVEIAS ' system from February 2000 through October 2004 was used to estimate residual TPHg Time Ool Asmt&SCM doc 12 March 17,2005 ' 770T 41658 00 0015