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Ms.Lori Duncan C A M B R I A September 9,2005 COC Background California SSTL Maximum Current Level MCL(Ng/L) Determined Detected Maximum (Ng/L) by RSCA Concentration Detected Analysis (yg/L) Concentration L /L TPHq <50 NA NA 170,000 100,000 Benzene <0.50 1 14 22,000 2,100 Toluene <0.50 150 10,000 31,000 4,900 Eth (benzene <0.50 700 5 100 3,700 2,800 X lanes <0.50 1,750 100,000 27 000 20,000 © pg/L=Micrograms per liter(ppb) NA NA=Not applicable The background levels for all organic chemicals are assumed to be less than the standard analytical reporting limit. Cambria proposes that the SSTLs for BTEX compounds determined by the RBCA analysis be adopted as the groundwater clean-up goals for the subject site. As discussed above, based on the complex mixture and varied nature of TPHg, there is no calculated SSTL for it. In addition, there is no MCL for TPHg in groundwater. Guidance from the San Francisco Bay RWQCB (Screening For Environmental Concerns at Sites With Contaminated Soil and Groundwater [Interim Final — February 2005)) states that "screening levels for benzene and related light-weight hydrocarbon compounds are considered to provide adequate additional protection of drinking water concerns for gasoline-impacted groundwater when used in conjunction with the TPH screening level of 100 ug/L." Therefore, Cambria proposes a groundwater clean-up goal of 100 ppb for TPHg. Current maximum concentrations of TPHg,benzene, toluene, ethylbenzene, and xylenes detected in groundwater beneath the site are 100,000, 2,100, 4,900, 2,800, and 20,000 ppb, respectively. At this time toluene, ethylbenzene, and xylenes are below the proposed clean-up levels, and TPHg and benzene are above them. Although significant reductions in groundwater concentrations of COCs can be attained by various remedial alternatives, attainment of the approved groundwater clean-up levels may prove to be technically or economically infeasible. Thus, groundwater clean-up may be limited to that which is technically or economically feasible. Soil Clean-up Levels Per the Basin Plan, clean-up levels for soils which threaten water quality should be equal to background concentrations, unless background levels are technologically or economically infeasible to achieve. Where background levels are infeasible to achieve, soil clean-up levels are established to ensure that remaining leachable/mobile constituents of concern will not threaten to 10