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F. F to decline and would reach the conservative WQO in a reasonable timeframe (approximately 30 il years) even if a substantial portion of the source had not been removed. The second graph in ttachment G-2 shows that the TPH concentrations are currently under WQOs since the A g excavation. The projected attenuation of TPHg from March 2008 forward projects that TPHg ' will continue to remain below WQOs. h Benzene The California Department of Public Health has issued a maximum contamination level (MCL)°'! 4 for benzene in groundwater which is 1 µg/L. The current benzene concentrations are below II laboratory detection limits. An attenuation graph was generated to show that benzene would have reached the MCL in a reasonable timeframe (approximately 17 years) even if a substantial portion of the source had not been removed. The attenuation graph is included in Attachment G-3 MTBE The MCL for MTBE is 13 µg/L and the MCL is used in this assessment. The attenuation graph included in Attachment G-4 shows that MTBE has reached and will stay below the MCL. h TRPH—Long Chain Petroleum Hydrocarbons The longer chain hydrocarbon concentrations have been declining and the graph in Attachment i G-5 shows that the concentrations will reach WQOs in a reasonable timeframe (7 years) and are�� under WQOs since the excavation. I is 3.4.2.2 Groundwater Models ,3 RSCA Chemical Release Model— UST B RBCA Tool Kit calculations were performed for petroleum hydrocarbons sourced from the: Bunker. The model assumed there was a potential off-site residential user at a distance of 1,50011 feet, a TPH concentration of 3.0 mg/L, and 18 pounds remaining in soil after excavation. The, model projected that petroleum hydrocarbons would attenuate to 0.1 mg/L at 150 feet from the'! UST B location(Minney, 2010). I' h RBCA Chemical Release Model—Motor/H draulic Oil UST RBCA Tool Kit calculations were performed for petroleum hydrocarbons sourced from they Motor/Hydraulic Oil UST. The model assumed there was a potential off-site residential user at a' distance of 1,500 feet. The model projected that petroleum hydrocarbons would attenuate to 0.111 mg/L at 150 feet from the UST A location (Minney, 2010). The excavation of the area around.1 UST B included removing the contaminated soil from beneath the former Motor/Hydraulic 0111 r UST. RBCA Chemical Release Model—Bunker RBCA Tool Kit calculations were performed for petroleum hydrocarbons sourced from the] Bunker. The model assumed there was a potential off-site residential user at a distance of 1,500.1 feet, a TPH concentration of 2.5 mg/L, and 6 pounds remaining in soil after excavation. The' model projected that petroleum hydrocarbons would attenuate to 0.1 mg/L at 150 feet from the" UST A location (Minney, 2010). j I� - 13 - i;