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i i7. I I� 3.4 Groundwater Plumes : Monitoring well (MW-A) was installed downgradient from UST A and was monitored since '' June 1999. Monitoring well (MW-B) was installed downgradient from UST B and the Bunker. :, MW-B was installed in a location to detect migration from both UST B and the Bunker. MW-B 1 has been monitored since September 1998. Monitoring well MW-BR was installed to replace MW-B which was destroyed prior to excavation activities. MW-BR is located approximately 15 feet further downgradient from the location of MW-B and is in a location to detect migration from both UST B and the Bunker. The discussion below illustrates that there is no significant 11 mass of petroleum related constituents remaining in groundwater. Historical data from these wells has been generated by Mr. Minney and are included in Table 7. i 3.4.1 Fate and Transport—UST A i Historical analytical results (Table 7) show that MTBE was detected in the well in 2008 and it was never detected above the taste and odor threshold (RWQCB-SFB, 2008). No other analyzed 11 constituent was reported above laboratory detection limits. The attenuation of MTBE over time{I is graphically presented in Attachment G-1. ' RBCA .Tool Kit for Chemical Releases, Version 2.52 (RBCA Tool Kit) calculations werel.: performed for petroleum hydrocarbons sourced from UST A. The model assumed there was a.I potential off-site residential user at a distance of 1,500 feet. The model projected that petroleum] F` hydrocarbons would attenuate to 0.1 mg/L at 50 feet from the UST A location (Minney, 2010). 3.4.2 Fate and Transport—UST B, Motor/Hydraulic Oil UST and Bunker Monitoring well MW-B was in a position to detect downgradient contamination migration] I sourced from UST B, the Motor/Hydraulic Oil UST, and the Bunker. The replacemenU monitoring well MW-BR was installed directly downgradient from these potential sources. ' p 3.4.2.1 Groundwater Attenuation Projections Historical data collected from MW-B shows that TPHg, TPHd, BTEX, and MTBE were detected; above RWQCB San Francisco Bay Region Environmental Screening Levels (ESLs). No other II ; analyzed constituent was reported above ESLs (Table 7). No analyzed constituents have been detected in the replacement monitoring well MW-BR (Table 6). ! 1 TPHg Regal tory drinking water standards for TPH and petroleum in general have not been developed. i California Region Water Quality Control Board — San Francisco Bay Region used thel groundwater screening level of 100 micrograms per liter (µg/L) (RWQCB-SFB, 2008). For the'! purposes of this assessment the TPH as diesel taste and odor threshold of 100 µg/L was used as, the water quality objective (WQO). `""'�r� TPHg concentrations have been declining as depicted in the projection of TPHg attenuation= it graphs included in Attachment G-2. The first graph indicates TPHg concentrations will continue - 12- �;