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Mr. Sam Brenneke, Shell Oil Products US - 2 - <br /> 6 July 2012 <br /> Shell Stockton Terminal <br /> conducted liquid phase hydrocarbon recovery; installed and operated a groundwater <br /> extraction system; installed and operated a soil vapor extraction (SVE) system; <br /> conducted oxygen injection; installed and operated a dual phase extraction (DPE) <br /> system; and installed and operated an air sparge (AS) system. <br /> The FS/CAP evaluated several potential remedial alternatives at the Site including DPE, <br /> AS/SVE, excavation, and monitored natural attenuation (MNA). DPE was eliminated <br /> from further consideration because of the difficulty in disposing of the effluent water, <br /> which contains elevated levels of tert butyl alcohol (TBA). In 2009, Central Valley Water <br /> Board staff approved a work plan for the installation of an SVE/AS system at the Site. <br /> Shell later concluded that the installation of the system is not justified given the lack of <br /> complete exposure pathways. Excavation was removed from consideration because <br /> the Site is an operating facility. The FS/CAP recommends MNA. <br /> The Letter responds to the Board staff 19 April 2012 letter regarding the trend graphs <br /> included in the FS/CAP. The Letter provides the rationale for the use of partial data <br /> sets to evaluate the concentration trends. Shell states that the trend analyses and <br /> subsequent estimates of when petroleum hydrocarbon concentrations will reach water <br /> quality objectives (WQOs) presented in the FS/CAP are accurate. The Letter states <br /> that "if the source of the release is finite the concentration will eventually decrease from <br /> the maximum [concentration] to below the concentration of concern." Shell created <br /> degradation trend graphs for each well using the maximum detected concentration <br /> through the most recent sampling date. The Letter states that the "concentrations may , <br /> increase one or more times before showing consistent attenuation towards the <br /> concentration objective." Shell used the graphs to estimate when petroleum <br /> hydrocarbons will reach WQOs. <br /> Our comments are: <br /> 1. The Site has been a fuel terminal since the 1940s, wells were installed around <br /> 1994 and several wells contained concentrations above the WQO during well <br /> installation. Therefore, the first arrival time for each well is unknown and the <br /> overall maximum concentration may also not be known. The Site soil contains <br /> petroleum hydrocarbons that may continue to leach into the groundwater. <br /> Central Valley Water Board staff concur that time is required for the analyte to <br /> reach the receptor; however, the Site has stored fuel for about 70 years, and the <br /> arrival time at each receptor is unknown. Board staff believes Shell should <br /> include all available data in the trend graphs unless there is justification for using <br /> a subset of data (e.g., remedial measures impacted the rate to reach WQOs). <br /> The Letter does not contain sufficient justification for the data sets used by CRA <br /> to estimate the time to reach WQOs. <br /> 2. Central Valley Water Board staff do not concur with the degradation estimates <br /> presented in the FS/CAP. Overall, Shell underestimates the time required to <br />