SITE INFORMATION AND CORRESPONDENCE

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Shell No. 204-7524-4404 3 • o uecernoer two 3011 W. Benjamin Holt Drive ckton 2. The lack of data beyond the current monitoring well network (all downgradient wells are impacted by MtBE) shows the extent of the MtBE is not defined laterally in the apparent downgradient direction. Shell claimed that Caltrans probably will not allow an investigation within the right of way of Interstate 5 to complete the delineation of the MtBE plume, although Caltrans did allow installation of monitoring wells in the southbound off-ramp next to the Site. 3. MtBE concentrations in MW-1, MW-2, MW-5, MW-11 and MW-12 continue to rise and fall in a sequence that mimics a moving plume; and do not exhibit a downward trend to meet Water Quality Objectives (WQOs) and restore beneficial uses of groundwater (Graph 1). The data indicate the presence of an unstable groundwater plume with continued leaching of MtBE contamination from Site soil to groundwater. Shell did not provide an estimate of time, or model groundwater for MtBE fate and transport, to show when WQOs would be met. In conclusion, the soil and groundwater results and quarterly monitoring show the following: • Groundwater monitoring results for MtBE were compared from 1996 to 2006, and have not shown a declining concentration trend in all monitoring wells for MtBE. The lack of declining concentrations indicates that there is an ongoing discharge to groundwater from residual soil contamination. A declining concentration Irend is a requirement for a low risk Site closure. Active remediation to address residual soil contamination may necessary to eliminate continued leaching to groundwater. • The vertical extent of the hydrocarbon plume was determined by data from deep monitoring well MW-2-82. The lateral extent of the MtBE groundwater pollution has not been defined in the apparent downgradient direction to the northeast. The groundwater plume may not be stable, and appears to be moving offsite under Interstate 5 towards the northeast. Stable plumes, and definition of vertical and lateral extents, are additional closure requirements. Additional groundwater investigation and continued monitoring are necessary to define the lateral extent of the plume and evaluate the stability of the groundwater plume in the northeastern direction. • Constituents of concern detected in groundwater exceed WQOs (5 ug/L) for MtBE. Therefore, SCEHD's closure recommendation is not appropriate at this time, and I do not concur with the closure recommendation. Regional Board staff discussed their concerns with SJCEHD in meetings on 13 September 2006 and on 13 November 2006. SJCEHD and Regional Board staff were unable to come to an agreement that closure was not appropriate at this time. Attachments