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A M B R I A Figure 4 illustrates an increasing TBA trend corresponding to a decreasing MTBE trend in well MW4 following each GWE event A similar trend is observed in piezometer P-1 (Figure 5) Cambria anticipates an initial increase in TBA concentrations in the area around the USTs as MTBE continues to naturally degrade The TBA plume remains localized around the area of the USTs, does not appear to be migrating, and will also degrade once MTBE is diminished WORK PLAN Interim Remediation. In the August 17, 2004 letter, SJCEHD requests that Shell conduct interim remediation to reduce the dissolved mass and to establish hydraulic control at this site As documented above, over 8 pounds of MTBE have been removed from the site Subsurface and MTBE concentrations have been reduced dramatically following the interim GWE Shell conducted These data suggest that this objective of interim remediation has been achieve Groundwater drawdown data and the transmissivities derived from both the October 2002 aquifer pump test and September 2003 GWE event demonstrate that it ma not be economically feasible to achieve hydraulic control at the site Additionally, as Cambria has noted previously, because the nearest receptor is located 0 45 miles from the site, there is no immediate or imminent threat to a sensitive receptor As discussed above and in SJCEHD's August 17, 2004 letter, TBA is an intermediate biodegradation product of MTBE Shell Global Solutions (US), Inc , a research division of Shell, conducted a study of TBA at selected UST sites in Orange County, California Their final report dated November 14, 2003, and slides from a corresponding presentation are included herein as Appendix C, for reference In addition to discussing the possible origins of TBA in groundwater, this report shows ',J3 —biodegradatioir—will—Iikely---follow--MTBE— biodegradation It is typical for TBA to increase as MTBE decreases Then, following a lag time, the TBA will also decrease Groundwater analytical data indicate that this is occurring at this site (see results for MW-1 and P-2 on Figure 5) Also, historical groundwater analytical data for all the wells are included as Appendix D Groundwater samples collected from wells MW-1 through MW-4, MW-7, and MW-11 and piezometers P-1 and P-2 have historically contained MTBE and TBA MTBE concentrations demonstrate a decreasing trend in all of these wells TBA concentrations are also decreasing (or have decreased to below detection limits) in wells MW-1, MW-2, MW-3, MW-7, MW-11, and P-2 TBA concentrations are currently increasing in only MW-4 and P-1, which are located in the presumed source area As discussed above, we anticipate a decrease in TBA concentrations ito follow the decrease in MTBE concentrations already witnessed in these wells 0955 8