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Stal tec • September 15, 2008 Page 6 of 9 Reference: Response to RWQCB July 30, 2008 Letter November 2006, and from August 2007 to February 2008. The February 2008 detection of 9.4 ug/L is the highest TBA concentration ever observed in AR/MW-18B." Regarding the history of analytical data for well AR/MW-18B, the well was installed in third quarter 2002 and first sampled in fourth quarter 2002. The initial analyses included TPH, BTEX, and fuel oxygenates. Low levels TPHd (61 ug/L; anomalous chromatogram) and benzene (2.0 ug/L) were detected in this first event; and all seven fuel oxygenates were below normal reporting limits. In accordance with the existing Monitoring and Reporting Program (MRP) 5-01-819, analyses for fuel oxygenates (except MTBE) were discontinued based on these results. The replacement MRP (R5- 2004-0823), did not require fuel oxygenate analyses if they had already been discontinued based on analyses collected under the previous MRP. This well (AR/MW-1813) was analyzed again for the seven oxygenates during fourth quarter 2005 to evaluate groundwater quality in this area. This is the last sampling date included in the pre-OS system historical average that constitutes the compliance limits for remedial observation wells. A TBA concentration of 8.2 ug/L was detected in fourth quarter 2005; and consistent with the MRP, quarterly analysis for fuel oxygenates were subsequently started again at this well (with the exception of one quarterly event). In the ten subsequent sampling events to date, TBA has been detected eight times at concentrations near the reporting limit (5.0 ug/L), ranging from 5.2 to 9.8 ug/L. These concentrations are marginally different than the reporting limit of 5.0 ug/L and do not display a clear increasing trend that would warrant modifying the established trigger compounds in the EMP. Statistical analysis of this small data set with tolerance intervals methods (assuming a normal distribution) indicates these detections are not statistically different than the reporting limit of 5.0, even at the low confidence of 90 percent. Further, the 11 quarterly events in the gap between the first and second analysis at well AR/MW-18B would likely display a normal data scatter similar to the ten more recent events. This would increase the pre-OS system historical average to above the reporting limit of 5.0 ug/L. In addition, as discussed above, the vast majority of TBA in the plume and offsite areas is the result of incomplete biodegradation. Therefore, these detections near the reporting limit are more likely evidence of in-situ biodegradation than of the failure of the barrier arrays. In the RWQCB's July 10, 2008 email to Stantec, the RWQCB recommended establishing 12 ug/L as the TBA compliance concentration for the trigger wells and if it was exceeded for three consecutive quarters it would require implementation of the contingency plan. It is important to understand that the implementation of the contingency plan for the remedial observation wells and for the trigger wells was always envisioned as a phased implementation. As the plume potentially moved further and further away from the terminals on Port of Stockton property, it would first potentially result in implementing the contingency plan for the remedial observation wells. Then, if off-site migration continued to the vicinity of the trigger wells and the trigger well criteria were exceeded, it would potentially result in the implementation of the contingency plan for the trigger wells. This is documented in SECOR's June 27, 2003 Response and Addendum in the section entitled Contingency Plan as repeated below with underlining added for clarity. I:\STTC-Stockton\Reports\Resp to RWQCB July 30,2008\Response to RWQCB July 30,2008 Letter fnal.doc