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Statrtec � • September 15, 2008 Page 5 of 9 Reference: Response to RWQCB July 30, 2008 Letter TBA AS A COMPLIANCE ANALYTE As mentioned above in the Introduction Section, the second issue relates to RWQCB concerns regarding TBA and recommendations that it be included in the above-described list of compliance analytes. Because the objective of the EMP was to monitor the potential off-site migration of the hydrocarbon plume (by comparisons with pre-OS system historical averages) the compliance analytes including the highly mobile MTBE are more than adequate. There are several reasons for not including TBA within the compliance analytes. One important reason is the lack of an adequate data set. Monitoring and Reporting Program (MRP) 93-807 did not require analyses for fuel oxygenates. This MRP was replaced by MRP 5-01-819 (effective March 28, 2001), which required an initial round of fuel oxygenate analyses that were to be discontinued if results were non-detect. The four remedial observation wells (PS/WC-1S, PS/WC-1M, PS/MW-18, and PS/MW-20) in existence when MRP 5-01-819 was issued were actually sampled in February 2001, prior to the effective date. The remaining three remedial observation wells (AR/MW-18B, AR/MW-21A, and AR/MW-21B) were installed and sampled in fourth quarter 2002. Therefore, only 2 of the 7 remedial observation wells (PS/MW-18 and PS/WC 1 M) have sufficient data to allow calculation of representative pre-OS system historical averages. Of the remaining five wells, three have been analyzed for fuel oxygenates (other MTBE) 2 out of 13 quarters; and two wells have been analyzed for fuel oxygenates 4 and 5 out of 17 quarters, respectively. Therefore, the historical data sets are inadequate to provide representative pre-OS sparge system historical averages and are in compliance with previous MRPs and the current MRP R5-2004-0823. From a geochemical perspective, TBA is not a primary component of gasoline, such as BTEX, and is not added to gasoline as a fuel oxygenate, such as MTBE or ethanol. TBA is an incidental byproduct of the production of MTBE and is only present in some gasolines at trace amounts. Typically, the manufacturing process for MTBE (variably present at zero to ten percent in gasoline) results in TBA at a fraction of one percent of the MTBE present. More importantly, the vast bulk of TBA present in a groundwater plume is the byproduct of incomplete biodegradation of MTBE. Therefore, adding TBA to the compliance analytes for either the remedial observation wells or the trigger wells does not achieve the objective of the EMP, to monitor the effectiveness of the OS barrier arrays. Because TBA is only present in some gasolines and at trace levels (less than a fraction of one percent of the MTBE present) increases in TBA concentrations in remedial observation or trigger wells are much more likely to document the accumulation of biodegradation byproducts of MTBE, than the failure of the barrier array system. Furthermore, MTBE and TBA were present downgradient from the East System ozone barrier array prior to its installation. Near remedial observation wells PS/MW-18 (A-zone) and PS/WC-1M (B-zone), MTBE had ranged from 55 to 120 ug/L and TBA from 11 to 2,700 ug/L prior to installation of the array. Therefore, increases in TBA in trigger wells further downgradient could be due to the migration of TBA already downgradient of the arrays or biodegradation of MTBE already downgradient of the arrays, rather than indicating the OS barrier array are not effective. The quarterly evaluation of existing compliance analytes as part of the EMP is more than adequate to achieve this objective. Also, Comment Number 2 of the July 30, 2008 RWQCB letter requested a discussion of historical and recent TBA data. The RWQCB pointed out that in remedial observation well AR/MW-186, "...The data trends show that TBA sustained concentration increases over three consecutive quarters, in excess of the only existing historical data point of <5.0 ug/L, from May 2006 to lASTTC-Stockton\Reports\Resp to RWQCB July 30,2008\Response to RWQCB July 30,2008 Letter final.doc