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Statttec • s September 15, 2008 Page 4 of 9 Reference: Response to RWQCB July 30, 2008 Letter Groundwater Zone Remedial Observation Wells Mid-Point Trigger Observation Wells Wells A PS/MW-18, PS/WC-1S, OW-2A, AR/MW-21A None OW-4A B AR/MW-186, AR/MW-21 B, PS/MW-20 OW-26, PS/WC-1 AA OW-48 The compliance concentration limits for the trigger wells were originally agreed to be the California primary maximum contaminant levels (MCLs) as shown in the table below from the 2003 Response and Addendum: Constituent Water Quality Source Limit in pg/L Ob'ective Benzene Chemical Constituents California Primary MCL 1.0 Ethytbenzene Chemical Constituents California Primary MCL 700 Toluene Chemical Constituents California Primary MCL 150 X enes Chemical Constituents California Primary MCL 1,750 MTBE Chemical Constituents California Primary MCL 13 As documented in 2003 Response and Addendum, it was also agreed that because of the long industrialized history of the Port of Stockton, the distance from the source areas, and concentrations of total petroleum hydrocarbons (TPH) already detected in the trigger wells, TPHd and TPHg would not be included as compliance analytes for the trigger wells. Subsequently, a January 6, 2005 electronic communication from the RWQCB indicated that consistent with RWQCB policy, a more appropriate list of compliance limits for the trigger wells would be the California water quality objectives shown below. The water quality objectives are the lowest of many various numerical and narrative limits designed to protect all designated beneficial uses for a particular water body and constituent. Constituent Water Quality Source Trigger Well Compliance Objective Limits /L Benzene Chemical Constituents OEHHA 0.15 0.5 default level Ethylbenzene Chemical Constituents USEPA 29 Toluene Chemical Constituents USEPA 42 Xylenes Chemical Constituents USEPA 17 MTBE Chemical Constituents CA DHS, 5.0 Seconder MCL Note: The default compliance limit for benzene is the method reporting limit of 0.5 pg/L per RWQCB electronic communication dated January 6, 2005. Thus, BTEX and MTBE have always been the compliance analytes for the EMP at the remedial observation and trigger wells, and the above water quality objectives were established as compliance concentration limits for the trigger wells. The objective of the EMP was to evaluate the effectiveness of the barrier arrays. These compliance analytes and compliance limits have been evaluated in all quarterly monitoring reports since the first required evaluation conducted in third quarter 2006, consistent with the 2003 Response and Addendum. I:\STTC-Stockton\Reports\Resp to RWQCB July 30,2008\Response to RWQCB July 30,2008 Letter fnal.doc