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SECOR Elbenzene Water Quality Source Trigger Well Objective Compliance Limits (EgIL) Chemical Constituents OEHHA 0.15 Chemical Constituents USEPA 29 Chemical Constituents USEPA 42 Chemical Constituents USEPA 17 MTBE Chemical Constituents CA DHS, Secondary 5.0 MCL Constituent Water Quality Source Remedial Objective Observation and Mid- point Well Compliance Limits /L Benzene Chemical Constituents California Primary MCL 1.0 Eth Ibenzene Chemical Constituents California Primary MCL 700 Toluene Chemical Constituents California Primary MCL 150 Xylenes Chemical Constituents California Primary MCL 1,750 MTBE Chemical Constituents California Primary MCL 13 • For the "A" zone and "B" zone trigger wells, if concentrations detected in these wells exceed the above compliance concentrations for three consecutive quarters, SECOR will implement the contingency plan outlined in the June 27, 2003 letter referenced above. The evaluation of first quarter 2006 results is presented in the following section. Analytical data from the ozone injection well monitoring program is summarized in Table 11. 9.4 Evaluation of Groundwater Concentration Trends During the fourth quarter 2005, SECOR collected pre-purge and post-purge dissolved oxygen (DO) and oxidation reduction potential (ORP) readings from the wells listed in Section 9.3. A summary of these results and laboratory analyses for these wells are included in Table 11. The concentrations reported during the first quarter 2006 were compared with the compliance values for the remedial observation and mid-point observation wells stated above. The benzene concentrations in "B" zone remedial observation well AR/MW-18B (3.7 pg/1) and PS/WC-1M (540 pg/1) exceed the benzene compliance concentration of 1.0 pg/1. The MTBE concentration in "A" zone remedial observation well PS/MW-18 (24 pg/1) and in the "B" zone remedial observation well PS/WC-1M (51 pg/1) exceed the MTBE compliance level of 13 pg/I. Dissolved hydrocarbon concentrations in the mid-point and trigger wells were below compliance concentrations during first quarter 2006. Concentrations of reported analytes decreased or remained the same in "A" zone remedial observation wells PS/MW-18, PS/WC-1S, and AR/MW-21A, "B" zone remedial observation well AR/MW-21B, "B" zone mid-point observation well PS/MW-20, "A" zone trigger wells OW-2A and • OW-4A, and "B" zone trigger wells OW-2B and OW-4B. Benzene increased slightly in well STTC IQ06 Final Report 19 May 1,2006 770T.43673.12.0002