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Semi-Annual Groundwater Monitoring and Quarterly Remediation Progress Report—First Quarter 2013 April 30,2013 Project#52.75118.2390 Page 6 Dissolved MTBE Distribution MTBE has historically been detected in C-Zone well MW-11 with the highest concentration reported at 310 pg/L in February 2002. During the First Quarter 2013, MTBE was detected in C-Zone well MW-11 at 3.8 pg/L. Dissolved TBA Distribution TBA has historically been detected in C-Zone well MW-11 with the highest concentration reported at 26,000 pg/L in February 2007. During First Quarter 2013, TBA was detected at a concentration of 9,800 pg/L in C-Zone well MW-11. Off-Site Contaminant Distribution TPPH, BTEX, MTBE, and TBA were not detected off-site within C-Zone wells S-16 land S-17 according to the CRA provided sampling results. 4.4 PURGE AND RINSATE WATER DISPOSAL i Three 55-gallon drums of purged groundwater were generated during the First Quarter 2013 groundwater sampling event, and were temporarily stored on-site. The drums were removed by an approved waste hauler on March 13, 2013 and disposed of at the Crosby and Overton disposal facility in Long Beach, California. 4.5r SUMMARYiRECOMMENDATIONS� Groundwater monitoring wells MW-3 and MW-7, immediately north of the former UST complex, had historically contained the highest concentrations.of petroleum hydrocarbons beneath the .site. The highest remaining petroleum hydrocarbon and fuel oxygenate impacts at the site appear to be observed in C-Zone monitoring well MW-11 with GRO detected at 260 pg/L, TBA detected at 9,800 pg/L, and ETBE detected at 0.76 pg/L during the First Quarter 2013. A review of historical groundwater.monitoring results and historical monthly groundwater analytical results for well MW-3 and historical monitoring results for well MW-7 since the start-up of the OS system in December 2002 indicate significant decreases in MTBE concentrations to low to non-detectable concentrations; decreasing TBA concentration trends have also been observed in these wells. Decreasing TBA concentration trends, and non-detect MTBE have also been observed in other A-Zone wells located in the vicinity of the OS wells (the OS wells are all screened within the A-Zone), indicating the OS systeml was effective in mitigating constituents of concern in the A-Zone on-site and additional OS is not needed. 4 Cardno recommends evaluating the site for closure. i i i i I I