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\✓ SNOW, First Semiannual 2012 Groundwater Monitoring Report Former Texaco Service Station Chevron Site ID 309075 June 21, 2012 consistent with other groundwater elevations; therefore, the groundwater elevation was not used for contouring nor was it included in the results presented above. "r Groundwater elevations in site monitoring wells have reached a historic high since monitoring began during the third quarter of 2008. A hydrograph depicting changes in groundwater elevations over time is presented as Figure 1 of Attachment E. Separate phase hydrocarbons (SPH) were not observed during the first semiannual 2012 monitoring and sampling event, nor have they historically been observed at the site. V 5.1.2 Groundwater Sampling Results Analytical results from the most recent semiannual groundwater sampling event are presented in Tables 1.1 and 1.2; historical analytical results are presented in Tables 1, 2 and 3 of Attachment F. A site concentration map of TPH-MO with SGC, TPH-DRO with SGC, and TPH-GRO is presented as Figure 4 and a site concentration map of benzene, MTBE, and 1,2-DCA is presented as Figure 5. Maximum and minimum concentrations of petroleum hydrocarbon constituents detected in groundwater samples collected during the first semiannual event of 2012 are presented in the table below: Frequency Range of California of Concentration of Detected Primary Frequency of Constituent Detection Concentrations MCLS Exceedances Exceedances Above the MDL' (Ng/L)2 (Ng/L) (N9/L) by Well `r TPH-M04 4/7 77- 180 -- -- -- TPH-DR04 2/7 110- 180 -- -- -- "' TPH-G RO 117 99 -- -- Benzene 1/7 2 1 1/1 2 (MW-6D) TBA 2/7 2- 12 -- -- 1,2-DCA 1/7 230 0.5 1/1 230 (MW-1) Notes: 1. MDL=method detection limit. 2. Ng/L= microgram per liter, equivalent to part per billion(ppb). 3. MCL=maximum contaminant level. 4. TPH-MO/TPH-DRO with silica gel cleanup(10-gram_ column). l F The groundwater sample collected from MW-6DD.ontained a concentration of benzene above the California Primary maximum contaminant-level (MCL). Benzene has not been detected V above the California Primary MCL in other site wells and was last detected at the same concentration in 2010 from MW-6D. The groundwater sample collected from MW-1 contained a concentration of 1,2-DCA above the California Primary MCL. 1,2-DCA is stable, not migrating "d and has not been detected in other site wells since groundwater monitoring began in 2006. g:taproject\env\chevron\309075\6 deli verables\periodic monitoring\2012\1sa12\text\309075 1sa12 gmrfln 06212012.doc 5