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u , `"VII r Second Semiannua/2010 "roundwaterMonitoring Report Former Texaco Service Station Chevron Site ID 309075 November 22, 2011 presented as Figure 1 of Attachment D. The groundwater elevation at MW-1 was not consistent with other groundwater elevations; therefore, the groundwater elevation was not used for contouring nor was it included in the results presented above. r 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 2011 monitoring and sampling event, nor have they historically been observed at the site. 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, 3, 4.1 and 4.2 of Attachment F. A site concentration map of TPH-MO (with 10-gram column silica gel cleanup), TPH-DRO (with 10-gram column silica gel cleanup) 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 2011 are presented in the table below: Frequency Range of California of Detected Primary Frequency of Concentration of Constituent Detection Concentrations MCLS Exceedances Exceedances Above the MDL' (Ng/L)2 (Ng/L) (Ng/L) by Well TPH_M04 7/7 48—990 -- -- __ TPH-DR04 3/7 61 -92 -- -- __ TPH-GRO 1/7 500 -- -- __ TBA 1/7 10 -- -- __ 1,2-DCA 1/7 550 0.5 1/1 550 (MW-1) Notes: 1. MDL=method detection limit. 2.pg/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). Ms. Dawn A. Zemo, P.G., of Zemo & Associates LLC, evaluated the nature of the organic material measured as extractable TPH-MO and TPH-DRO in groundwater samples collected from this site. Ms. Zemo used the chromatograms and laboratory data collected during the first semiannual groundwater monitoring and sampling event and reported findings in the letter report Forensic Evaluation of Organics in Groundwater Measured as Extractable `TPH"dated September 20, 2011 (Attachment G). A summary of the findings are as follows: "The TPH-MO and TPH-DRO, with and without silica gel cleanup, detections being measured in groundwater V g:\aproject\env\chevron\309075\6 deliverables\periodic monitoring\2011\11sa11\temp pdf\text\309075 1sal 1 gmr fin 11222011.doc 5 r