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..r NNW, ..r' <br /> First Semiannual 2012 Groundwater Monitoring Report <br /> Former Texaco Service Station <br /> `r Chevron Site ID 309075 <br /> June 21, 2012 <br /> • Methyl tertiary butyl ether (MTBE), tertiary butyl alcohol (TBA), di-isopropyl ether (DIPE), <br /> ethyl tertiary butyl ether (ETBE), tertiary amyl methyl ether (TAME), <br /> 1,2-dichloroethane (1,2-DCA), ethylene dibromide (EDB), also known as <br /> 1,2-dibromoethane, and ethanol by USEPA Method 8260B <br /> • Methanol by USEPA Method 8015B <br /> Quality assurance/quality control (QA/QC) samples, inclusive of trip blanks, were submitted for <br /> laboratory analysis. A laboratory supplied trip blank accompanied each sample delivery group. <br /> Trip blank samples were analyzed for TPH-GRO and BTEX. Analytes were not detected in the <br /> trip blank at concentrations at or above the method detection limits (MDL). The laboratory <br /> analytical report and chain-of-custody record for the semiannual groundwater sampling event <br /> ,r are included in the Lancaster analytical results package (Attachment C). <br /> 4.3 Remedial System Status <br /> A remedial system is not associated with the site. <br /> 4.4 QA/QC <br /> Quality assurance/quality control (QA/QC) samples, inclusive of trip blanks, were submitted for <br /> laboratory analysis. A laboratory supplied trip blank accompanied each sample delivery group. <br /> Trip blank samples were analyzed for TPH-GRO, BTEX and MTBE. <br /> 5.0 DISCUSSIONS, CONCLUSIONS AND RECOMMENDATIONS <br /> V <br /> 5.1 Discussion <br /> 5.1.1 Groundwater Monitoring Results <br /> Depth-to-water measurements were subtracted from surveyed top of casing elevations to <br /> calculate the groundwater elevation at each monitoring well. Depth-to-water measurements and <br /> calculated groundwater elevations are presented in Table 1.1. Calculated groundwater elevation <br /> data was used to construct a groundwater elevation contour map of the site, presented as <br /> Figure 3. <br /> Groundwater elevations across the site ranged from 18.85 (MW-6D)to 19.07 (MW-3) feet below <br /> mean sea level (BMSL). On average, groundwater elevations at site monitoring wells decreased <br /> 2.28 feet from the second semiannual 2011 monitoring event. The groundwater flow direction <br /> across the site was toward the northeast at an approximate horizontal hydraulic gradient of <br /> 0.002 feet per foot (ft/ft), as shown on the groundwater elevation contour map presented as <br /> Figure 3. The predominant historical groundwater flow directions across the site have been <br /> toward the southwest and east-northeast, as depicted on the groundwater flow direction rose <br /> diagram presented as Figure 1 of Attachment D. The groundwater elevation at MW-6 was not <br /> g:laprojed\envlchevron130907516 deliverables\periodic monitoring\2012\1sa12\teM\3090751sa12 gmr fin 06212012.doc `/ 4 <br />