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P CQ)H 53 Dissolved Concentration Trends SECOR constructed Concentration versus Time Graphs for MW-1 through MW-5 and MW- 6D, which depict the decreasing trend of MtBE and TBA concentrations in groundwater over time The only well to show any increase in concentrations is MW-2 for which MtBE concentrations have increased from 1,700 ug/L in October 2003 to 2,900 ug/L in July 2004 Groundwater concentrations from MW-7 through MW-9 were not graphed as there are not enough data points 6.0 ADJACENT LEAKY UNDERGROUND STORAGE TANK SITES ISECOR reviewed SJCEHD files to obtain information related to leaky underground storage tank (LUFT) sites located near the subject site A total of four LUFT sites were found during ' the file review 0 2420 Grant Line Road, Grant Line Food and Gas, • 2375 Grant Line Road, Shell Gasoline Station, ' • 2450 Toste Road, Former Curtiss Pontiac Buick, and • 2353 Toste Road, Tracy Marine and Sales ' Due to the proximity of the Shell and Grant Line Food and Gas sites, their groundwater monitoring wells and the 7-Eleven groundwater monitoring wells were surveyed jointly and plotted on an aerial photograph (Figure 2) The well locations are also shown on Figure 2 Details related to the two adjacent sites are presented below 2420 Grant Line Road — Former Grant Line Food and Gas The site is an active Valero gas station with eight monitoring wells 1MW-1 through MW-8 (Figure 2) Valero is not responsible for the release or the groundwater wells associated with the Former Grant Line Food and Gas Groundwater monitoring and sampling have been performed since at least 1994, according to Advanced GeoEnviron mental, Inc 's I Quarterly Report, First Quarter 2003, dated May 21, 2003 (Attachment G) Depth to water at the site has ranged between approximately 8 and 10 5 feet bgs The groundwater flow direction at the site has been to the north-northwest for ten of the twelve monitoring events provided in the quarterly report The maximum concentration of dissolved MtBE was reported at 360 uglL in well MW-8 located northeast of the underground storage tanks (Figure 2) The groundwater has not been monitored or sampled since the third quarter 2003 • 2375 Grant Line Road — Shell Gasoline Station The site is an active Shell gas I station with four USTs and four groundwater monitoring wells (MW-1 through MW-4) Monitoring and sampling have been performed since February 2003 The site is located north (downgradient) of the 7-Eleven site across Grant Line Road Based on ' the third quarter 2004 groundwater monitoring report (Appendix G), and the groundwater gradient map as calculated by SECOR (Figure 6), the groundwater gradient at the site is northward In the third quarter 2004, maximum MtBE concentrations were reported in well MW-1, located north (downgradient) of Shell's USTs, at 2,600 pg/L MW-4, located upgradient of Shell's USTs, contained 45 pg/L of MtBE (Figure 10) 32262 Assessment&Feasibility Report&SCM doc g September 30,2004 I77EL 32262 04 0502