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------------ Report:Groundwater-quality Monitoring—October 20-21,2005, 7500 West Eleventh Street, Tracy, CA. Page 9 4 `7':.; 2.0 OCTOBER 2005 GROUNDWATER-QUALITY MONITORING ROUND 1'f On October 20 and 21, 2005, SJC conducted the nineteenth round of groundwater-quality monitoring at the 7500 West Eleventh Street site. The depth to groundwater was -j measured in all of the 27 groundwater quality and floating product monitoring wells r' ? extant on the site. The scope of groundwater sampling conducted in October 2005 was designed to i incorporate both the findings from the natural attenuation studies that had been conducted on the site in March 2005 and the results of a Monitoring and Remediation Optimization System(MAROS) study, which was also conducted at that time in the manner advocated by the United States Environmental Protection Agency (US-EPA) and the California Environmental Protection Agency (Cal-EPA) (The San Joaquin Company 2006). Although the results of the MAROS and natural attenuations studies called for quarterly recovery of groundwater samples from only Monitoring Wells MW-3, MW-7 and MW- 13, SJC, to ensure an abundance of conservatism, recommended that, for a period of one year following the March 2005 sampling round, additional wells located along the long axis of the primary plume, at key locations on its western and eastern periphery and in the secondary plume of affected groundwater that is present on the site should be sampled. Accordingly, the array of wells sampled in the October 2005 monitoring round included Monitoring Wells MW-2, MW-3, MW-4, MW-5, MW-7, MW-10, MW-11, MW-12, MW-13, MW-16, MW-17, MW-18, MW-19, MWFP-1, MWFP-2, MWFP-4 and MWFP- <:"1 5 (see Figure 3 for locations). 2.1 Groundwater Elevations and FIow Direction To initiate the October 2005 sampling program, the depth to groundwater in each of the 27 monitoring wells was measured using a conductivity probe on October 20, 2005. The water table elevations were computed relative to mean sea level(MSL). E�1 The groundwater elevations presented in Table 1 were used to generate the groundwater contours shown on Figure 2. However, because, as is noted in Table 3, an apparent 0.14 ft. of floating product was detected in Monitoring Well MW-7, the groundwater elevation in that well was corrected for the purpose of drawing the groundwater contours to allow ': _( for the depression of the water table in the vicinity of that well by the hydrocarbon fuel products floating on the surface of the water. The correction was derived as follows: Assume Specific Gravity of Diesel=0.84 �. Assume Specific Gravity of Gasoline= 0.74 I, t For the purpose of computing the required adjustment, it was assumed that relative proportions of diesel and gasoline in the floating product in Monitoring Well MW-7 are in the same ratio as the concentrations of diesel and gasoline in the sample of groundwater recovered from that well on March 9, 2005. They were,respectively, 18,000 µg/L and 15,000 µg/L. (See Table 2.) i SJC �` 1