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r� Report;Groundwater-qualityMonitoring—July 29,2002: 7500 West Eleventh Street, Tracy, CA, Page b �a �(. 2.0 JULY 29, 2002 GROUNDWATER SAMPLING AND ANALYSIS i On July 29, 2002, the seventh round of groundwater-quality monitoring was conducted. Monitoring Wells MW-1 through MW-3, MW-3A, MW-313, MW-4 through MW-12 and MW-12A, at the locations shown on Figure 2 were sampled. r , 2.1. Groundwater Elevations and Flow Direction To initiate the July 29, 2002 sampling program, the depth to groundwater in each of the fifteen monitoring wells was measured using a conductivity probe. The water table elevations were computed relative to mean sea level (MSL). Those measurements and the computed groundwater table elevations are recorded in Table 1, In the period between April 11, 200, the date on which the depths to groundwater in the monitoring wells were last measured, and July 29, 2002, the groundwater table had risen, on average, 0.9 ft. However, the rise in the water table was not uniform over the whole of the area monitored. Groundwater rose more that 1 ft. in the monitoring wells along and to the south of West Eleventh Street, but it rose no more than 0.62 ft. in the northern area of the primary plume of affected groundwater It is noteworthy that the elevation of the groundwater table rose over the period April 11 to July 29, 2002 even though no precipitation fell anywhere in the Sa Joaquin Valley during that time. Such changes in groundwater elevation unrelated to weather conditions are commonly seen in the Tracy area and are caused by importation of large volumes of water to the area via the Delta-Mendota canal and by water from deep agricultural wells being discharged to shallow aquifers in the seasons when crops require irrigation. 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 2, an apparent 0,18 f 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 !: I products floating on the surface of the water. The correction was derived as follows: Assume Specific Gravity of Diesel=0.84 r 1 Assume Specific Gravity of Gasoline= 0.74 Assume 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 December 13, 2001. They were, respectively, 800,000 gg/L and 61,000 gg/L. (See Table 2.) Thus, the estimated specific gravity of the floating product in well MW-7 is: } pp = 0.83 sic I