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Report; Groundwater-quality Monitoring—April 15,2003: 7500 West Eleventh Street, Tracy, CA. Page 6 ,. .,� 2.0 APRIL 15, 2003 GROUNDWATER SAMPLING AND ANALYSIS On April 15, 2003, the tenth round of groundwater-quality monitoring was conducted. Monitoring Wells MW-1 through MW-3,MW-3A, MW-3B, MW-4 through MW-12, and MW-12A, which are at the locations shown on Figure 2, were sampled. i. '- 2.1 Groundwater Elevations and Flow Direction To initiate the April 15, 2003 sampling program, the depth to groundwater in each of the �:_j fifteen monitoring wells was measured using a conductivity probe. The water table elevations were computed relative to mean sea level (MSL). Those measurements and j the computed groundwater table elevations are recorded in Table 1, i In the period from January 20, 2003, the date on which the depths to groundwater in the monitoring wells were last measured, to April 15, 2003, the groundwater table rose between 0.04 ft. and 0.54 ft in the groundwater-quality monitoring wells. This is " consistent with the seasonal patter of agricultural irrigation that dominates groundwater =� levels in the area around Tracy. When crops are being irrigated, very large volumes of water are imported to the fields around Tracy via the Delta-Mendota canal, and water pumped from deep agricultural wells is being discharged to shallow aquifers. This activity begins in the spring of each year and typically continues through early- to mid-September, when it stops and ' J1> groundwater levels in shallow aquifers begin to fall. Except in unusually wet winters, levels continue to fall until some time in February or March. Inspection of Table 1 shows i that the rises and falls in the groundwater table elevation beneath the 7500 West Eleventh Street site usually occur with little or no relationship to seasonal precipitation in the San Joaquin Valley. 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 0.52 ft. of floating product was detected in Monitoring Well MW-7, the groundwater elevation :Y 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 ` a Assume Specific Gravity of Gasoline=0.74 I 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, 500,000 µg/L and 61,000 µg/L. (See Table 2.) Thus, the estimated specific gravity of the floating product in well MW-7 is: sic