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-quality Monitoring—October 25, 2002: 7500 West Eleventh Street, Tracy, CA Page 6 Report. Groundwater 2.0 OCTOBER 25, 2002 GROUNDWATER SAMPLING AND ANALYSIS _ On October 25, 2002, the eighth 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, which are at the locations shown on Figure 2, were sampled. j, 2.1 Groundwater Elevations and Flow Direction To initiate the October 25, 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. '7 In the period between July 29, 2002, the date on which the depths to groundwater in the monitoring wells were last measured, and October 25, 2002, the groundwater table had ` . fallen, on average, 1.05 ft. However, the fall in the water table was not uniform over the whole of the area monitored. Groundwater fell more that 1.29 ft. in Monitoring Well MW-12, which is located close to the southern boundary of the West Eleventh Street property, but it fell no more than 0.70 ft. in Monitoring Well MW-11, located to the north of West EIeventh Street at the northern down-gradient limit the of the primary plume of affected groundwater. The type of seasonal change in groundwater elevation noted above commonly occurs in the Tracy area. It is not directly related to changes in local weather conditions, but is caused by agricultural irrigation practices. When crops are being irrigated, very large volumes of water are imported to the area via the Delta-Mendota canal, and water 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 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 I shows that the fall in the groundwater table elevation beneath the 7500 West Eleventh Street site seen between the groundwater monitoring round conducted on July 29 and that of October 25, 2002 was preceded by a mean rise of 0.90 ft. in the water table elevation t' that occurred over the period from April 11 to July 29, 2002, during which time no precipitation fell anywhere 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 0.48 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 ,4 Assume Specific Gravity of Gasoline=0.74 sic