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Report:Groundwater-quality Monitoring—October 27, 2003: 7500 West Eleventh Street, Tracy, CA. Page 6 2.0 OCTOBER 27, 2003 GROUNDWATER SAMPLING AND ANALYSIS On October 27, 2003, SJC conducted the twelfth round of groundwater-quality monitoring at the 7500 West Eleventh Street site. In accordance with the scope of work directed by the SJCEHD (San Joaquin County Environmental Health Department 2003), the depth to groundwater was measured in all 15 extant wells but sampling for groundwater-quality analysis was conducted in only Monitoring Wells MW-2, MW-3, MW-3-A, MW-4, MW-5, MW-6, MW-9, MW-10, MW-11 and MW-12, which are at the locations shown on Figure 2. Because free product was floating on the surface of the �.. groundwater in MW-7, no sample was recovered from that well but the depth of floating product in it was measured and recorded. ,,. 2.1 Groundwater Elevations and Flow Direction To initiate the October 27, 2003 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 from July 30, 2003, the date on which the depths to groundwater in the monitoring wells were last measured, to October 27, 2003, the groundwater table fell +� between 0.81 ft. and 1.32 ft in the groundwater-quality monitoring wells. The observed changes in depths to groundwater between July 30 and October 27, 2003 are consistent with the seasonal pattern 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 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 that the rise and fall in the groundwater table elevation beneath the 7500 West Eleventh Street site usually occurs with little or no relationship to seasonal precipitation in the San Joaquin Valley where little rain falls between May and September of each year. "� 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 0.55 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 sic