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+ i Report: Groundwater-quality Monitoring—July 30,2003: 7500 West Eleventh Street, Tracy, CSI. Page b rj • 2.0 JULY 30, 2003 GROUNDWATER SAMPLING AND ANALYSIS a On July 30, 2003, the eleventh round of groundwater-quality monitoring was conducted. 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-3, MW-4, MW-5, MW-6, MW-9 and MW-11, 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 free product in it was measured. 2.1 Groundwater Elevations and Flow Direction To initiate the July 30, 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 April 15, 2003, the date on which the depths to groundwater in the monitoring wells were last measured, to July 30, 2003, the groundwater table rose between 0.20 ft. and 0.56 ft in the groundwater-quality monitoring wells. This is 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 0.33 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