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' 4 Report:Groundwater-quality Monitoring—January 26,2004: 7500 West Eleventh Street, Tracy, CA. Page 6 2.0 JANUARY 26, 2004 GROUNDWATER SAMPLING AND ANALYSIS On January 26, 2004, SJC conducted the thirteenth 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-3, MW-4, MW-5, MW-6, MW-7, MW-10 and MW-11, which are at the locations shown on Figure 2, 2.1 Groundwater Elevations and Flow Direction To initiate the January 26, 2004 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 October 29, 2003, the date on which the depths to groundwater in the ion monitoring wells were last measured, to January 26, 2004, the groundwater table rose between 0.03 ft. and 0.15 ft in the 12.groundwater-quality monitoring wells that are screened in the near-surface aquifer beneath the site. (See Figures 3, 4 and 5 for aquifers i-- and well screen intervals.) In the monitoring wells that are screened in the next deepest aquifer(i.e., MW-3A and MW-12A), the piezometric head rose between 0.04 and 0.16 ft. In the deepest aquifer that is monitored at the site (Monitoring Well MW-3B) there was a O-W decrease in piezometric head of 0.10 ft. The observed changes in depths to groundwater in the groundwater-quality monitoring wells that were observed between October 2003 and January 2.004 are consistent with the seasonal pattern of rainfall in the Tracy area. However, it is noted that, as has been previously recorded at this site, agricultural irrigation dominates variations in groundwater elevations 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 usually 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. The groundwater elevations in monitoring wells MW-3A, MW-3B and MW-12A were not considered when the groundwater contours were drawn because, although small in �- sic