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Report:Groundwater-quality Monitoring—July 14-15,2006, 7500 West Eleventh Street, Tracy, CA. Page 9 2.0 JULY 2006 GROUNDWATER-QUALITY MONITORING ROUND On July 14-15, 2006, SJC conducted the twenty-second round of groundwater-quality monitoring at the 7500 West Eleventh Street Site. 2.1 Groundwater Elevations and Flow Direction Prior to recovery of samples of groundwater from the monitoring wells, the depth to groundwater in 26 of the 27 monitoring wells at the site was measured using a conductivity probe on July 8, 2006. The water table elevations were computed relative to mean sea level (MSL). Depth to groundwater measurement was not made in Monitoring Well MW-2 because, at the time that the field work was conducted, it could not be found. It apparently has been buried under rock paving laid down for the new Arch Lumber, Inc. storage lot that has been established in the 7500 West Eleventh Street property. In the period from March 29, 2006, the date on which the depths to groundwater in the monitoring wells were last measured, to July 8, 2006, the groundwater table in the majority of wells that are screened in the near-surface aquifer beneath the site rose some 0.5 to 0.9 ft., although there was an apparently anomalous rise of 1.24 ft. in the groundwater table in Monitoring Well MW-5. The rising groundwater reached its highest elevations recorded at the site since June 2005, which is consistent with the response of the groundwater regime to regional agricultural irrigation processes that result in the �t water table in the shallow aquifer being at its highest in June or July but at its lowest in February or March (The San Joaquin Company 2000b). However, in Monitoring Well MW-16, the elevation of the groundwater fell 4.40 ft. compared to its elevation when measured March 29, 2006. That phenomenon is related to the waxing and waning of a groundwater mound located in the area around that well, which is discussed in Section 2.1.1, below. In the monitoring wells that are screened in the first aquifer that is located deeper than the near-surface aquifer (i.e., Monitoring Wells MW-3A and 12A), the piezometric head rose an average of 0.9 ft. In the deepest aquifer that is monitored at the site (by Monitoring Well MW-3B), the piezometric head rose by only 0.23 ft. between March and July 2006. 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 the differences between the elevations of the groundwater in Wells MW-3, MW-3A and MW-3B and in MW-12 and MW-12A are sufficiently great to indicate that the groundwater monitored by the shallow well at the locations of each of those well clusters has a different piezometric pressure from that of the deeper wells in the same cluster. This demonstrates that, at least locally, the shallow aquifer is hydraulically isolated from the aquifers monitored by the deeper wells. SJC