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Report: Groundwater-quality Monitoring—March 29-31,2006, 7500 West Eleventh Street, Tracy, CA Page 10 3- 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. 2,1.1 Groundwater Mound * A significant feature of the groundwater contours shown on Figure 2 is the groundwater mound centered near Monitoring Well MW-16. The existence of that mound was first observed in the groundwater measurements made on February 24, 2005 (The San Joaquin Company 2006d). As was noted at that time, SJC believes that it can be attributed to a failing septic tank leach field that is located in an area to the east of the Capitol Furniture building at 7501 West Eleventh Street. The depth to groundwater in Monitoring Well MW-16 on March 29, 2006 was measured at 2.60 ft. BGS, which is 1.45 ft. higher than the depth to groundwater last measured in that well on December 20, 2005. A depth to groundwater of 2.60 ft. in that well yields a groundwater elevation of 43.88 ft. MSL, which is the same as was the case when the .-. groundwater mound was first observed on February 24, 2005. Its continued existence 4, produced the highly complex pattern of groundwater contours that are shown on Figure 2. However, despite the continuing presence of the mound of groundwater, at the scale of the site as a whole, the general direction and gradient of groundwater flow remains to the north-northeast at approximately 0.003 ft/ft, as generally has been the case throughout the years that groundwater depths have been monitored at the site. 2.2 Purging of Groundwater-quality Monitoring Wells ' After the depths to groundwater in the seventeen groundwater-quality monitoring wells to �. be sampled were measured, a small-diameter, submersible pump was used to purge each of stagnant water. The pumped water was discharged into 5-gallon pails, each of which was, in turn, discharged into a 55-gallon drum. The water in the drum was periodically discharged into a 1,100-gallon holding tank on the 7500 West Eleventh Street property. During the purging procedure, the temperature and electrical conductivity of the stream ip- of purge water were monitored by checking those parameters periodically using a multi- . function electronic meter. Purging continued until both parameters stabilized (Le., variations between measurements were less than 10%) or, in the case of wells screened above the water table, until a minimum of 15 gallons of groundwater had been removed, whichever was greater. The array of parametric results for each well is recorded in SJC's field notes (see Field Notes, Appendix B). 6- v �' SJ C