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1 and clays, is found directly below the pavement and fill, and ranges to between approximately 10 and 15 feet bgs Below the clay/silt layer and to the depth explored (approximately 33 feet bgs at site 1 and 38 feet bgs at site 2), the soils consist of unconsolidated sand with minor amounts of silt and gravel Geologic cross sections for former tank sites 1 and 2 are ' provided in Figures 3 and 4, respectively ' 14 GROUND WATER HYDROGEOLOGY Ground water flow at each former tank site was evaluated using the three monitoring wells constructed at each former tank site and the results of monitoring well measurements collected at surrounding sites in the city of ' Tracy Monitoring well construction data are summarized in Table 1 and historic water levels measured in the site wells are summarized in Table 2 In general, the ground water potentiometric surface at the property ranges ' from approximately 4 5 to 7 feet bgs Review of the ground water potentiometric surface maps prepared for sites 1 and 2 indicates that ' ground water flow direction is toward the north-northwest,consistent with regional flow (see Figure 5) At former tank site 1, ground water flow direction appears to vary throughout the year Ground water elevations in the southernmost well (MW-3) have been higher than the other two wells at this site during all ' sampling events completed This indicates that flow 1s toward the north- northwest However, elevation measurements at MW-4 and MW-1 indicate that at different times of the year, the elevations in MW-4 and ' MW-1 vary such that the flow direction between these two wells changes When MW-4 has a higher elevation than MW-1, the flow direction appears ' consistent with the regional north-northwest flow direction When MW-1 has a lugher elevation than MW-4, the flow appears to be from MW-1 toward MW-4, indicating that MW-4 is a low point (as shown in Figure 5) ' One possible explanation for this observed behavior is that the building footings affect the ground water table in this area As stated above, ' ground water in this area is found at shallow depths (4 5 to 7 feet bgs) A building is present in the regional downgradient direction from MW-1 ' (i e , north-northwest) If the footings of the building extend into the saturated zone immediately downgradient of MW-1, the water table at MW-1 may be artificially elevated This would explain the behavior ' observed during some sampling events at the site Another possible explanation is that MW-1 is screened to a greater depth than MW-4 At this greater depth, MW-1 contacts the third layer of soils ' ERM 3 HEINZ USA 3374 60.3/27/00