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I environments The present day surface outcrop of geologic units in the Stockton area reflects the Late Tertiary alluvial valley fill derived from the San Joaquin River The fill probably extends to a depth of several hundred feet Site-specific geologic information in the vicinity of the property is ' limited to that which was encountered during investigations of the former tank sites Three general soil types were encountered The first ' layer (encountered only at Site #2) is a thin layer of fill ranging from the ground surface to two to three feet below ground surface (bgs) The second layer, found below the fill at Site #2, and directly below the ' pavement at Site #1 ranges to a depth between approximately 10 and 20 feet bgs The soils in this layer consist of semi consolidated silts and clays Below the clay/silt layer and to the depth explored ' (approximately 25 feet bgs), the soils consist of unconsolidated sand with minor amounts of silt and gravel Ground Water Hydrology Y gY Investigation and tank removal activities in the site area indicate the presence of confined shallow ground water conditions During drilling activities performed by ERM-West and previous investigators, ground water was typically encountered in borings at depths ranging from 3 5 to 19 feet bgs In these borings, ground water was generally first encountered after drilling through a silt and clay zone into sandy soils The fine-grained silts and clays generally serve to confine the ground water to the sandy soils The confined nature of the ground water was confirmed after construction and development of monitoring wells at both sites indicated that stabilized water levels tended to be higher than initial water levels In general, the ground water potentiometric Isurface at the site is at a depth of approximately 4 to 6 feet bgs Two borings placed into previously disturbed soils associated with ' backfill of the former tank excavations showed anomalies which were not characteristic of confined ground water conditions At Site #1, ground water was first encountered during the construction of ' monitoring well MW-5 at a depth of approximately 65 bgs, the silt- clay/sand contact was encountered at a depth of 11 feet bgs Similarly, during the construction of MW-2A at Site #2, ground water was first ' encountered at a depth of 3 5 feet bgs, but the silt-clay/sand contact was encountered at a depth of 15 feet bgs During the removal of the tanks at both sites, ground water was reported to be present in the excavations Apparently, the ground water entered the tank excavations as a result of a localized ground water flow gradient induced by the creation of the pit during tank removal This 2-2 I