ARCHIVED REPORTS_XR0007892 - Laserfiche WebLink

Home Browse Search

' GedU40c Setting The West Weber property lies within the Great Valley geomorphic province of California The Valley is an asymmetric synchlinonum containing up to 60,000 feet of sediment along the western margin. The sediments consist of marine and nonmanne late Triassic to early Tertiary clastic material. Late Tertiary to Present Age sediments are exclusively of non-manne clastic deposition in a variety of alluvial-fluvial 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 developed during investigations of the former tank sites. Two general soil types were encountered From the ground surface to a depth ranging between approximately 10 and 20 feet below ground surface (bgs), the soils consist of semiconsolidated silts and clays. Below this 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 Investigation and tank removal activities in the site area indicate the ' presence of confined shallow ground water conditions at Former Tank Sites #1 and #2. 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 always first encountered after drilling through a silt and clay zone into sandy soils. The fine- grained silts and clays 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 surface 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 6.5 bgs, but the silt-clay/sand contact was not encountered until a depth of 11 feet bgs ' had been reached Similarly, during the construction of MW-2A at Site 42, ground water was first encountered at a depth of 3.5 feet bgs, but the silt-clay/sand contact was not encountered until a depth of 2-2 r