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KEI-P89-1104.R7 December 10, 1990 Page 9 HYDROLOGY AND GEOLOGY The water table stabilized in the monitoring wells at depths ranging from 57 . 79 to 58. 60 feet below ground surface. The ground water flow direction appeared to be toward the east at a hydraulic gradient of approximately 0. 001 on October 26, 1990, (based on water level data collected from the four monitoring wells prior to purging and sampling) . Discussions with Mr. Harlin Knoll of SJCPHS indicate that the below-site-level depths to ground water, although not readily explainable, is not unprecedented. The sub3ect site is located adjacent to the Sacramento-San Joaquin delta in the Central Valley geomorphic province. The Central Valley is a large, northwestward-trending, asymmetric structural trough that has been filled with as much as six vertical miles of sediment in the San Joaquin Valley. Based on review of regional geologic maps (U.S. Geological Survey Miscellaneous Field Studies Map MF-1401 "Geologic Maps of the Sacramento-San Joaquin Delta, California" by Brian F. Atwater, 1982) , the subject site is underlain by Holocene and/or upper Pleistocene alluvium identified as Alluvium of Calaveras River and Vicinity (Qcr) . The Calaveras River alluvium was deposited by the Calaveras River, Bear Creek and several lesser streams, and has not been described in detail. The results of our subsurface study indicate that the site is underlain by fill materials which are about 1 foot thick, except in the vicinity of MW3 at the former fuel tank pit where pea gravel backfill materials were encountered to a depth of about 27 feet below grade. Immediately underlying the fill materials (except in the vicinity of MW3) is a zone of highly expansive silty clay extending to depths below grade of about 5. 5 to 7. 5 feet. This expansive clay zone is inturn underlain by a lenticular sequence of silty clay and clayey silt materials extending to depths below grade of about 25 to 30. 5 feet. However, in MW1, this fine- grained sequence contains an approximately 2 . 5 foot thick lens of poorly graded sand extending to a depth of about 13 feet. The fine-grained sequence is inturn underlain by a predominantly sandy sequence extending to depths below grade of about 35. 5 to 45.5 feet, which is about 7 .5 to 8 feet thick at MW2 and MW3 , and is about 15. 5 to 16 feet thick at MW1 and MW4 . This predominantly sandy sequence is characterized by silty sand, and poorly graded to well graded sand but locally includes lenses of gravelly sand and sandy silt. In MW4, two silt lenses, each about 4 . 5 feet thick, are also included within this sandy sequence with only traces of • silt encountered in MW2 and MW3 . This sandy sequence is inturn underlain by a second relatively thick fine-grained sequence, which consists predominantly of silty clay and clayey silt lenses and