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KEI-P89-1104 .QR2 August 7 , 1991 Page 8 HYDROLOGY AND GEOLOGY Based on the water level data gathered during the quarter, ground water flow direction appeared to be relatively flat, but trending toward the east-southeast at an average gradient of 0. 001 on June 26 , 1991, relatively unchanged from the flow direction reported on March 26 , 1991 . water levels have steadily decreased during the quarter, showing a net decrease of 0. 80 to 0 . 86 feet in all wells since March 26, 1991. The measured depth to ground water at the site on June 26, 1991 ranged between 57 . 35 and 58 . 15 feet. The sub]ect 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 sub3ect 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 1s inturn underlain by a lenticular sequence of silty clay and clayey salt 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 extends to depths below grade of about 53 . 5 to 68 . 5 feet and