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KEI-P89-1104 .QR4 <br /> February 3 , 1992 <br /> Page 8 <br /> Destruction of the abandoned water well was completed on January <br /> 24 , 1992 , per KEI 's work plan (KEI-P89-1104 .P4) dated July 19, <br /> 1990. <br /> HYDROLOGY AND GEOLOGY <br /> Based on the water level data gathered on December 30, 1991, the <br /> ground water flow direction appeared to be relatively flat, but <br /> trending toward the east-southeast at an average gradient of 0.002 . <br /> This flow direction is relatively unchanged from the flow direction <br /> reported for the last three quarters. Water levels have steadily <br /> increased during the quarter, showing a net increase of 2 . 80 to <br /> 2 . 85 feet in all wells since September 27 , 1991. The measured <br /> depth to ground water at the site on December 30, 1991, ranged <br /> between 55. 45 and 56. 20 feet below grade. <br /> The subject site is located adjacent to the Sacramento-San Joaquin <br /> delta in the Central Valley geomorphic province. The Central <br /> Valley is a large, northwestward-trending, asymmetric structural <br /> trough that has been filled with as much as six vertical miles of <br /> sediment in the San Joaquin Valley. Based on review of regional <br /> geologic maps (U.S. Geological Survey Miscellaneous Field Studies <br /> Map MF-1401 "Geologic Maps of the Sacramento-San Joaquin Delta, <br /> California" by Brian F. Atwater, 1982) , the subject site is <br /> underlain by Holocene and/or upper Pleistocene alluvium identified <br /> as Alluvium of Calaveras River and Vicinity (Qcr) . The Calaveras <br /> River alluvium was deposited by the Calaveras River, Bear Creek and <br /> several lesser streams, and has not been described in detail. <br /> The results of our subsurface study indicate that the site is <br /> underlain by fill materials that are about 1 foot thick, except in <br /> the vicinity of MW3 at the former fuel tank pit, where pea gravel <br /> backfill materials were encountered to a depth of about 27 feet <br /> below grade. Immediately underlying the fill materials (except in <br /> the vicinity of MW3) is a zone of highly expansive silty clay <br /> extending to depths below grade of about 5. 5 to 7 . 5 feet. This <br /> expansive clay zone is inturn underlain by a lenticular sequence of <br /> silty clay and clayey silt materials extending to depths below <br /> grade of about 25 to 30. 5 feet. However, in MW1, this fine- <br /> grained sequence contains an approximately 2 . 5 foot thick lens of <br /> poorly graded sand that extends to a depth of about 13 feet below <br /> grade. The fine-grained sequence is inturn underlain by a <br /> predominantly sandy sequence extending to depths below grade of <br /> about 35.5 to 45. 5 feet, which is about 7 . 5 to 8 feet thick at MW2 <br /> and MW3 , and is about 15. 5 to 16 feet thick at MW1 and MW4 . This <br /> predominantly sandy sequence is characterized by silty sand and <br /> poorly graded to well graded sand, but locally includes lenses of <br /> gravelly sand and sandy silt. In MW4, two silt lenses, each about <br /> 4. 5 feet thick, are also included within this sandy sequence (only <br />