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. KEI-P91-0202 .R6 January 23 , 1992 Page 7 table contours. Ground water flow direction appeared to be toward the southwest on January 3 , 1992, with a hydraulic gradient of approximately 0. 002 (based on water level data collected from the monitoring wells prior to purging) . The subject site is situated near the approximate center of the Central Valley geomorphic province of California, and is situated adjacent to and east of the Sacramento-San Joaquin Delta area. The Central Valley is a large, northwestward-trending , asymmetric structural trough that has been filled with up to six vertical miles of sediment in the San Joaquin Valley and up to 10 miles of sediment in the Sacramento 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 the Pleistocene Modesto Formation (Qm) . The Modesto Formation in the vicinity of the site is described as typically consisting of loose sand and silt (which . is chiefly fluvial) representing an alluvial fan deposit of the Mokelumne River (which has a reported average thickness of about 10 to 15 feet and is commonly underlain by compact, unweathered silt) . The Modesto Formation is considered to be underlain by the Pleistocene Riverbank Formation. The results of our previous subsurface study (log of borings MWI, MW2 , and MW3) indicate that the subject site is underlain predomi- nantly by sand (well graded, poorly graded, and silty) with interbedded zones of silt which extend to depths below grade of approximately 79. 5 feet. This interbedded sand and silt zone is inturn underlain by clay materials that generally extend to the maximum depths explored (82 to 86 feet below grade) , except for MW3 , where an approximately 4 foot thick zone of sand was encoun- tered at about 81 to 85 feet below grade. Also, at the base of the relatively thick sequence of interbedded sand and silt is an approximately 2 to 5 foot thick zone of silty to well graded gravel , except at MW2 , where an approximately 2 foot thick lens of poorly graded sand was encountered between the gravel and the clay materials. In addition, within the relatively thick sequence of sand and silt, an approximate 19 to 20 foot thick uninterrupted zone of sand was encountered that extends to a depth of about 61. 5 to 62 feet below grade across the site. Additionally, ground water was initially encountered during drilling activities within both sand and silt materials at approximately 4. 5 to 5.-5 feet above the relatively consistent gravel bed discussed above. The results of our current subsurface study (log of borings MW4 through MW9 , EBI and EB2) indicate that the subject site is underlain predominantly by interbedded silt and sand beds which