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60 Y KEI-P91-0202.R5 October 16, 1991 page 6 on water level data collected from the three monitoring wells prior to purging and sampling) • is situated near the approximate center of the The subject siteeomorphic province of California, and is situated Central Valley g asymmetric adjacent to and east of the eat northwestwa rd-trending,lta area. The central valley is a large, to six vertical structural trough that has been filled with UP to 10 miles of miles of sediment in the San Joaquin Valley and up sediment in the Sacramento Valley. Geological survey maps (tJ.S. Gealog of the Based on review of regional geologic rimaps "Geologic Maps Miscellaneous Field Studies Map Brian F. Atwater, the Pleistocene Modesto Sacramento-San Joaquin Delta, California" Y 1982) , the subject site is underlain by which Formation {Qm) • The Modesto Formation int � sandnand silt,of site is described as typ ically consisting the is chiefly fluvial, representiongt�dn alluvial thickness of about 10 Mokelumne River which has aunderlain by compact, unweathered silt. to 15 feet and is commonly The Modesto Formation is considered to be underlain by the Pleistocene Riverbank Formation. s MWIr Io of boring subject site is underlain predomi- The results of our recent subsurface study g with MW2, and MW': indicate =aced the poorly graded, or silty sand) nantly by sand (well g + to depths below grade of interbedded zones of silt which extend sand p extend to the approximais tely 79.5 feet. This interbeddedenerallyd silt zDefor inturn underlain by clay materials which g rade except maximum depths explored (82 to 86 feet }�elow grade) MW3, where an approximately 4 foot thick zone o Also, at the base of the tered at about 81 to 85 feet below grade. , sequence of interbedded sand and silt is an thick to well graded , relatively to 5 foot thick zone of silty approximately 2 where an approximately 2 foot thick lens of gravel, except at MW2, ravel and the clay poorly graded sand was encountered betw thin the relatively thick materials, as discussed above. approximately 19 to 20 foot thick sequence of sand and silt, to a depth uninterrupted zone 2feet oa below grade across the s nd was enc 'tends Additional- of about 61.5 to encountered during drilling lY, ground water was initially ials a app roximately 4.5 bed discussed _ activities within both sand and silt mater to 5.5_ feet- above the---.relatively consistentgravel -- - above.