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y KEI-P91-0202 .QR1 May 4, 1992 Page 6 gallons by the use of a surface pump. During purging operations, the field parameters pH, temperature, and electrical conductivity were monitored and are presented in Table la. After a minimum of four casing volumes had been purged, and the field parameters were observed to stabilize, water samples were then collected by the use of a clean Teflon bailer. The samples were decanted into clean VOA vials that were then sealed with Teflon-lined screw caps and stored in a cooler, on ice, until delivery to the state-certified laboratory. HYDROLOGY AND GEOLOGY Based on the water level data gathered on April 7 , 1992 , the ,;g!roun watflow rectia appeared to be predominantly towar e southwes as s own on the attached Site Plan, Figure 1 . The average "ydraulic gradient at the site is approximately 0.002 . Water levels have fluctuated during the quarter, showing a net increase of between 2 . 20 feet and 2 .25 feet in all wells since January 3 , 1992 , except for well MW5, in which a net decrease of 0. 31 feet had been observed. The measured depth to ground water at the site on April 7 , 1992 ranged between 61. 54 feet and 71.41 feet below grade. 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 (the log of borings MW1JLM2, and MW3) indicate that the subject site is underlain pr. domnaritly by_ wand (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 in