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r ' 3.0 SITE HYDROGEOLOGY 3.1 Stratigraphy The site is underlain by Quaternary alluvial deposits to a depth of a few hundred feet. ' Various investigators have subdivided these deposits into several formations and members, although disagreements exist as to the correlation and age of some units. Most correlations ' are based on the presence of unconformities and paleosol horizons, although a few radiometric age determinations have been made (Figure 4). The thicknesses and correlations shown in Figure 4 combine data from Cherven and Blum (1977), Davis and Hall (1959), Marchand (1977), Marchand and Allwardt(1977), and Woodward-Clyde Consultants(1975). All of the formations consist of arkosic to lithic sand, gravel, silt, and clay derived from erosion of metamorphic,volcanic, and granitic sources in the Sierra Nevada. Coarse-grained sediment (stippled in Figure 4) was deposited in stream channels on alluvial fans, and is interbedded with finer-grained sediment (dash-dot pattern) that was deposited in overbank rand lake environments (e.g. the Corcoran Clay member of the Turlock Lake Formation). The near-surface deposits are part of the Modesto Formation (previously mapped as the ' upper part of the Victor Formation). According to Marchand and Allwardt (1977), the Modesto Formation ranges between 30 and 120 feet in thickness. It overlies the weathered and eroded upper surface of the Riverbank Formation. The brown to light reddish-brown paleosol that separates the two units is a relatively widespread stratigraphic marker that is commonly several feet thick. Both formations are late Pleistocene in age; the Riverbank ranges from approximately 100,000 yrs to 270,000 yrs. Carbon-14 dating of wood fragments rfrom the upper part of the Modesto has yielded an age of 14,000 yrs before present. Two fluvial sequences have been penetrated by the drill at this site (Figure 5). Both 1 sequences may be part of the Modesto Formation. The upper sequence extends from the surface to a depth of approximately 40 feet. The lower contact of this sequence varies somewhat, due to interbedding with silt or clayey silt in some wells (e.g. MW-2). The upper ' sequence generally coarsens downward from yellow or red silty fine-grained sand and sandy silt to medium-grained, yellow or brown sand. Coarse-grained sand mixed with silt and clay is locally present near the base of this sequence. The upper sequence appears to have high tporosity and permeability, but has not contained water since this investigation began in 1987. ' An interval of silt and/or clay forms the top of the lower sequence. This fine-grained interval lies at a depth of between 35 or 40 feet and approximately 70 feet. A few feet of silty sand may be present within this interval in some borings, but exact thicknesses cannot be ' determined on the basis of 5-foot sample spacing. The lithologic contacts shown in Figure 5 are only approximate. This silt/clay interval coarsens downward in MW-1, MW-4, MW-5, -7, and W-8 through clayey sand to silty or fine-grained sand. Presumably this sand ' continues to coarsen downward, but the lower part of the sequence has not been penetrated. First groundwater occurs in this sequence, which has high porosity and permeability. The Gca3ogral Audis Se ,ln<. ARA PA,Rn.23-WA 47 13 1