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i Groundwater was encountered at 32 feet bg s Details of the initial assessment are summarized in WaterWork's, "Preliminary Assessment Report", dated January 12, 1990 Due to the presence of elevated levels of hydrocarbons beneath the three tanks, the San Joaquin County Public Health Services, Environmental Health Division (PHS/EHD) ' requested additional work be performed at the site 2.3 Regional Geological Setting and Hydrogeology The site is located in the northeastern portion of San Joaquin Valley approximately 25 miles west of the Sierra Nevada foothills and east of the Coast Ranges The topography in the I vicinity of the site is relatively flat Site elevation is approximately 15 feet (mean sea level datum) ' The property is underlain by Late Pliocene and Quaternary deposits of mostly arkosic sand, silt, and clay mapped as non-manne deposits in the Geologic Map of California These most recent deposits probably represent run-off from the surrounding highlands and local ' fluvial deposits The Turlock Lake, the Riverbank and the Modesto Formations belong to this group (Figure 3) The Modesto Formation of Late Pleistocene is the youngest and occurs at the surface in the Stockton area This formation is approximately 100 feet thick ' here but thickens westward toward the axis of the San Joaquin Valley (The geological data were obtained from the San Jose Sheet - GeoIogical Map of California, 1966, California Divisions of Mines and Geology and from United States Geological Survey Open-File Report 77-748, 1977) I The site lies within the limits of San Joaquin Valley Water Basin (Basin No 5-22) of the San Joaquin Basin Hydrologic Study Area, which is drained pnmanly by the San Joaquin River Two aquifers are recognized to exist at the site, a deep and a shallow aquifer The ' deeper aquifer contains connate water occurring in the semi-consolidated to consolidated Merhten and Valley Spring Formations The fresh water bearing formation, consisting of alluvial sediments occurs in a shallower depth These two aquifers are hydraulically ' separated from each other ' 3.0 PROCEDURES 3.1 Drilling and Sampling Five soil bonngs (B1 through B5) were drilled on February 11-14, 1994 Bonng B1 was advanced to a depth of 75 feet and completed as groundwater monitoring well MW-1 ' Borings B2 and B3 were advanced to a depth of 74 feet and completed as groundwater monitoring wells MW-2 and MW-3, respectively Bonng B4 was advanced to a depth of 70 feet and was also completed as groundwater monitoring well MW-4 Boring B5 was ' advanced to a depth of 55 feet and backfilled with Portland cement to the surface Auger returns were placed in 55-gallon DOT drums and stored on site Larry.Aub QIP,MI,S 4