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Ms.Devra Lewis • June 7,2001 Page 3 2.0 SITE CONDITIONS The Site, which covers approximately 10 acres, is located within the San Joaquin Valley, west of the San Joaquin River. The Site is bounded on the west by South Banta Road, on the south by agricultural lands, and by California Northern Railroad Company right-of-way along the northeast property line(Figure 1). The surrounding area consists primarily of agricultural usage, with some mixed-use light industrial and residential areas. 2.1 Regional Geology and Hydrogeology The Site's location in the San Joaquin Valley overlies recent Quaternary deposits and the Tulare Formation. Recent Quaternary silt and clay alluvial deposits are predominant in the upper 30 feet of the subsurface. These shallow subsurface deposits are difficult to distinguish from the underlying deposits of the Tulare Formation. The Tulare Formation has been divided into Upper, Middle, and Lower Members in the vicinity of the Site. The Upper Tulare deposits display rapid variation in grain size and consist of interbedded gravels, sands, silts, and clays. The Upper Tulare reaches a thickness of 200 feet in the northwest part of the San Joaquin Valley near Tracy and pinches out at both the eastern and western margins of the valley. The Middle Tulare,which occurs at a depth of approximately 220 bgs, is the very poorly transmissive Corcoran Clay Member, and is estimated to be 220 to 250 feet thick. The Lower Tulare Member has not been encountered in any of the recent groundwater investigations conducted in the Tracy area. The Upper and Lower Members are transmissive zones that are important to the water supply in the San Joaquin Valley. Subsurface investigations at DDJC-Tracy have extensively mapped alluvial and fluvial stratigraphy of the Upper Tulare Member and overlying deposits. The Upper Tulare Member has been divided into four geologic units,designated"horizons": Above Upper,Upper,Middle, and Lower. For the purposes of this report the Above Upper sands are included in the Upper Horizon designation. These horizons were deposited as a result of changes in fluvial flow regime and sediment load over time. The depth, thickness, and lithology of the horizons vary across the area. The horizons consist of fine-to medium-grained deposits of sand, and gravel with varying amounts of sand, separated by fine-grained deposits of silt and clay. Groundwater elevation maps from the Site and from DDJC-Tracy monitoring data indicate that groundwater flow is generally north-northeast. Evidence from groundwater elevation data, hydraulic gradients,and contaminant concentration at DDJC-Tracy indicate that groundwater in the four horizons is in hydraulic communication between the horizons. The clay layers separating these horizons do not prevent vertical movement of groundwater. Since September 1992, vertical hydraulic gradients between the Upper and Middle Horizons have historically been downward;however, vertical gradients between the Upper and Middle Horizons showed no consistent direction in 1998. Site Geology and Hydrogeology There are currently eight groundwater monitoring wells(MW-1,MW-2,MW-3,MW-4,MW-5,MW-6, MW-7, and MW-8) associated with the Site(Figure 2). The monitoring wells are screened in the Upper Horizon over the interval from 10 to 30 feet bgs. Monitoring wells MW-1 through MW-4 were installed in October, 1989. Monitoring wells MW-7 and MW-8 were installed in June 1997. Monitoring wells MW-5 and MW-6 were installed offsite in the railroad right-of-way in June, 1999. The groundwater gradient at the Site has historically been to the north-northeast. Depth to groundwater measured in May 2001 ranged from 12.39 feet bgs in MW-7 to 15.39 feet bgs in MW-3. HAPioneer-Tracy\WorkPlan200 I\workplan.doc SECOR International Inc.