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' S E C O R Cleanup Plan October 27, 2003 t Page 2 Stratigraphy is relatively consistent across the site. The local geology consists of recent unconsolidated alluvial and fluvial deposits to the total depths explored of approximately 60 feet ' below ground surface (bgs). These sediments are a mixture of gravels, sands, silts and clays and are principal mixtures of sandy-silts and silty-clays. Specifically, lean clay extends from the surface down to approximately 10 feet bgs, which is underlain by silty sand to poorly graded sand to a depth ' of approximately 30 feet bgs. Records of borings adjacent to TW-5 and TW-6 indicated sandy lean clay extending to approximately 10 feet bgs with silty sand to poorly graded sand to a total explored depth of approximately 30 feet bgs. Generalized geological cross sections are included as Figures 3 ' and 4 (USGS, 1998). 2.2 Hydrogeology ' The principlal source of groundwater in the area is the Tulare formation. The Tulare formation is separated into three zones, the Upper Tulare Aquifer, the Corcoran Clay Layer, and the Lower Tulare Aquifer(USGS, 1998). The Upper Tulare Aquifer lies from approximately 5 to 200 feet bgs. The Corcoran Clay consists of low permeability silty, diatomaceous clay, is generally encountered at 200 feet bgs, and is approximately 100 feet thick. The confined Lower Tulare Aquifer lies below the Corcoran Clay (USGS, 1998). Site assessment activities completed at the site indicate that the Upper Tulare Aquifer maybe locally ' subdivided into the Upper A zone located from 5 to 15 feet bgs, the A zone located from 15 to 35 feet bgs, and the B zone encountered as shallow as 50 feet bgs (SECOR, 2003b). The lower boundary of the B-zone has not been explored. ' 2.2.1 Groundwater Flow Rates, Directions, Recharge, and Discharge A 2003 Aquifer Testing Report, (SECOR, 2003c) suggests hydrogeologic communication exists between the Upper A zone and A zone onsite. However, locally, there is no distinction between the Upper A and A zones. Groundwater beneath the site generally flows to the north with gradients ranging from approximately 0.0008 feet per foot(ft/ft)to 0.06 ft/ft(SECOR, 2003d). ' Aquifer test results indicate an average transmissivity (T) and specific yield (Sy) of approximately 381 square feet per day (ft2/day) and 0.41 cubic feet per cubic feet (ft3 per ft), respectively. Based on a typical saturated thickness of 20 feet in the A zone, the average conductivity (K) was approximately ' 19 feet per day (ft/day) or 0.0067 centimeters per second (cm/sec). The pumping well radius of influence (ROI) was calculated to be approximately 83 feet. ' Historical groundwater gradient data suggests that shallow groundwater flows toward and may discharge to Tom Paine Slough and the Paradise Cut. Groundwater in the area appears to recharge from rain fall. The average rain fall for the city of Tracy is 7.9 inches annually (http://www.worldclimate.com). ' 2.2.2 Groundwater Use The San Joaquin groundwater basin underlies the site. Beneficial uses of groundwater in the area include municipal, irrigation, stock watering, process and service supply (SECOR, 2003a). • 1 ' IAChevron\1001621\REPO RTS\2003\2003-Cleanup Plan\Cleanup Plan.doc