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r <br /> r <br /> 1.2.2 Hydrogeology <br /> The principal source of groundwater in the area is the Tulare formation. The Tulare formation is separated into <br /> three zones,the Upper Tulare Aquifer,the Corcoran Clay Layer, and the Lower Tulare Aquifer(USGS, 1998a). <br /> The Upper Tulare Aquifer lies from approximately 5 to 200 feet bgs. The Corcoran Clay, consisting of low <br /> permeability silty diatomaceous clay, is generally encountered at 200 feet bgs, and is approximately 100 feet <br /> ' thick. The confined Lower Tulare Aquifer lies below the Corcoran Clay. Based on regional geology, there <br /> appears to be no perched aquifers under the site (USGS, 1998a). <br /> Site assessment activities completed to date indicate that soils underlying the site consist primarily of sandy silt <br /> for the top 5 feet, silt and sandy silt from approximately 5 to 10 feet bgs, and sand and silt units with clay lenses <br /> ranging from 1 to 5 feet in thickness beyond 10 feet bgs. Hydrogeologic data collected at the site also indicate <br /> ' that the Upper Tulare Aquifer is subdivided into the following three flow zones: <br /> 1. The Upper A zone located from 5 to 15 feet bgs; <br /> 2. The A zone located from 15 to 35 feet bgs; and <br /> 3. The B zone encountered as shallow as 50 feet bgs. <br /> These three flow zones of the Upper Tulare Aquifer underlying the site are separated by apparently <br /> ' discontinuous layers of less permeable silt and clay (SECOR, 2003b). <br /> 1.2.2.1 Groundwater Flow Rate, Direction, Recharge, and Discharge <br /> A 2003 Aquifer Testing Report, (SECOR, 2003c) suggests hydrogeologic communication exists between the <br /> Upper A zone and A zone onsite. Locally, there is little distinction between the Upper A and A zones. The <br /> groundwater flow in the Upper A is towards the northeast at an approximate hydraulic gradient of 0.003 foot per <br /> foot (ft/ft) (Figure 5). The groundwater flow direction in the A zone is towards the east at an approximate <br /> hydraulic gradient of 0.01 ft/ft(Figure 6). The groundwater flow direction in the B zone is towards the north at <br /> an approximate hydraulic gradient of 0.01 ft/ft (Figure 7). Groundwater flow direction and hydraulic gradient <br /> data are presented in Appendix B. A rose diagram showing the historical gradient and flow direction of shallow <br /> groundwater(i.e., Upper A and A zones) is included as Figure 8. <br /> Results of the aquifer testing indicated the Upper A zone could sustain a pumping rate of 0.9 gallons per minute <br /> (gpm) and that the A zone could sustain a pumping rate of 9.25 gpm. An average value of approximately 294 <br /> square feet per day and 0.41 was estimated for average transmissivity and specific yield, respectively. Based on <br /> the A zone saturated thickness of 20 feet, the average hydraulic conductivity estimated was 15 feet per day <br /> (0.0052 centimeter per second)(SECOR, 2003c). <br /> rHistorical groundwater gradient data suggests that shallow groundwater flows toward and may discharge to Tom <br /> Paine Slough (located approximately 0.75 mile northeast of the site) and/or the Paradise Cut (located <br /> approximately 1.5 miles northeast of the site). Groundwater recharge in the site vicinity appears to be <br /> predominately from rain fall. The average rain fall for the City of Tracy (City) is 7.9 inches annually <br /> (http://www.worldclimatecom/cgi-bin/data.p?ref'--N37W 121+2200+04899C). <br /> 1 <br /> BLASLAND, BOUCK&LEE, INC. <br /> 3124106 — engineers,scientists,economists 1-2 <br /> J:\D0006\44634 00161022_Conceptual Site Model.doc <br />