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estimated specific storage value for the site is 0.15 which for a water table aquifer is <br /> equivalent to the specific yield. <br /> III. FLOW SIMULATION <br /> The U.S. Geological Survey's three dimensional finite difference groundwater flow <br /> model, MODFLOW, (McDonald and Harbaugh, 1988) was used to simulate the <br /> groundwater flow regime in the pond area. The first step in the flow simulation was to <br /> develop a conceptual model and various input data files containing the site specific <br /> information. Conceptually, the model domain contains a 4800 feet x 3900 feet (0.671 <br /> square mile) section of the study area which includes the San Joaquin River on the west <br /> and a portion of the Oakwood Lake on the south (Figure 1). The size of the conceptual <br /> model was chosen carefully to minimize the computation effort. The finite difference <br /> grid of the modeled area is shown in Figure 2. Areally, the model grid is 59 rows by <br /> 59 columns with each model cell 100 feet x 100 feet away from the percolation pond <br /> area and 50 feet x 50 feet in the pond area. Such a discretization was done to achieve <br /> numerical stability and better model accuracy. Boundary conditions are that of no flow <br /> boundaries along the AB, CD, ED and EF side; river head boundary along the AF side <br /> and a constant head boundary along BC (Figure 2). The purpose of defining no flow <br /> boundaries is to define a control volume for modeling purposes. <br /> _ The second step in the flow simulation was to check model accuracy and sensitivity to <br /> various input parameters by performing numerous transient simulations. The flow <br /> simulations begin with an arbitrary time zero and with a river elevation of 4 feet (13.0 <br /> ft. depth), representing normal average river stage. The model area is subjected to the <br /> localized uniform recharge rate due to the percolation ponds, as previously discussed. <br /> The test runs showed that the three most influential aquifer parameters in the model are <br /> hydraulic conductivity, storage coefficient, and the aquifer thickness. The simulation of <br /> groundwater mound development over time during a normal year showed initial <br /> mounding of about 2 feet the first month. As expected, the model depicts that under <br /> APRIL 1994 CCIP(SIMULATION OF PERCOLATION POND PERFORMANCE PAGE 5 <br />