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about 320 ft. Transverse dispersivity is about 10 percent of longitudinal <br />dispersivity, so 32 ft was used for transverse dispersivity. Although it may be <br />anticipated that vertical dispersivity is about 10 percent of transverse dispersivity, <br />a value of zero was used for vertical dispersivity so that spreading of the solute is <br />minimal across the second layer. This has the effect of causing higher <br />concentrations of fluoride to be predicted in the top layer. Further explanation on <br />choosing this parameter is discussed in the next section in relation to the fate and <br />transport model. <br />• Precipitation is relatively low for San Luis, so recharge to the aquifer from rainfall <br />was not considered. <br />Water pumped from the West Pit is proposed to be transported via pipelines to the North <br />and South pivots for irrigation. The remaining crop requirement (not satisfied by the <br />West Pit water) would be pumped from two irrigation wells, which are located in close <br />vicinity to the pivots (Well Permit Nos. 19564-F and 21590-F). Since the West Pit water <br />contains fluoride concentrations above background levels in the valley fill aquifer, for <br />purposes of predicting expected fluoride levels in the ground water down gradient of the <br />irrigation pivots, conservative assumptions were made which affect the rate of transport <br />of the fluoride entering the ground water. The assumption was made that fluoride is <br />essentially non-reactive, meaning that there is no sorption or retardation of the fluoride to <br />other minerals or elements. The background concentration of fluoride is assumed to be <br />zero for modeling purposes since the main concern is in the concentration increase due to <br />input of fluoride from West Pit water. <br />Numerical Model <br />The three-dimensional ground water flow model was constructed using MODFLOW. <br />MODFLOW uses a finite-difference scheme to solve the governing equation for ground <br />water flow based on mass balance. MODFLOW solves the basic flow equation using a <br />finite-difference method which necessitates dividing the area of interest into grid blocks <br />and head is calculated at the center of each grid block or node. The grid cell size was <br />Y7' -3-