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A transient model was then created from the steady-state model with a stress period of six <br />months and a monthly time step that represents the irrigation season. The final head <br />values from the steady-state model were used for the initial heads in the transient model. <br />In the transient case, the proposed future irrigation practices on the Salazar Ranch were <br />simulated, where West Pit water is piped to the North and South center pivots and the <br />existing irrigation wells will be used to provide a full irrigation water supply. Therefore, <br />a portion of the irrigation water supply is provided from water pumped and stored in the <br />West Pit, with additional irrigation water provided by the existing irrigation wells. It is <br />estimated that up to 42,930 cubic feet per day (R3 /d) will be provided from the West Pit, <br />and pumping rates for irrigation wells 19564-F and 21590-F are 9,541 ft3/d and 11,930 <br />ft3/d, respectively. The irrigation wells were pumped only for the irrigation season (six <br />months) and the total irrigated acreage of the North and South pivots was 126 acres (ac) <br />and 155 ac, respectively. <br />Consistent with historic irrigation practices, total recharge of water beneath the North and <br />South pivots is 39.1 ac-ft per season and 48.1 ac-ft per season, respectively, based on a 15 <br />percent return flow from applied water. The corresponding recharge was distributed <br />equally into the grid cells, with 29 cells representing the North pivot and 40 cells <br />representing the South pivot, as shown in Figure 1. This equates to 160.8 ft3/d per cell <br />and 143.5 ft3/d per cell of recharge for the North and South pivots, respectively. <br />Once the ground water flow model was constructed, a solute transport model was <br />constructed using MT3DMS to evaluate the expected changes in fluoride concentration at <br />the SLM-2 well with time. MT3DMS is designed to be used in conjunction with <br />MODFLOW, and it uses the finite-difference method to solve the governing equation for <br />solute transport. The finite-difference method used to solve for concentrations of a solute <br />requires that the area of interest be divided into grid cells, and since the MT3DMS is used <br />in conjunction with MODFLOW, the grid size must be the same as previously described. <br />It is important to note that the concentration of fluoride entering the ground water table <br />through recharge at each grid cell on both pivots is evenly spread throughout the first <br />-5-