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layer upon entry. Vertical dispersivity is chosen as zero so that additional early-time <br />spreading into the second layer is limited. <br />Several observation points were added to the model to record fluoride concentration <br />levels after each time step. One point was located on the South pivot to track fluoride <br />concentrations at its initial entry point to the aquifer. Since the SLM-2 well is slightly <br />offset from the principal axis of ground water flow, we evaluated steady-state fluoride <br />concentrations along the main axis of ground water flow and at the SLM-2 well. <br />Therefore, a second observation point was located at approximately 3200 ft southwest of <br />the South pivot along the principal flow axis, where concentration could be expected to <br />be highest. The final observation point was located at SLM-2 to record the expected <br />fluoride concentration with time. SLM-2 is also located approximately 3,200 ft <br />downgradient of the South pivot. <br />While only 15 percent of the applied water is return flow to the local ground water table, <br />it is estimated that the fluoride mass applied, minus the estimated uptake by the alfalfa <br />crop and soil adsorption, will be transported to the ground water table. The fluoride mass <br />input to the model is 0.136 pounds per day (lb/d) per cell on the North pivot and 0.122 <br />lb/d per cell on the South pivot during the irrigation season. This same mass was applied <br />in every irrigation season during the model run. The model was nrn until solute <br />concentrations achieved steady-state at the SLM-2 well. The total time for the model run <br />was 85 years; however, steady-state concentrations were reached prior to the end of the <br />model run. <br />Results <br />MODFLOW and MT3DMS provided estimated changes in fluoride concentrations with <br />time downgradient of the irrigation pivots. Since the input of fluoride mass to the ground <br />water system was constant over time, fluoride concentrations reached steady-state <br />conditions, i.e., the concentrations over time didn't change once they were at steady state. <br />Based on historic water quality data from the SLM- wells, ambient fluoride <br />