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approximately 1.5 inches per year for the Tucson South area. This rate was used as guidance when <br /> assigning recharge rates in the model to account for deep percolation of precipitation. <br /> Recharge rates due to both precipitation and irrigation were varied seasonally and spatially. As a <br /> rationale for estimating the recharge in areas where crop irrigation is applied,it was assumed that 2.5 <br /> feet would be applied per acre per irrigation year and that 60 percent of that water would be consumed <br /> by crops. Of the remaining water,it was assumed that approximately one-half would be Iost as runoff <br /> and one-half would percolate below the root zone to the water table. The resulting estimated recharge <br /> from irrigation would be on the order of six inches per irrigation season. An irrigation rate was applied <br /> in addition to precipitation recharge on existing irrigated lands northwest of the South Platte River. <br /> For simplicity,the seasonal model runs were divided into"wet"and"dry"seasons. During the"dry" <br /> season(October 1 through March 31),all non-irrigated active cells in the mapped valley fill received a <br /> total of one-sixth of the annual recharge(0.25 inches),and the remaining five-sixths(1.25 inches)were <br /> distributed over the wet season(April 1 to September 30). For irrigated lands,the dry season recharge <br /> rate was set equal to twice the non-irrigated land rate(0.5 inches)to account for lag effects,and the <br /> remaining amount was spread evenly over the wet season. <br /> 3.0 MODEL SIMULATIONS <br /> This section provides an overview of the process used for model calibration and simulation of the effects <br /> of mining and the results of these simulations. <br /> 3.1 Simulation Approach <br /> The approach to simulating aquifer conditions in the Tucson South area and the effects of mining <br /> consisted of three basic steps: <br /> 1) Using average annual values,the model was run under steady state conditions. Aquifer <br /> parameters and boundary conditions were adjusted as needed during this step to roughly <br /> calibrate the model to the"observed"groundwater levels based upon regional USGS mapping. <br /> This step also served to fix bugs and test the robustness of the model. <br /> 2) The transient model was then run for several years under cyclical dry and wet season scenarios <br /> to allow the model to adjust to seasonal fluctuations. Area recharge rates,ditch,river and creek <br /> conditions were adjusted to account for the seasonal effects.Aquifer parameters and boundary <br /> conditions were adjusted as needed during this step to calibrate the model to the"observed" <br /> groundwater levels based upon regional USGS mapping, supplemented by recent site-specific <br /> data from monitoring wells installed by Tetra Tech RMC around the site. <br /> 3) The slurry walls,dewatered mines,and associated features around the Tucson South mine were <br /> then inserted into the model for various scenarios.The wet and dry annual cycles were simulated <br /> for a timeframe sufficient to achieve new seasonal quasi-equilibrium conditions and assess post- <br /> mine effects. <br /> -5- August 2004 <br /> L\3919 019\TS GW Model\TS Report\Tuaon South Rpt Draft.doo <br />