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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 imgation is applied, it was assumed that 2.5 <br />feet would be applied per acre per imgation 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 lost 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-imgated 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 Annroach <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 />I:U919_019\TS GW ModcftTS Rcpon\Tuuon South_Rp<_rhaft.doc <br />