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2.4 Recharge <br />Recharge was applied in the model to account for rainfall and snow melt that infiltrates into the ground <br />and reaches the water table. Precipitation recharge innorth-central Colorado has been reported to be <br />about five percent of the total precipitation (RMC, 1991). This translates into an annual recharge rate of <br />approximately one inch per year for the Hazeltine area. This rate was used as guidance when assigning <br />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 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 />During model calibration, an even higher recharge rate was applied along the northwest boundary to <br />represent additional recharge from hillside runoff and seepage from ditches on the bedrock. <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). All non-irrigated lands in the eolian deposits <br />above the valley were recharged at 0.25 inches during the dry season and 0.75 inches during the wet <br />season. For imgated lands, the dry season recharge rate was set equal to twice the non-imgated land <br />mte (0.5 inches) to account for lag effects, and the remaining amount was spread evenly over the wet <br />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 Hazeltine area and the effects of mining consisted <br />of three basic steps: <br />Using average annual values, the model was run under transient, but unchanging, conditions for a <br />time period sufficiently long to achieve aquasi-steady state water surface. Aquifer parameters and <br />boundary conditions were adjusted as needed during this step to calibrate the model to the <br />"observed" groundwater levels based upon regional USGS mapping, as supplemented by other <br />recent site-specific data from local wells. Those wells include monitoring wells installed by Tetra <br />Tech RMC around the site, wells on Aggregate Industries' property to the south, and wells <br />associated with the Rocky Mountain Arsenal. <br />2. The transient model was then run for several years under cyclical dry and wet season scenarios to <br />allow the model to adjust to seasonal fluctuations. Area recharge rates and certain ditch, river and <br />creek conditions were adjusted to account for the seasonal effects. <br />- 4 - July 2004 <br />I:U919 OIB\GW ModcRRcponWaztltine Rp[) 2l doc <br />