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f <br /> 3.2 Pre Mine Simulation Results <br /> Pre-mine simulations were run to calibrate the model under average annual conditions and to also <br /> implement seasonal variations in the hydrologic inputs. Provided below is a summary of the results of <br /> these simulations. <br /> Quasi-Steady State Calibration <br /> Transient model simulations were made for varying lengths of time to ensure that the water table <br /> conditions had reached a quasi-equilibrium condition and comparisons were made between simulated <br /> and"observed"water table conditions mapped over the entire domain and at specific well locations. <br /> Calibration was performed by testing varying hydraulic conductivity values,river/ditch parameters, and <br /> recharge to obtain a reasonable match with water levels measured in local area wells and with regional <br /> water level mapping. Levels measured in local area wells in March 2004 were in general agreement <br /> with the regional water table mapping. As would be expected,some exceptions occur in the vicinity of <br /> lined water storage reservoirs installed in recent years. <br /> Seasonal Simulations <br /> After the quasi-steady state calibration was complete,the model was re-started and simulations of <br /> cyclical wet and dry seasons were performed to establish baseline seasonal conditions prior to <br /> simulation of the effects of the Tucson South Mine. For the simulation of wet and dry season variations, <br /> changes were made to various river and ditch cells as well as area recharge. As described in Section 2.4, <br /> for simplicity the annual simulations were divided into two stress periods;a"wet"season(April 1 to <br /> September 30)and a"dry"season(October 1 to March 31). For these seasonal baseline simulations, <br /> recharge rates developed during calibration were distributed between the two periods as described in <br /> Section 2.4. River and ditch cells were modified to mimic conditions during wet and dry seasons. <br /> Ditches that do not transmit flow during the non-irrigation season were turned off during the dry season <br /> portions of the simulations or allowed to only receive flow if the head conditions dictated it. The stage <br /> level in the South Platte River was reduced during the dry season. <br /> The seasonal simulations were run for a period of six years to establish a quasi-equilibrium condition for <br /> each season. The quasi-steady water table conditions at the end of the wet and dry periods were used as <br /> a basis for comparing the effects due to mining at the same time periods. <br /> Figure 4 illustrates the comparison of simulated water levels during the calibration stage(scenario 1) <br /> with the general regional water table mapping performed by the USGS(Robson, 1996)supplemented <br /> with water level readings from monitoring wells on site. Some of the minor differences between the <br /> simulated and mapped water-table can be attributed to seasonality and the effects of recently installed <br /> lined reservoirs. The match between simulated and observed water levels shown on Figure 4 and table 2 <br /> was deemed adequate for the purpose of assessing relative differences due to the Tucson South mining <br /> operation. Overall,the simulated head levels are quite close to,and the groundwater flow directions and <br /> gradients are consistent with,the"observed"levels. <br /> This match was obtained using aquifer parameters and hydrologic inputs that were consistent with the <br /> expected values based on literature and site observations. The effects of changing hydraulic <br /> conductivities,recharge rates and river cell parameters were tested. The valley-fill alluvium and the <br /> alluvial terrace deposits were reported and observed to be similar,and the model adequately represented <br /> the observed groundwater conditions using a uniform value for the two deposits. The hydraulic <br /> -6 - August 2004 <br /> I:\4919 01ATS GW ModenTS Report\Tueson South Rpt Drakdoc <br />