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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 />uasi-Steadv State Calibration <br />Transient model simulations were made for varying lengths of time to ensure that the water table <br />conditions had reached aquasi-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 aquasi-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.U919_019\TS GW Modct\TS Repon\TUaon South R~ Dtaftdoc <br />