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<br /> <br /> <br />~J <br /> <br /> <br /> <br /> <br />Il <br />1 <br /> <br /> <br />1 <br /> <br /> <br /> <br /> <br /> <br />1 ' <br />thickness of the aquifer, about 13 feet, was utilized to estimate the hydraulic conductivity of the <br />aquifer at this location. The estimated hydraulic conductivity was estimated to be about 692 gpd/ftZ• <br />which confirms the results of the pumping test on PZ-26-PW. The hydraulic conductivity values <br />from the two pumping tests were averaged and used for our well impact analysis model, as <br />summarized below. Accordingly, a hydraulic conductivity value of about 878 gpd/ft'- was used in <br />the model. <br />4.3 Ground Water Model Concept <br />The finite difference groundwater model was built using MODFLOW and GIvIS. The, model <br />includes the extent of the alluvial aquifer in the vicinity of the proposed mine, as presented in <br />Appendix J. The model was utilized to simulate water levels in [he aquifer before, during and after <br />mining. Water levels at steady-state (equilibrium) conditions were examined for each of these <br />scenarios. Initially, modeled water levels were calibrated to mapped water levels as indicated by <br />piezometer data. Simulated water levels during and after mining were compared witl> simulated <br />water levels before mining. This comparison provided an estimate of water level impacts to nearby <br />wells resulting from the proposed mine. The model was built for steady state simt.lations only; <br />accordingly, model inputs did not include a storage coefficient, and the model was calibrated to <br />stead}' state conditions only. <br />The model was based on winter-time conditions, when water levels are lowest and the irrigation <br />ditches, which provide recharge to the aquifer, are not flon'ing. Accordingly, the irritation ditches <br />were not included in the model. This approach is representative of a worst-case scenario in that it <br />does not include historic recharge occurring to the aquifer during the irrigation season. <br />General head boundaries were included at the upper and lower boundaries and at tributaries flowing <br />into the aquifer within the modeled stream reach. Streams in the modeled reach, including the St. <br />Vrain River and the South Branch were modeled as MODFLOW "rivers". Existin;; and planned <br />lakes were simulated by inputting zones of very high hydraulic conductivity for unliled ponds and <br />-13- <br />ltidz/~-l;roCi/ni .Istii.'vm t. hn. <br />