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aorvr ar rtirvrtv3 z zz ve i}•ri iii z~za a.3rv rrURV- r r~ o <br />in the model grid area hounded by rows 10 and 16 and collumns 7 and <br />9. The area of the pond represented in the model is approximately <br />20 acres. Evaporation from the pond was simulated with pumping <br />wells (each pumping at a rate of 24o cubic feet per day) which were <br />placed in each of 8 cells bounded by the above rows and collumns. <br />Pumping wells were assumed to fully penetrate the unconfined <br />reclamation fill. <br />Comparison of Numerical Modeling Results With Conceotual Modeling <br />Results of the preliminary steady state numerical simulation using <br />MODFLOW compare favorably with the conceptual analysis. A copy of <br />the simulation output is included as attachment 2. The output of <br />hydraulic heads indicates that drawdown of the water table surface <br />is most steep in the northern portion of the reclaimed pit while <br />levels flatten taWard the south. Significant drawdown is aparrent <br />in the northern portion of the reclaimed pit in the vicinity of the <br />pond and near the northern pit boundary. Groundwater levels near <br />the southern boundary of the reclaimed pit are shown in the MODFLOW <br />output as being elevated when compared with regional levels. This <br />affect is primarily due to grid affects and high contrast between <br />K of the reclaimed fill and K of the native material. In reality <br />this contrast in K will not be as pronounced. Although water levels <br />may be expected to be somewhat elevated near the southern boundary, <br />surface discharges are not expected. <br />In the unlikley event that surface flow from the reclaimed southern <br />mine pit occurs, any surface flows will be conrolled through the <br />use of drainage ditches. <br />SUMMARY AND CONCLUSIONS <br />• Based on ideal assumptions, the pumping rates to meet <br />dewatering objectives were calculated for 5, 10, 15, 20, <br />and 25 years. This rough analysis indicates that <br />dewatering rates should not exceed 300 gpm over the <br />period of mine development. The potential presence of <br />water bearing fractures in underlying bedrock could <br />increase the dewatering requirements. <br />• The State of Colorado Division of Water Resources <br />Analytical Stream Depletion Madel (Shroeder, 1987} was <br />used to determine the time for dewatering activities at <br />Dowe Flats to affect the St. Vrain. It was determined <br />that dewatering could potentially impact the 5t. Vrain at <br />the end of abput 1 year of dewatering. However, the <br />amount of stream depletion (a total of approximately 40 <br />acre-feet after 5 years) is probably insignificant <br />compared to the estimated volume of flow at any point <br />6 <br />