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Response to DRMS Comments <br />Varra Coulson Mine Operations <br />Greeley, Colorado <br />Page 2 <br />cause of diminished well use /yield an appropriate remedy will be implemented. This can <br />include but is not limited to providing supplied water, reconfiguration of the well's pump and <br />the drilling of a new well. The timing of mitigation, if required, will be dependent on measures <br />implemented and may involve factors outside of the control of Varra. If a complaint is deemed <br />valid the timing of mitigation will be conveyed to DRMS and the well owner. <br />18. All registered wells within 600 feet were identified as available from the Colorado Division of <br />Water Resources. Field inspections have not been conducted to verify the presence of <br />registered wells. <br />19. The drawdown of approximately 16 feet referenced by DRMS (at 380 feet) is accurate. The <br />drawdown referenced in the dewatering report was a model predicted change in the <br />observation well (not the aquifer) at that location. The model was reconstructed and it was <br />observed that the top of the well screen in the model parameters was located in bedrock. The <br />well screen was moved into the coarse alluvium and all simulations were rerun and the <br />predicted drawdown at the observation well was on the order of 14 feet. The calibration <br />graph and model generated groundwater contours are presented as Attachment A. <br />As requested all model calibration runs are presented in Attachment B. Varra recently <br />provided water level data that was not available at the time of model construction. Based on <br />these data it was observed that leakage from the Greeley # 3 Ditch was over estimated. The <br />leakage was approximated based on trial and error runs until good agreement between <br />measured water levels and predicted water levels was achieved. The drawdowns depicted <br />between the River + Ditch and River + Ditch + Durham model runs depict the influence of the <br />Durham dewatering. A review of the model runs shows a drawdown of approximately three <br />feet at 1,500 (MW -6) feet south of the mine face. <br />20. We disagree that the Shadow /Mounding analysis does not reflect predicted effects from the <br />proposed Coulson reclamation. The sites are nearly identical in scale, hydrogeology and liner <br />construction. The use of field data in lieu of model predictions should be given substantial <br />consideration. <br />We did, however, modify the numerical model used for drawdown predictions to simulate a <br />low permeability barrier at the Coulson mine. A barrier wall was constructed in the model <br />around the mine face and a constant head boundary was assigned inside the barrier wall to <br />simulate a full pond. The assumptions used in the model were as follows. <br />• Barrier wall thickness of seven feet with a permeability of 1 x 10- 6cm /s. <br />• The constant head boundary inside the barrier wall was assigned an elevation of <br />4027 feet. <br />• The barrier wall is fully penetrating. <br />• All simulations were steady state. <br />In order to evaluate the influence of the pond liner, simulations were run showing the <br />hydrology of the site without influences from the Durham mine, Greeley Number 3 Ditch and <br />..�w�s ltil`1 1► ►►r ♦tar 1 ► urt. 1 � rl 1 ► 1lin�. 1 / `I `►I,! //1 ► // "iN <br />