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Mr. Carl Mount <br />September 14, 2005 <br />Page 2 <br />Proposed trigger. The July 2003 monitoring and mitigation plan by Wright Water Engineers, Inc. <br />(WWE) proposed that an impact to groundwater will be considered to have occun•ed when post- <br />mining data show: 1) a lowering of the average (growing or non-growing season) groundwater level <br />by more than 2 feet, or 2) a lowering of two or more consecutive readings by more than 2 feet, <br />compared to pre-mining data at the same time of year. The 2-foot trigger was recommended based <br />on the modeling conducted that showed a maximum change of 2 feet in groundwater levels from the <br />operation. <br />The monitoring data (Tables 1 and 2) show that groundwater did not fluctuate greatly over the <br />monitoring period. The maximum fluctuation was 1.83 feet during the growing season, and 1.85 <br />feet during the non-growing season, both at MW-10. The largest annual fluctuation was 2.76 at <br />MW-10. These findings, coupled with the modeling results, indicate that the trigger of 2-feet <br />proposed in the permitting process is reasonable for average groundwater levels. However, 2-feet is <br />too small and within the measured range of variation in groundwater levels for consecutive <br />readings. A trigger of 2-feet for two or more consecutive readings could cause it to be reached due <br />to existing changes in groundwater levels. Data collected indicate that this part of the trigger should <br />be 3-feet. A period of two months is too short to indicate real changes in groundwater. Also, the <br />original trigger only addressed the lowering of groundwater levels and not mounding effects, which <br />could also occur. Therefore, it is proposed that the text describing the trigger for the permit be <br />changed as follows: <br />"A potential impact to groundwater will have occurred when post-mining <br />groundwater monitoring data show: 1) a change in the average (growing or non- <br />growing season) groundwater by more than 2-feet, or 2) a change in three or more <br />consecutive readings by more than 3-feet compared to data taken the same time of <br />year prior to mining." <br />Modeling results. Modeling was completed using the U.S. Geological Survey (USGS) <br />MODFLOW-2000 model to evaluate potential effects on groundwater from the proposed operation. <br />The results of this modeling aze contained in the March 2003 W WE report. <br />MODFLOW is afinite-difference model that was used to predict steady-state conditions. The <br />model was calibrated to published pre-mining water levels in the vicinity of the Heit Pit. The model <br />was then used to assess changes in groundwater levels with a lined pit in place. The modeling <br />predicted a maximum mounding of groundwater on the west side of the pit of 2 feet, and a <br />maximum drawdown or "shadow" effect north of the pit of 3 feet. The model also showed that <br />groundwater levels returned to near pre-mining elevations, with no shadow effect, within <br />approximately 1,000 feet north of the pit. Additional details of the modeling can be found in the <br />March 2003 report. <br />The MODFLOW model is capable of transient analysis using a portion, if not all, of the data <br />obtained over the past eighteen months (Table 1). However, the average piezometric surface <br />represented by the monitoring data (Figures 2A and 2B) compazes favorably with the pre-mining <br />water levels generated by the steady-state model runs. This comparison shows similarity in the <br />shape of the groundwater table, the average slope of the groundwater table across the Heit Pit (0.25 <br />