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Grr1NS lZ 1J l_T JAN CX L�SSOCIAT PSS <br />Hydrogeological and Environmental Consultants <br />Another modeling simulation shows groundwater levels rising 4 to 6 feet in the same area <br />around the pits. Although the mounding occurs on the west side of the river, flows are <br />diverted around the constrictions and raises water levels in the channel and on the east <br />side of the river with corresponding impact to the landowners located on the east side of <br />the river. As stated in the U.S.G.S. report (Arnold, 2010), the South Platte acts a sink to <br />groundwater when water levels are elevated in the alluvium. Hence, mounding can cause <br />increased groundwater discharge to the river, thus contributing to higher base flows in the <br />river. As such, a lined pit slurry wall will not only impact groundwater levels, but will <br />also elevate surface water flows in the river for any given storm flow. <br />Although the contribution of alluvial groundwater or "bank flow" in this study has not <br />been quantitatively determined by numerical modeling, it may be concluded that <br />additional alluvial discharge to the surface water system is occurring adjacent to the <br />Bachofer property, thereby contributing to the increase in river level. Due to the distance <br />to the gage downstream at the Fort Lupton gage, this localized discharge, which is part of <br />the combined bank flow and open -channel flow, would not affect the gage reading. <br />Hence, the results of the surface water model should be considered conservatively low, <br />since they don't reflect the total flow from local discharge to the river from groundwater. <br />0 <br />Rebuttal to D&A Model Results <br />Pre -Mining Topography <br />The pre -mining topographic map used by Deere and Ault in their 2008 modeling report <br />does not represent pre -mining conditions at the mine site. This topographic surface was <br />created from the more recent 2008 aerial survey data by altering natural and man-made <br />features from the surface. Hence, these data represent conditions after the mining began <br />and the land surface was significantly altered. To recreate a pre -mining surface from <br />present day data is mostly guess work and highly subjective. It cannot reflect a true pre - <br />mining picture of the land surface in this area along the South Platte River. <br />Model Not Calibrated to Site Conditions <br />The results of the D&A model for both 2008 topographic data and pre -mining data are <br />not indicative of or reflect present flows and stage levels in the river or established <br />baseline floodplain elevations. Model results show only negligible changes between <br />present and pre -mine simulations near Bachofer property under 100 -year flow event of <br />29,000 cfs. Yet the Bachofer property has been flooding above the 100 -year elevation for <br />the past 10 years or more with flows that are significantly lower than 29,000 cfs. Water <br />surface elevations from the model results should be tied to actual measured elevations at <br />the mine site that correspond to the 100 -year event. These results suggest that the model <br />is not calibrated to any real-time hydrologic data and as such is unreliable. <br />Manning's Coefficient <br />The D&A model used the same Manning's number (0.035) for both the channel and <br />overbank values. While this value is reasonable for the channel roughness and is similar <br />to the value that was determined by the U.S. Army Corps of Engineers and used in the <br />FIS for Fort Lupton, the FIS used a value that ranged from 0.045 to 0.120 for the <br />-12- <br />