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i .: <br />The runoff from the Bourg mine area is approximately 14 <br />percent of the total Mann Draw runoff. <br />The mining operation will impact the surface hydrology of <br />Mann Draw in two major aspects. <br />1. Runoff from the mine area will be increased from mining <br />activities. <br />2. A portion of the runoff from the mine area will be <br />stored permanently in the sedimentation ponds and lost <br />to evaporation. <br />The effects of the second can be measured quantitatively. The <br />total dead storage at the mine site in the four ponds will be <br />6.9 acre-feet. Average net annual pond evaporation at the <br />site (total less precipitation) is 21.4 inches, estimated <br />from a regression analysis of pan evaporation data. The total <br />surface area of the ponds at dead storage is 2.37 acres. The <br />average annual volume of water lost to evaporation would, <br />therefore, be 4.23 acre-feet. This amounts to 30 percent of <br />the on-site runoff and 4 percent of the total runoff at the <br />mouth of Mann Draw. <br />This impact will be alleviated somewhat by the expected <br />increased runoff from the mine site. After reclamation of the <br />site and removal of ponds, runoff may be expected to be <br />greater than for pre-mining conditions. Experimental data in <br />various studies indicate that runoff may be increased from <br />reclaimed mine sites. However, adequate information is not <br />available for a quantitative prediction of how much or for <br />how long runoff will be greater than for natural conditions. <br />Eeonard Rice Consull~ng Waler Engineers, Inc <br />I-37 <br /> <br />