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5/23/2012 Interoffice Memo <br />we have made considerable progress in both improving modeling accuracy and requiring on -the- <br />ground reclamation practices specifically to address this issue. The Division and Colowyo has put in a <br />great deal of time and effort solely focused on preventing any water quality violations as well as <br />ensuring to the extent possible future storm runoff that would jeopardize the safety of the public users of <br />Highway 13 below Prospect pond. <br />Colowyo most recently updated the approved curve numbers at the mine through technical revision <br />#73 (TR -73, June 2009). We asked at that time why they were proposing changes to the curve <br />numbers and they indicated it was for "day- forward modeling ". We also asked them to support the <br />changes to the curve numbers and they did by providing tables 7 and 8. Also they said the proposed <br />"range of values" would not be used but the "non -bold, non - italic (new) values would be used for all <br />future designs ". <br />Based on their response, our adequacy questions were about the soils being C soils not B soils as they <br />originally proposed. We also asked them to use their own site - specific vegetation results to support the <br />85% cover assumption. Colowyo subsequently made the changes to correctly identify the soils as C <br />soils and also appropriately reduced the cover percentage based their own vegetation sampling results. <br />After review of the changes, the Division approved TR -73. <br />Now Colowyo has indicated that they are not "comfortable with the outcome of TR -73 with respect to <br />curve numbers on reclaimed lands" (email from K. Blunt, 5/8/2012) and would like to yet again change <br />curve number values to lower values that would result in modeling results that estimate less storm <br />runoff. <br />The results of numerous storm events in the East Pit have shown that modeling done prior to the most <br />recent modeling in the East Pit had underestimated the amount of storm runoff generated from the <br />disturbed and reclaimed lands in the East Pit. While there is no current evidence to prove it, TR -73 <br />curve number changes, as well as modeling runs for three different future time periods that include <br />stock ponds as a secondary runoff control structures, in addition to Prospect pond, have resulted in <br />possibly the most accurate modeling done to this point. <br />Colowyo had previously asked the Division to review the Strifer and Rhodes (CSU) report, which we <br />did. This report was done to characterize erodibility and erosion potential on reclaimed coal lands and <br />included sample sites at eleven surface mines in Colorado. Specific to Colowyo, the sampling was <br />done on 11 acres of lands reclaimed at Colowyo in 1978. Colowyo now has thousands of acres of <br />reclaimed lands. Without going into details regarding the validity of the study or the conclusions, the <br />author states on page 14 "However, any generalizations about the similarities of surface hydrology or <br />regarded coal mined land from state to state, between areas within a mining region, or even between <br />sites within a mine are risky due to critical site specificity ". <br />The Division is always interested in improving modeling results when those improvements are based <br />on refining the modeling input parameters based on high quality, scientifically -based information <br />specific to the watershed. At this point, Colowyo has not provided anything new or scientific to support <br />modeling changes. <br />KAG <br />7/24/98 2 <br />