2013-04-24_REVISION - C1981019 (2)

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I recommend that we approve Colowyo's TR -95 since the new curve numbers are based on the second best methodology for assessing post - mining soils. Regarding the argument that Colowyo just changed their curve numbers less than four years ago, and it is not appropriate to let them change again, I offer the following points: • The numbers determined in TR -73 were based on the poorest method of selecting soil types, i.e., using pre - mining soil types. • Colowyo submitted TR -73 largely because of the problems in the Prospect basin. They were prudent to make the model more conservative in this basin because of the history of problems and because of the threats to public safety below the Prospect Pond. However, it was not necessary, in my professional judgment, to apply these conservative curve numbers to other basins at the mine site. • In documents from the time frame of the related Notice of Violation (CV- 2008 -004), Kent Gorham discussed problems with the model not matching actual conditions on the ground. However, from what I have discerned, the cause of this difference is other factors (such as the area draining to the pond) rather than curve numbers. For example, in a memo to Jim Stark on 11/13/2008, Kent states that the model assumes 533 acres draining to Prospect Pond, but the actual area that drains to the pond (for the scenario in question) is 700 acres. Another important factor related to this subject is compaction and soil handling procedures. In order to classify their reclaimed lands as B soils, Colowyo needs to continue their commitment to ripping the overburden prior to laying down topsoil (page 2.05 -27 in PAP) and continue to roughen topsoil surfaces (page 2.05 -29 in PAP). These procedures will help counteract the effect on infiltration due to compaction by large construction equipment during grading and other reclamation activities. It might be prudent to ask Colowyo to provide documentation of these practices in a quarterly report.