2013-04-24_REVISION - C1981019 (2)

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Interoffice Memo Date: April 24, 2013 To: File for Colowyo Mine (C- 1981 -019) Cc: Dan Hernandez From: Rob Zuber RE: Colowyo Hydrologic Curve Numbers (TR -95) The following is my rational for approving TR -95, which is an application to update the hydrologic curve numbers associated with reclaimed mined lands (Exhibit 7 of the Colowyo Permit Application Package). It should be noted that TR -95 does not impact modeling for watersheds in the East Pit area, including those that drain to Prospect Pond, Gulch A Pond, or Streeter Pond. I have reviewed literature related to identifying soil types, which are used for selecting hydrologic curve numbers to be used in SEDCAD models. This includes the National Engineering Handbook, Part 630; the Strifller and Rhodes report (sponsored by OSM and completed in 1981). Based on this literature as well as my professional judgment, I have determined the following options for identifying soil types, in order of preference: 1. Based on site - specific infiltration studies 2. Based on studies at nearby sites. 3. Based on pre- mining soils at the site. I have been encouraging Colowyo to perform the site - specific infiltration studies, and they indicated at one point in the adequacy process that they might be amenable to performing the study. However, now they are saying that their management will not spend the money on any studies. To my knowledge, we have not required other operators to do this, so I do not feel comfortable requiring Colowyo to perform the study. The second option is what Colowyo is proposing. They are sighting a study performed by Colorado State University staff (and sponsored by OSM) that indicates that the regraded spoil areas on a portion of Colowyo and at several other coal mines in Colorado have the hydrologic properties (namely infiltration) of Hydrologic Soil Group A and B soils. The third option is not a good method. As it states in the National Engineering Handbook (page 7 -5 in Part 630), "As a result of construction and other disturbances, the soil profile can be altered from its natural state and the listed group assignments generally no longer apply, nor can any supposition based on the natural soil be made that will accurately describe the hydrologic properties of the disturbed soil." Unfortunately, when TR -73 was approved in 2009, this method was the primary justification for classifying Colowyo's spoil piles as Hydrologic Soil Group C.