2009-09-17_REVISION - C1980007 - Laserfiche WebLink

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MOUNTAIN COAL V COMPANY L.L.C. A Subsidiary of Arch Westem Resources, LLC September 17, 2009 West Elk Mine P.O. Box 591 5174 Highway 133 Somerset, CO 81434 (970) 929-5015 Fax (970) 929-5595 Mr. Tom Kaldenbach Colorado Division of Reclamation Mining and Safety Office of Mined Land Reclamation 1313 Sherman Street, Room 215 Denver, Colorado 80203 Re: Mountain Coal Company, LLC, West Elk Mine; Permit No. C-1980-007; Technical Revision No. 119, Enhancing MCC's Stormwater and Water Storage Management, Adequacy Comments Responses Dear Mr. Kaldenbach: Mountain Coal Company, LLC (MCC) provides the following responses to the adequacy comments on Technical Revision No. 119 per your letter of September 14, 2009. The responses are provided in the same order as presented in your letter. 1. Yesterday, MCC submitted a stability analysis (by e-mail) completed by Barr Engineering for MCC documenting compliance with Rule 2.05.3(4)(a)(ii)(B) and 4.05.9(8)(b)). It is my understanding that this analysis is currently under review by the CDRMS Engineer. 2. Upon review of the SedCAD+ model, MCC found that MB-3 showed 0.0 ml/1 for peak settleable solids for both the 10-year and 25-year, 24-hour storm events, but the MB-5E value for the 10-year, 24-hour event was greater than 0.5 ml/1 as you'd noted. Please consider that the SEDCAD+ modeling assumes that the pond discharge outlets are Den during storm events. However, the ponds are designed at a larger capacity than the 10-year storm event volume, and MCC's pond operations practices include having all outlet structures closed at all times (except when discharging) allowing the capture and treatment of storm event water. Water in the ponds is monitored to insure compliance with discharge limitations per MCC's CDPS permit prior to discharge. This is stated in the SedCAD+ model as follows. "West Elk Mine has gated valves on the primary spillways of all ponds. These valves are typically closed. This allows the mine to increase their storage, if required, which will in turn provide a longer time period for sediment to settle. The mine can thus better regulate the suspended sediment concentration before water is discharged. As a result, the SEDCAD+ results for the ponds are conservative. " Nonetheless, MCC's CDPS permits allow for an exemption to the 0.5 ml/1 daily max limitation for settleable solids as follows; "Any discharge or increase in volume of a discharge caused by precipitation within any 24- hour period greater than a 10-year, 24-hour event (or series of storms or snowmelt equivalent volume) may comply with the following ... Settleable Solids, ml/I - Report". 3. The reference to "two inlet pipes" has been corrected in the attached permit text pages to "two 12-inch gated drain pipes" to clarify that these are outlets (see text excerpt below). 4. The following text has been revised to clarify how the two independent cells of Pond MB-5E will be operated, including which outlets will be primarily used for pond discharges.