2015-10-06_PERMIT FILE - C1981019A (3)

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RULE 4 PERFORMANCE STANDARDS 4.05.7 Discharge Structures The sedimentation ponds at Colowyo are designed to treat the theoretical 10 -year, 24-hour storm event in accordance with Rule 4.05.6(3)(a). As such, the general operation of the ponds will be a passive discharge system where water is allowed to discharge automatically as necessary. Colowyo will sample discharges as appropriate to remain in compliance with applicable CDPS Permit requirements. Pond dewatering through a manual headgate may be performed as necessary to lower the water level depending on operational requirements. Manual dewatering of ponds will meet applicable CDPS Permit standards. Discharge from sedimentation ponds will be controlled by energy dissipaters and flow check devices where necessary. All embankment ponds utilize separate principal and emergency spillways with the emergency spillway located at a minimum of 1 foot above the elevation of the maximum water surface during the discharge of the 10 -year, 24-hour storm event through the principal spillway. The principal spillways are designed for the 10 -year, 24-hour storm event and the emergency spillways are designed to pass the 25 -year, 24-hour storm event in accordance with Rule 4.05.6(5). The design requirements for existing ponds can be found on each of the pond as -built drawings or in Exhibit 7, Item 15 of the existing permit document. All embankment sedimentation ponds will provide a non -clogging dewatering device or conduit spillway to remove water storage from inflow. Design requirements for the currently proposed ponds can be found in Exhibit 7, Item 23. For compliance purposes, in systems that incorporate ponds in a series, CDPS effluent quality parameters will only apply to the last pond in the series that directly discharges into the receiving stream or drainage. Out of pit designed ponds internal to the last pond in the series will be inspected and maintained in the same manner as the "compliance" pond to ensure proper sediment control and design performance. 4.05.8 Acid-forming and Toxic -Forming Spoil Acid forming materials do not exist in significant quantities within the overburden to be removed by the mining operations. A discussion on the overburden at the Colowyo operation has been conducted as set forth in Section 2.04.6. A discussion of the overburden monitoring plan is set forth in Section 2.05. Acid -Base Accounting shows that 19 out of 4,212 feet of analyzed over - and inter -burden has a net acid -generating potential, and the average acid -neutralizing potential to acid -generating potential ratio is strongly weighted toward acid -neutralizing in each borehole (Exhibit 6, Item 9). 4.05.9 Post -Mining Impoundments Colowyo constructs small impoundments on reclaimed areas in accordance with Section 4.05.9 of the CMLRD regulations for Coal Mining, 3/21/01. These small impoundments are essential and basic to the management of the rangeland post -mining land use of livestock grazing and wildlife habitat. The design of post -mining impoundments provides for structures having a vertical height less than five feet from the bottom of the channel to the bottom of the spillway and impound less than two acre-feet of water. As such they are exempt from Division of Water Resources, Office of State Engineer requirements. Water harvesting ditches may also be used to enhance the function of the impoundments, which is consistent with practices employed on adjacent rangelands. The impoundments collect surface runoff from precipitation events and snowmelt from reclaimed areas: Thei impoundments do not result in the diminution of the quality or quantity of wa.er for dowr. stream water users. Colowyo is the holder of water rights immediately downstream. During periods of low precipitation, the impoundments may be dry, which is corsistenc with Collom — Rule 4, Page 10 Revision Date: 9/28/11 Revision No.: PR -03