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Center Ridge Permanent Impoundment <br />Octobtr, 1991 <br />Page 3 <br />• <br />The inspection report will include any appearances of instability, structural weakness, hazardous <br />conditions, depth and elevation of impounded waters, existing storage capacity, and any other <br />aspects of the structure affecting stability. A copy of all reports and inspections shall be retained at <br />the mine site. <br />EMERGENCY PROCEDURES -- If any examination or inspection discloses that a potential hazard <br />exists, the person who examined the impoundment will promptly inform the CMLRD of the <br />findings and of the emergency procedures formulated for public protection and remedial action. If <br />adequate procedures cannot be formulated or implemented, the CMLRD will be notified <br />immediately. <br />Pond Justification <br />This permanent impoundment will be a developed water resource, consistent with the post-mining <br />land use and reclamation plan map of Permit C-80-001. Postmining uses of the water in this <br />impoundment will include, but not be limited to, stock and terrestrial wildlife watering, and fish <br />and aquatic wildlife development. <br />Historically, the Center Ridge sediment pond has had a stable permanent pool. This condition is <br />not expected to change with the permanent impoundment. The configuration of the impoundment is <br />adequate to support the postmining land uses of rangeland and wildlife habitat <br />• Water quality in this structure is expected to be adequate based upon past monitoring results. The <br />previous three years of NPDES monitoring reports for discharge point 003 (NPDES Pemvt No. 0- <br />00032638) indicate acceptable water quality levels for pH, total suspended solids, total dissolved <br />solids, settleable solids, oil & grease, total recoverable iron and total recoverable manganese <br />(copies of these reports are on file at [he mine site and the CMLRD offices). Therefore, it is <br />expected that this impoundment will continue to have adequate water quality for the post-mining <br />land use and not result in any diminution of water quality for downstream users. <br />The ratio of watershed azea (in acres) to impoundment permanent pool volume (in acre-feet) is <br />approximately 159:1. At minimum, a 60:1 ratio is generally recommended by the Soil <br />Conservation Service for northwest Colorado (Figure 10, SCS Agricultural Handbook number <br />590). Consequently, the size of the impoundment should be adequate and suitable for the intended <br />purpose. <br />This impoundment will not result in diminution of quality or quantity of water uses downstream <br />since the impoundment is currently at its permanent pool elevation. Even if the water level in the <br />impoundment needs to be pumped down sometime in the future to facilitate maintenance, there <br />should be very little lag time associated with the water level in the impoundment reaching the <br />permanent pool elevation. This permanent impoundment will have a positive effect on downstream <br />users in that peak flood flows will be retarded and released downstream at a slow and steady rate. <br />The impoundment has adequate and safe access for the proposed water users (livestock and <br />wildlife). <br />The Soil Conservation Service generally recommends that a seven foot minimum depth of water be <br />maintained in ponds in moist subhumid areas (Figure 11, SCS Agricultural Handbook #590). <br />• Consequently, when sediment control is no longer needed, the mute will ensure that the permanent <br />impoundment has at least seven feet of depth. <br />App. 4.6-0-3 <br />September 7, 1993 <br />