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RULE 4 PERFORMANCE STANDARDS <br />be employed to control water within pits and /or route water between pits. The ultimate destination of such <br />water will be for operation's use (i.e. dust control), evaporation, or seepage into the backfilled spoil areas. <br />In addition to the mining, reclamation, and treatment methods described and referenced in this Section, <br />further protection of the hydrologic balance will be established by an on -going plan for monitoring potential <br />changes in surface water quality and quantity and groundwater quality. This monitoring plan is described <br />under Section 4.05.13 and the monitoring locations are graphically shown on Map 10A. Excess spoil valley <br />fill areas are located up -dip from mining and reclamation areas and periodic monitoring for seeps and springs <br />and periodic monitoring of piezometer wells will detect the formation of spoil springs. <br />4.05.2 Water Quality Standards and Effluent Limitations <br />The plan for protection and control of drainage and sediment described in 2.05.6 provides that surface <br />drainage from the disturbed area within the permit area will be passed through sedimentation control <br />structures. All ponds will be constructed and maintained to contain or treat the volume design for a 10 -year, <br />24 -hour precipitation event. The accumulation of sediment in the ponds will be monitored quarterly. In <br />addition, grab samples of water, as required, will be collected from pond discharges to measure the <br />effectiveness in meeting the applicable Colorado and Federal water quality standards. <br />The proposed sedimentation ponds have been designed and will be constructed and maintained to effectively <br />trap sediment from runoff resulting from precipitation events up to and including the 10 -year, 24 -hour <br />precipitation event. <br />Drainage from the mining area, after treatment in sedimentation ponds, is not anticipated to exceed the <br />effluent limitations of any federal or Colorado agency requirements. Baseline groundwater quality is <br />discussed in Section 2.04.7. No acid mine drainage of pH equal to or less than 6.0 is expected. For further <br />details relating to the sediment pond discharges, refer to the NPDES reports found in the 1983- through 1990 <br />Annual Reports. <br />Historically, Colowyo has experienced no pH problems with water discharges sampled in the vicinity of the <br />Colowyo operations. As reported in Section 2.04.7, all pH values of water samples taken in the vicinity of the <br />Colowyo operations have ranged between 7.2 and 8.5; therefore, it is anticipated that no acid mine drainage <br />will occur as the operations move to southward to South Taylor and west to Lower Wilson. <br />4.05.3 Diversions and Conveyance of a Watershed Less than One Square Mile <br />The drainage and sediment control measures described under Section 2.05.6 and presented in Erosion and <br />Sediment Control Structures (Exhibit 7, Item 20) provides for temporary diversion of surface drainages within <br />the permit area. A system of temporary ditches will be used to divert runoff from disturbed areas to sediment <br />ponds. Temporary diversions will be constructed to pass at a minimum the runoff from the precipitation <br />event with a two -year recurrence interval. <br />The temporary diversions drain watersheds less than one square mile in size and serve to reduce the <br />contribution of suspended solids to runoff. The diversions will be constructed with a minimum gradient to <br />pass the design flow and will be stabilized with grasses or riprap. If not removed by mining, upon completion <br />of mining and at an appropriate point mandated in the Coal Regulations of the Colorado Mined Land <br />Reclamation Board, the temporary diversions will be reclaimed as required in Section 4.05.17. <br />The only stream channels that will be impacted by the South Taylor pit are headwaters tributary to Taylor <br />Creek and West Fork Good Spring Creek, which are intermittent and drain watersheds less than one square <br />mile. There will be no upstream diversions of these streams since mining will extend to the top of the <br />South Taylor /Lower Wilson — Rule 4, Page 5 Revision Date: 7/21/14 <br />Revision No.: MR -135 <br />