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RULE 4 PERFORMANCE STANDARDS <br />It is Colowyo's contention that the models represent nothing more than the best hydrologic estimates for a <br />described worst-case condition. The intent of the modeling is to aid in the design of sedimentation ponds to <br />predict compliance with applicable effluent standards. A primary limitation of the modeling and subsequent <br />designs is the available existing topography, which is very coarse at a 25 -ft interval. Colowyo believes it <br />would be an inappropriate use of the SEDCAD models to use them as an enforcement tool for such operations <br />as topsoil stripping; backfilling, grading, reclamation, etc. Furthermore, more detailed topography must be <br />obtained to verify results prior to implementation. <br />The scenario used for the sedimentation ponds corresponds to an active, disturbed operation. In terms of <br />groundwater, Colowyo's pits have remained essentially dry. Pumping of pit water (precipitation induced <br />surface runoff) into sedimentation ponds is not anticipated. Discharges from the ponds will remain in <br />compliance with Colowyo's CDPS Discharge Permit. The use of flocculants in sedimentation ponds may <br />also be used in accordance with the provisions of the CDPS Permit. <br />Sediment will be removed from all sedimentation ponds on an as needed basis or when the sediment level will <br />not allow effective treatment of the runoff resulting from the 10 -year, 24-hour precipitation event in <br />accordance with Rule 4.05.2. Quarterly inspections will note the level of sediment in each pond. Ponds will <br />typically be cleaned of sediment when water levels are lowest, and the least amount of precipitation is <br />expected. The removed sediment will be used as topsoil or subsoil if it meets the suitability criteria discussed <br />under Section 2.04.9. The Division will be notified of this determination if the material is selected as <br />overburden material that can be substituted for or as a supplement to topsoil. <br />All sedimentation ponds will be designed so that the minimum elevation at the top of the settled embankment <br />is at least one foot above the elevation of the water surface in the pond with the emergency spillway flowing <br />at design depth. <br />Colowyo will design, construct, and maintain the sedimentation ponds to prevent short-circuiting to the extent <br />possible. As a general rule, the inflow to the ponds will be at the opposite end from the outflow area. The <br />constructed height of the sedimentation pond embankment will be designed to allow for settling. During <br />construction, a registered professional engineer will ensure that the appropriate embankment height is <br />accomplished. For all sedimentation ponds, the entire embankment, including the surrounding areas disturbed <br />by construction, will be seeded after the embankment is completed, using the Topsoil Stockpile/Pond <br />Embankment seed mix described below. Areas in which revegetation is not successful or, where rills and <br />gullies develop, will be repaired and revegetated. <br />Colowyo will inspect the condition of each sediment pond, sediment trap, or future postmining stock reservoir <br />on a quarterly basis. All of these types of structures meet the requirements of an impoundment, and the <br />inspection procedures will meet the requirements under Rule 4.05.9 (17). Previously, Colowyo has received a <br />waiver from quarterly inspections for several existing stock reservoirs within the current permit area as <br />described under Section 4.05.9. This waiver changed the inspection frequency to annual. Following <br />construction of any future postmining stock reservoir in the South Taylor area, Colowyo may request a <br />similar waiver but until that is approved, the quarterly frequency would apply. Results of all impoundment <br />inspections will be submitted annually. <br />South Taylor/Lower Wilson — Rule 4, Pa,ge 5 Revision Date: 4/7/17 <br />Revision No.: RN -07 <br />