2020-03-31_PERMIT FILE - C1981019A (6)

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RULE 4 PERFORMANCE STANDARDS This application contains calculations used to determine runoff volumes and flow rates for a theoretical 10 -year, 24-hour precipitation event and subsequent sediment volumes. The precipitation data were obtained from the NOA Atlas 2, Volume 3 for Colorado; soil types were obtained from the Soil Conservation Service, and are shown on the Soils - South Map (Map 6) and the Soils - North Map (Map 5). The ongoing mining activities within each watershed of the permit area will create constantly changing hydrologic conditions. The design models are generally based on a static, theoretical scenario, utilizing SEDCAD+ Version III which considers a final reclamation and pit closure scenario in each watershed. Please see Map 41 - Watershed Boundaries for Hydrologic Modeling for a delineation of the areas used for these modeling purposes as well as the individual maps associated with each SEDCAD run. The dates indicated on Map 41 are for development of the worst case scenario for hydrologic modeling and are not a definitive schedule for mining and reclamation activities. It is Colowyo's contention that the proposed models represent nothing more than our best hydrologic estimates for a described range. of conditions. According to Applied -Hydrology and Sedimentology for Disturbed Areas, Barfield, Warner and Haan, 1981, "Hydrologic calculations are estimates, with the error in these estimates increasing as the degree of approximation increases or as the estimation procedure is applied beyond the range of conditions for which it is intended." Accordingly, it would be inappropriate for these modeling scenarios to be used or relied upon other than for their intended purpose: as an aid in the design of sedimentation pond structures to predict compliance with applicable effluent standards. Colowyo believes it would be an inappropriate use of the SEDCAD models to use them as an enforcement tool for such operations as topsoil stripping; backfilling, grading, reclamation, etc. The scenario used for the Work area pond and loadout ponds correspond to an active, disturbed operation. In terms of ground water; Colowyo's pits have remained essentially dry. Pumping of pit water (precipitation induced surface runoff) into sedimentation ponds is unlikely but, nevertheless should it be necessary to pump, ponds 001, 002, 007; 008, 009 and 010 are designed to handle a small base flow. Colowyo's CDPS Discharge Permit (to be renewed in 1998) will provide for pumping of pit water from both precipitation induced surface runoff and groundwater sources should they be -encountered. Pit pumping rates will be controlled so that there will be no outflow thru the emergency spillway from the combined flows of pit pumping and up to the l Oyear, 24-hour storm event. Discharges from Colowyo's ponds will remain in compliance with the applicable CDPS Discharge Permit requirements. The use of flocculents in sedimentation ponds may also be used in accordance with the provisions of the CDPS Permit. Sediment will be removed from all sedimentation ponds on an as needed basis or when the sediment level will not allow effective treatment of the runoff resulting from the 10 -year, 24-hour precipitation event in accordance with Rule 4.05.2. Quarterly inspections will note the level of sediment in each pond. Ponds will typically be cleaned of sediment when water levels are lowest, and the least amount of precipitation is expected. The removed sediment will be used as topsoil or subsoil if it meets the suitability criteria discussed under Section 2.04.9. The Division will be notified of this determination if the material is selected as overburden material that can be substituted for or as a supplement to topsoil. Rule 4 Performance Standards 4-11 Revision Date: 5/25/18 Revision No.: RN -07