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Mr. Forrest Luke <br /> Page 6 of 7 <br /> July 2020 <br /> 6. Please provide the Division with discussion on how Trapper plans to backfill and <br /> regrade to ensure spoil participle distribution matches that which has been modeled <br /> using the "depth-based" approach. <br /> On 16 July DRMS received TMI's response to the above question and is reviewing the information <br /> submitted. <br /> Rule 4.27.4(2) Limited Variances—Watershed Improvements <br /> DRMS July 2020 <br /> DRMS notes that for the validity of the above mentioned watershed improvement rule to apply,the <br /> improvement shall be in accordance with the approval by the appropriate State Agency. As TMI has <br /> a water quality permit in place comprising approved and permitted NPDES outfalls, QW monitoring <br /> wells and must meet the prescribed water quality standards as per their permit, and as there are no <br /> requested changes as per the water quality infrastructure at TMI DRMS finds that adequate <br /> protections are in place. <br /> The requirements of the above rule are adequately met. <br /> Trapper submitted the following documentation applicable to the above mentioned rule: <br /> 1. Evaluation ofPre Mine and Post Mine Sur ace Hydrology Characteristics Associated with <br /> the Trapper Mine Approximate Original Contour Variance Application <br /> 2. Flume Gulch Pre Mine Hydrology and SedimentoloU <br /> 3. Flume Gulch Post Mine Hydroloa and Sedimentolo <br /> 4. Johnson Gulch Pre Mine HydrolM and SedimentolM <br /> 5. Johnson Gulch Post Mine Hydrology and SedimentologE <br /> The above submittals comprise the analysis for the demonstration that runoff from the affected <br /> area will result in equivalent or lesser sediment contribution as compared to baseline conditions <br /> and similar undisturbed lands. Trapper Mine uses a combination of SEDCAD modeling and the <br /> Universal Soil Loss Equation (LISLE). <br /> The variables used in the calculations are reasonable for making the comparison and fall on the <br /> side of conservative. The comparison therefore is technically valid. A chief cause of soil loss is <br /> transport of solids in suspension. The comparison indicates the affected drainage area does not <br /> yield more suspended solids than pre-mining conditions. <br /> The design storm modeled comprised the 100 year 24 hour storm for both pre and post mining in the <br /> Johnson Gulch and Flume Gulch watersheds. This same storm event is utilized by TMI in phase II <br />