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utilized baseline vegetation cover values from the permit application (Table 2.3 -13 and 4.4 -2). The <br />post- mining calculation uses vegetation cover data for the disturbed area collected in 2014. The <br />resulting calculation indicates post- mining soil loss rate is 31% of the pre- mining rate. The variables <br />used in the calculations are reasonable for making the comparison. The comparison therefore is <br />technically valid. A chief cause of soil loss is transport of solids in suspension; therefore, the <br />comparison indicates the untreated drainage, currently, does not yield more suspended solids than <br />prior to mining. <br />The USLE is an empirical formula that can be expected to give a rough approximation of the <br />expected sediment yield from a parcel of land. It is reasonable to believe that the USLE is useful for <br />discriminating between two yield rates that are significantly different, but is not useful for <br />discriminating between two yield rates of similar magnitude. For the bond release partials in SL -16, <br />Trapper calculated a pre- mining sediment yield rate of 3.38 tons per acre per year and a post- mining <br />rate of 1.05 tons per acre per year. The considerable difference between these two calculated rates <br />indicates the post- mining rate is likely less than the pre- mining rate. This conclusion is only valid if <br />Trapper used reasonable variables in the USLE calculations. <br />The Division believes the variables Trapper selected are reasonable. The variables are similar to <br />those the Division has found to be acceptable in Trapper's previous bond releases. The lands in SL- <br />16 are similar to the lands in previous bond releases. <br />Trapper's two sets of pre- mining and post- mining SEDCAD models are in Appendix 3 of the bond <br />release application. Trapper utilized baseline data for selecting parameters for the SEDCAD 4 <br />models of pre- mining conditions. Trapper utilized recent data in the bond release submittal for the <br />models of post- mining conditions. The parameters reasonably represent the conditions in the field for <br />the pre- mining conditions and the post- mining conditions. <br />Trapper used SEDCAD 4 to model 20 -acre study areas having sedimentologic characteristics <br />representative of each bond release parcel. SEDCAD 4 calculated the peak settleable solids <br />concentration of runoff flowing to a single discharge point at the down - stream end of each 20 -acre <br />study area. SEDCAD 4 was run for pre- and post- mining conditions. The Division believes use of <br />20 -acre study areas is a valid approach for modeling sedimentation in the reclaimed drainages in the <br />bond release area because it enables applying each block's actual vegetative cover data to the <br />SEDCAD run for each block. The total sediment load calculated for the pre- mining block A, F, K &Z <br />RSAB &C 20 -acre study area was 32.6 tons of sediment and 13.3 tons of sediment for post- mining <br />block A, F, K &Z RSAB &C. TMI's SEDCAD 4 modeling concluded that the calculated post- mining <br />peak settleable solids generated from each block is much less than the calculated pre- mining value. <br />Based on Trapper's SEDCAD 4 modeling using baseline data, the Division finds that the lands <br />proposed for Phase II bond release are not contributing suspended solids to streamflow or runoff <br />outside the permit area in excess of pre- mining levels. <br />Summary and Conclusions <br />Based upon a review of the mine permit, the applicant's bond release application, the October 15, 2014 <br />Page 9 of 10 <br />