2020-12-09_REVISION - C1994082 (11)

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In order to meet CDRMS Regulation 3.03.1(3)(b), SPL provided modeling information demonstrating sediment transport,comparing the reclaimed areas to the adjacent unmined areas using the RUSLE Revised Universal Soil Loss Equation). The RUSLE calculates expected average annual soil loss per acre of land(rate of erosion),based on slope,rainfall pattern, soil type and vegetation type from the equation ARKLSCP;where A (average annual soil loss)represents the product of the six variables defined below. The RUSLE is an empirical formula approximating expected sediment yield from a parcel of land. It is reasonable to hypothesize that the RUSLE is useful for discriminating between two significantly different yield rates. However, it is not useful for discriminating between two yield rates of similar magnitude. The variables used in the calculations are reasonable for making the comparison. The comparison therefore is technically valid. A chief cause of soil loss is transport of solids in suspension. The comparison indicates the total drainage area does not yield more suspended solids than pre-mining conditions. The definition for the RUSLE variables are presented below in Table 4: Table 4. R LISLE Variables and their Definitions. Only the "C" factor varies Variable Definitio between the pre-mining and the R Climatic erosivity factor due to precipitation post-mining calculations; all K Soil erodibility under a standard condition other variables are constant for LS Slope length and steepness factor pre-and post-mining. The C C Cover management/land use factor takes into account land P Support practices factor use,vegetative canopy cover, effective cover and grass type. The pre-mining calculation utilized baseline vegetation cover values from the permit application. Table 5,of the 24 November 2020 adequacy review response comprises a portion of SPLs' response to The Divisions 21 August 2020 adequacy questions by addressing the RUSLE variables and calculations for each bond release polygon. The average tons/acre/year of sediment yield 0.06 from the SL8 proposed bond release areas when compared to the Divisions RUSLE demonstration for the Yoast reference areas (as per SL7), 0.37 tons/acre/year show that the proposed SL8 bond release areas contribute less sediment then the surrounding reference areas meeting Rule 3.03.1(3)(b) requirements. As the reference areas remain the same for both the SL7 and SL8 bond release actions, The Division considers the variables and associated comparison valid. IV. PROPOSED DECISION The approved post-mining land use for the land applicable to SL8 is livestock grazing and wildlife habitat. The bond release block,the subject of this bond release application (SL8),has been reclaimed for livestock grazing and wildlife habitat. The revegetation plan was designed to achieve the approved post mining land use and appears to be on track to meet this objective based on the vegetation cover established within the bond release block. Based on the observations above, the Division proposes to approve SPL's request for a partial Phase II bond release for the Yoast Mine. This proposed decision will release the applicant from Phase II liability Permit Number C1994 082 Page 14 of 15