2014-11-24_REVISION - C1981010 (2)

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INTEROFFICE MEMORANDUM SUBJECT: C1981 -010 TRAPPER MINE SL -16 SEDIMENT CONTROL DEMONSTRATION REVIEW DATE: 11/21/2014 TO: JARED EBERT FROM: JASON MUSICK In accordance Rule 3.03.1(3)(b) and with Section IILA.7.a. of the Division's 1995 "Guideline Regarding Selected Coal Mine Bond Release Issues ", Trapper submitted two evaluations to demonstrate that runoff from each Bond Release Block will result in equivalent or lesser sediment contribution as compared to baseline conditions and similar undisturbed lands. Trapper's bond release application compares pre and post mining sediment yields by calculating the expected sediment yields for both conditions. Trapper used the USLE and SEDCAD4 calculation methods. The USLE calculates expected average annual soil loss per acre of land and SEDCAD 4 calculated the expected setteable solids concentration in runoff from a precipitation event. USLE Comparison: Only the "C" factor is varied between the premining and the postmining calculations; all other variables are constant for pre and post mining. The premining calculation utilized baseline vegetation cover values from the permit application (Table 2.3 -13 and 4.4 -2). The postmining calculation uses vegetation cover data for the disturbed area collected in 2014. The resulting calculation indicates postmining soil loss rate is 31 % of the premining rate. 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, therefore, the comparison indicates the untreated drainage, currently, does not yield more suspended solids than prior to mining. The USLE is an empirical formula that can be expected to give a rough approximation of the expected sediment yield from a parcel of land. It is reasonable to believe that the USLE is useful for discriminating between two yield rates that are significantly different, but is not useful for discriminating between two yield rates of similar magnitude. For the bond release partials in SL -16, Trapper calculated a premining sediment yield rate of 3.38 tons per acre per year and a postmining rate of 1.05 tons per acre per year. The considerable difference between these two calculated rates indicates the postmining rate is likely less than the premining rate. This conclusion is only valid if Trapper used reasonable variables in the USLE calculations. The Division believes the variables Trapper selected are reasonable. The variables are similar to those the Division has found to be acceptable in Trapper's previous bond releases. The lands in SL -16 are similar to the lands in previous bond releases. Trapper's basis for selecting each variable and the Divisions review of each variable is in the table below.