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SEDCAD 4. The USLE calculates the expected average annual soil loss per acre of land. SEDCAD <br />4 calculates the expected settleable solids concentration in runoff from a precipitation event. Page <br />3 of the Division's bond release Guideline explains that a predictive storm model may be used for <br />compazing [he suspended solids load in runoff in pre- and post-mining conditions. <br />Reliability of using USLE for comparing pre- and post-mining sediment yields. The USLE is <br />a simple empirical formula that can be expected to give a rough approximation of the expected <br />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 signiticantly different, but is not useful for <br />discriminating between two yield rates of similar magnitude. For the bond release parcels, Trapper <br />calculated apre- mining sediment yield rate of 2.05 tons per acre per year and apost-mining rate of <br />0.47 tons per acre per yeaz. The considerable difference between these two calculated rates indicates <br />the post-mining rate probably is less than the pre-mining rate. This conclusion is valid only if <br />Trapper used reasonable variables in its USLE calculations. <br />Reasonableness of variables Trapper used in USLE calculations. Trapper's basis for selecting <br />each variable is explained on pages 10 and 11 of the SL-02 submittal. The Division reviewed each <br />of the vaziables and summarized the results in a memo to the SL-02 file dated December l6, 1999. <br />As explained in the memo, the Division believes the variables Trapper selected aze reasonable. <br />Also, the variables are similar to those the Division selected in its review of the previous bond <br />release SL-O1. The lands in SL-02 are similar to those of SL-01 in terms of sedimentologic <br />characteristics. In the SL-01 review the Division verified the reasonablesness of the variables it used <br />in a USLE calculation by surveying sediment volumes in Trapper's ponds and then calculating the <br />soil loss rates that would have produced those volumes. Given the similar sedimentologic <br />chazacteristics in the SL-O1 and SL-02 areas, the Division's pond survey exercise conducted in the <br />review of SL-O1 lends support to Trapper's USLE calculation in the SL-02 application. <br />Reasonableness of parameters Trapper used in SEDCAD 4 runs. Trapper's SEDCAD 4 <br />computer program tuns are shown in Appendix 3 of the bond release submittal and aze sutnmazized <br />in Tables 1.4-1 and 2, on pages 13 and 14 of the submittal. Trapper used baseline data for selecting <br />parameters for the SEDCAD 4 tuns of pre-mining conditions. Trapper used recent data in the bond <br />release submittal and annual reclamation reports for the runs of post-mining conditions. The <br />Division reviewed each of the parameters and concluded the parameters reasonably represent the <br />conditions in the field for the pre-mining conditions and the post-mining conditions (please see the <br />memo to the SL-02 file dated December 16, 1999). <br />Substitution of SEDCAD's settleable solids for regulation's suspended solids. Section <br />3.03.1(3)(b) of the Regulations specifies that "suspended solids" be used in compazing pre- and post- <br />mining sediment yields from lands proposed for Phase II release. SEDCAD 4 does not calculate <br />suspended solids, but instead calculates total sediment and settleable solids. Trapper compared total <br />sediment for pre-mining and post-mining coditions. Trapper's use of total sediment, rather than <br />suspended solids, is valid for compazing pre- and post-mining sediment yield because the total <br />Trapper Mine <br />Phase II Bond Release Page 10 12/23/99 <br />