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SEDCAD 4. The LISLE 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 />8 of the Division's bond release Guideline explains that a predictive storm model may be used for <br />comparing the suspended solids load in runoff in pre- and post-mining conditions. <br />Reliability of using LISLE for comparing pre- and post-mining sediment yields. The LISLE 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 LISLE is useful for <br />discriminating between two yield rates that aze significantly 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 year. 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 LISLE calculations. ~ ~ v~,,;.,~ w>~ ~~--u9 <br />Reasonableness of variables Trapper used in LISLE calculations.l(I'rapper's basis for selecting <br />each variable is explained on pages ]0 and 11 of the SL-02 submittal. `The Division reviewed each <br />of the vaziables and summazized the results in a memo to the SL-02 file dated December 16, 1999. <br />As explained in the memo, the Division believes the vaziables Trapper selected are reasonable. <br />Also, the variables aze similar to those the Division selected in its review of the previous bond <br />release SL-O1. The lands in SL-02 aze similar to those of SL-O1 in terms of sedimentologic <br />characteristics. In the SL-Ol review the Division verified the reasonablesness of the variables it used <br />in a LISLE 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 similaz sedimentologic <br />characteristics 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 LISLE 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 are summarized <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 subtittal 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 l6, 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 comparing 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 settleable solids; consequently, Trapper used settleable solids <br />for calculating the quantity of sediment yielded by the bond release parcels. Settleable solids are the <br />coaser-grained fraction of the sediment load that settles out from a still body of water. Trapper's use <br />of settleable solids, rather than suspended solids, is valid for comparing pre- and post-mining <br />Trapper Mine <br />Phase II Bond Release Page 10 12/21/99 <br />