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COAL <br />S <br />COMPANY .� <br />�Tlr A E��¢G <br />EOPLE•PSODUCTION• <br />Second, the appropriate curve numbers to use for regraded lands are <br />largely determined by the assumptions made regarding infiltration rates. <br />CCC maintains there is overwhelming evidence to support the use of HSG <br />"B" soils for all reclaimed areas, and in fact the use of HSG B actually <br />constitutes a conservative assumption. As discussed below, a strong case <br />can be made for selecting HSG A, although that is not proposed here. <br />Referring to Section 630.0701 of the National Engineering Handbook, <br />each HSG is described, and the final sentence of each soil group <br />description provides a minimum infiltration rate. Specifically for deep <br />soils such as regraded spoils covered with topsoil, <br />Hydrologic Soil Group A has a minimum infiltration rate of 1.42 in/hr, <br />Hydrologic Soil Group B has a minimum infiltration rate of 0.57 in/hr, <br />Hydrologic Soil Group C has a minimum infiltration rate of 0.06 in/hr, <br />and Hydrologic Soil Group D has an infiltration rate of less than that. <br />Finally, the results presented in the 1981 Striffler and Rhodes study <br />clearly indicate that regardless of the mine location, the infiltration rates <br />observed are well beyond those normally assigned to a HSG C soil. <br />Although Colowyo's data is limited to 6 sites which would have had an <br />undisturbed mapped soil with HSG C, the infiltration rate observed in the <br />field tests was well beyond a Type C soil. In fact it appears more like a <br />HSG A soil. The average measured infiltration rate was 5.5 cm /hr (2.2 <br />inches /hr; Table 10 in Striffler and Rhodes). Figure 2 presents a plot of <br />the average infiltration on all 14 sites in that study. These are described <br />in Tables A -1 through A -14 of the Striffler and Rhodes report. There are <br />76 field infiltration tests altogether represented on Figure 2. Figure 2 is <br />for illustration purposes only and is not intended to be part of the TR -95 <br />revision. Eleven of the sites are clearly HSG B or A soils. Only two of the <br />sites approach a HSG C soil. <br />CCC never intended to use Striffler and Rhodes to extrapolate between <br />different mines or different regions, but the results of all of the tests <br />clearly indicate the infiltration rate of reclaimed lands is better <br />represented as HSG A or B. In Colowyo's case, the measured infiltration <br />rates suggest HSG A, with infiltration rates over 2 inches /hr. <br />A Western Fuels- Colorado, LLC mining property <br />