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Exhibit 7 Page 3 Revision Date: 4/28/22 <br /> Revision No.: TR-154 <br />“Disturbed Soils” in Chapter 7 of Part 630 of the NRCS National Engineering Handbook (NEH) <br />recommends that onsite investigations should be made to determine the HSG, including on-site <br />infiltration tests, grain size characteristics and lab permeability tests. A program meeting these <br />requirements was carried out and reported in Striffler and Rhodes (1981), and it included tests performed <br />at the Colowyo mine, as well as ten other Colorado coal mines. <br /> <br /> All of the infiltration tests performed at the Colowyo Mine reported in Table C-5 of that study indicated <br />infiltration rates well in excess of a “C” soil, and in all cases except one reflected rates that would be <br />associated with a HSG “A” soil. That exceptional case indicated infiltration rate between HSG “A” and <br />HSG B” soils. The selection of HSG”B” for the disturbed lands is therefore a conservative assumption, <br />more likely to overestimate than to underestimate runoff rates. <br /> <br />With the HSG determined as described above for undisturbed and disturbed land, the CN is then <br />dermined based on the land cover condition as described in numerous hydrologic references, all <br />ultimately based on NEH cover and condition tables. <br /> <br />The resulting hydrologic curve numbers (CN) for use in SEDCADTM4 for the various categories of land at <br />the mine are presented below in Table 1. Table 7 provides a scanned image of published curve numbers <br />typical for disturbed areas; these values will not be used directly but are only provided to support to Table <br />1. Table 8 provides a similar listing from the National Engineering Handbook (NEH-4) (USDA-SCS, <br />1985); these values are included within the drop down menus of the SEDCADTM4 model. Of the several <br />tables included here as Table 8, the last table presented here in Table 8 is applies to Range lands in arid <br />and semi-arid areas, and is the most appropriate set of Curve Numbers for the Colowyo Mine <br />applications. Values in Table 8 will not be used directly but are provided to support the values presented <br />below in Table 1.