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utilized in the upper four feet (includes replaced topsoil thickness) of the <br /> reclaimed soil profile. <br /> UNIT 4 Unsuitable root growth material. The lower two to three feet of interburden <br /> directly above the lower Dakota coal seam. This material consists predominantly <br /> of gray to very dark gray shale with a severe acidity limitation. This material <br /> will not be replaced in the upper four feet (includes replaced topsoil <br /> thickness) of the reclaimed soil profile. <br /> Nucla East Overburden and Interburden Characteristics. Overburden and interburden <br /> characteristics for Nucla East, are presented in Tab 6, Attachment 6-1, Geology and <br /> Overburden Assessment. Overburden and interburden suitability and lithologic units for <br /> Nucla East, described in Tab 6 (Attachment 6-1 ), are very similar to the shallow <br /> overburden units previously described for Nucla Mine. The only significant difference <br /> between mine areas is the mean thickness of Unit 1 which averages 5 feet at Nucla Mine <br /> compared to about 30 feet within the Nucla East permit area. Approximately 83 percent of <br /> the overburden and interburden material at Nucla East, consisting predominantly of Units 1 <br /> and 3, is classified as suitable for root growth and aquifer medium material (Tab 6, <br /> • Attachment 6-1 ). Unsuitable material was identified in the lower overburden and <br /> interburden (Units 2 and 4) within the proposed disturbance area (Tab 6, Appendix 6-5). <br /> Acid- and Toxic-Forming Materials at Nucla Mine Areas 1-3. Acid- and toxic-forming <br /> materials were identified in Nucla Mine Areas 1-3 as discussed in Tab 6 and listed in Tab <br /> 6, Appendices 6-4 and 6-5. Mitigation of all unsuitable zones within the underburden <br /> material (40 percent of all unsuitable zones which were identified) will be by avoidance, <br /> i.e., these zones will not be disturbed by mining activities. <br /> Normal overburden/interburden handling by dozers and scrapers will often effectively blend <br /> or dilute unsuitable parameter concentrations to acceptable levels or naturally bury <br /> unsuitable materials. The upper Dakota Coal seam and adjacent very dark gray, <br /> carbonaceous shale which occurs in the upper portion of the <br /> overburden/interburden/underburden (OIU) section may, by the normal mining process, often <br /> be buried in the adjacent pit. <br /> Where blending or normal burial are ineffective, the overburden (Unit 1) and sandstone <br /> interburden (Unit 3) will be selectively handled as a topdressing material to ensure that <br /> all unsuitable regraded spoil is covered. In these situations, unsuitable spoil will be <br /> topdressed with a minimum of four feet of suitable material (includes replaced topsoil <br /> thickness). <br /> 21-3 Revised 08/19/88 <br />