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Section 2.05.4(2)(d) Page 3 December 2021 (RN-02) <br />Overburden Thickness Map, shows the area that had sufficient overburden to contain the clay beds. <br />These clay beds were mined in the area where the overburden was the thickest. When these two <br />clay lenses were being excavated during mining, they were thoroughly mixed with coarser textured <br />overburden materials. The mixing and dilution process was accomplished in the backfill by: 1) the <br />overburden shovel was digging a full face through the clay beds, and 2) the haul trucks with the <br />diluted clay beds which were mixed and diluted during mining dumped their load along a dump <br />face which resulted in significant additional mixing and diluting of the clayey materials. After this <br />process, the concentration of the high clay material was minimal and compactability of the <br />regraded overburden was negligible. Where the clay beds were below the overburden zone they <br />were handled as less suitable material along with the rest of the overburden and were buried in the <br />backfill prior to placement of Bench 1. <br /> <br />Bench 1 Monitoring Program <br />To document the suitability of the Bench 1 as potential rooting material, a Bench 1 sampling and <br />analysis program will be implemented. A life of mine sampling grid has been established on 500 <br />foot centers and each sampling location on the grid corresponds to a letter/number combination. <br />Using this grid interval, each sampling location represents an area of about 5.7 acres in size. A <br />minimum of four feet of Bench 1 will be sampled prior to topsoil replacement with a hydraulic <br />soil sampler, a bucket auger, or other suitable equipment. Two representative samples, each <br />representing a two-foot depth increment will be collected. The vertical sampling increments will <br />be 0.0 to 2.0 feet and 2.0 to 4.0 feet. Bench 1 suitability parameters are presented on Table <br />2.05.4(2)(d)-1 and the results will be presented annually in the Annual Reclamation Report. <br /> <br />Table 2.05.4(2)(d)-1 Criteria for Evaluating Bench 1 Suitability <br />PARAMETER SUITABLITY <br />pH Between 6.1 and 8.5 <br />Neutralization Potential Ratio (NPR) ≥ 2 <br />Course Fragment Content (%) ≤ 20 <br />Sodium Absorption Ratio (SAR) ≤ 15 <br />Electrical Conductivity (mmhos/cm) ≤ 6 <br />Carbonate (%) ≤ 20 <br />Selenium (ppm) 1 ≤ 1 <br />Texture All soil textures except: s, ls, sc, sic, c <br />1This value for selenium was chosen based on guidance from Dave Dearstyne of NRCS and recent data that other states are using. <br />Mr. Dearstyne is currently working extensively (2008) with the Selenium Task Force on the western slope and is very <br />knowledgeable about this issue. Mr. Dearstyne noted that there is no concern for selenium unless Mancos shale strata are being <br />placed near the surface. At the NHN Mine, no Mancos shale has been encountered in the overburden. Also, all tests of water soluble <br />selenium in the pre-mine soil survey of NHN Mine show all values less than 0.1 ppm. Mr. Dearstyne believes a threshold suitability <br />level of 0.2 ppm in the topsoil material and 1.0 ppm in the subsoil material is adequate. These are the levels proposed. <br /> <br />Unsuitable Bench 1 Backfill Mitigation Plan <br />If the results of the sampling indicate that one or more of the unsuitable levels are reached, the <br />Division will be contacted. Also at that juncture, NHN will resample the area to determine the