2018-01-03_PERMIT FILE - C1981010A (16)

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Additional mixing of the backfilled spoil materials occurs during grading to the approximate original surface contour. Figure 3.5-2 shows that in the regraded spoil profile the middle zone overburden materials (dozer spoils) are placed on the bottom of the pit floor. This is followed by the dragline placing spoils on top of the dozer spoil, with truck/loader spoil material being placed on top of the overburden profile. Finally, during spoil regrading operations, the tops of multiple spoil ridges and truck dumps are flattened out with dozers, resulting in additional spoil mixing. Overburden Analvtical Results Seventy-six (76) overburden samples from the PR -7 Mine Expansion Area were analyzed for 30 physical and chemical parameters in this evaluation. A summary of the laboratory analyses performed and analytical methods used is provided in Table 2.7-8a Overburden Laboratory Analyses and Methods. The parameters analyzed included soil paste pH; electrical conductivity (EC); percent sand, silt and clay to determine soil texture; water soluble calcium (Ca), magnesium (Mg), and sodium (Na) to calculate the sodium adsorption ratio (SAR); saturation percentage (SP); water soluble selenium (Se) and boron (B); total organic carbon; extractable molybdenum (Mo), arsenic (As), iron (Fe), manganese (Mn), lead (Pb), and zinc (Zn); mercury (Hg); percent solids; extractable nitrate/nitrite as nitrogen, and sulfur forms (total, HCI, HNO3, organic, pyritic sulfide, sulfate). A summary of the laboratory analytical results are provided in Table 2.7-8c Overburden Geochemical and Physical Laboratory Analytical Results. A discussion of the parameters having currently approved plant growth medium suitability criteria is provided below. Soil Reactivity (pH). Soil reactivity or pH is a valuable indicator parameter to monitor reclamation efforts at mine sites. Circumneutral soil pH values (6 to 8) favor nutrient availability and microbiological activity in reclaimed soils. Overburden pH values ranged from 3.8 to 8.5, and averaged 7.52 ± 0.78. Approximately 95 percent (or about 200 feet) of the overburden falls within the suitable pH range of 5.5 to 8.5, while 2.77 percent (or 5.8 feet) is considered marginal (pH 5.0 to 5.5 and 8.5 to 9.0), and 1.91 percent (or 4 feet) is considered unsuitable (pH less than 5 or greater than 9.0). Samples 75 (marginal) and 76 (unsuitable) are located approximately 10 and 15 feet below the expected mining level and will not be affected by mining. This means that all of the truck/loader spoil, all of the dozer spoil and about 90 feet or 96 percent of the dragline spoil has suitable pH. Only Sample 30 (3.6 feet of dragline spoil) has a marginal pH. A detailed study, Selective Placement of Coal Strip Mine Overburden in Montana, III Spoil Mixing Phenomena, published by the Montana Agricultural Experiment Station (1978) evaluated the effects of dragline mixing of overburden with suspect suitability values. The comprehensive study concluded that "when the inhibitory material constituted less than (<) 5% of the overburden volume, such material was essentially not detectable in the resultant spoil pile" (referred to hereafter as the "5 percent dragline mixing criterion"). Based on these study results, the soil reactivity value of the overburden in the PR -7 Mine Expansion Area would be considered suitable after mixing during mining and reclamation. Verification of the likely homogenization of high and low pH values in reclaimed spoils is documented in the historic regraded spoil data presented in Appendix I Overburden Characteristics - 4.0 Analytical Comparison Between Regraded Spoils and Adjacent Overburden Drill Holes at Trapper Mine, 1985- 1987 and in the Annual Reclamation Reports from 1988 through 2000, where a total of 110 regraded 2-391v ;,;vision: ► R` 08 approved: