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

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spoil pH values are reported to average a pH of 7.28, which is consistent with the average pH (7.52) obtained in this evaluation. Electrical Conductivity (EC). EC is a measure of soil salinity; increases in soluble salts increases EC. Soils with excess salts reduce plant water availability and interfere with the growth of vegetation. Overburden EC values ranged from 0.328 to 5.3 millimhos per centimeter (mmhos/cm), and averaged 1.705 ± 0.905 mmhos/cm. The PR -7 overburden EC values generally indicate low salinity and fall within the WDEQ suitable EC range of 0 to 8 mmhos/cm. The average EC value is consistent with historic regraded spoil data presented in the 1989 through 2000 Annual Reclamation Reports where a total of 110 regraded spoil EC values were reported to average 1.98 mmhos/cm. Based on these results the probable salinity value of the overburden in the PR -7 Mine Expansion Area would lie within the suitable category after mixing during mining and reclamation. Soil Texture. Soil texture, or the relative amounts of sand, silt, and clay particles, affects mine reclamation and revegetation success. A suitable plant growth medium has enough sand for aeration and looseness to permit root growth and enough silt and clay for adequate nutrient and water holding capacity. The overburden soil texture results are graphically shown on Figure 2.7-4a which depicts U.S. Department of Agriculture soil textures based on sand, silt, and clay percentages. The overburden soil textures range from loamy sand to clay. The average overburden sand, silt, and clay percentages are 35.3 ± 28.9, 32.4 ± 18.1, and 32.3 ± 13.4, respectively, which is a silty clay loam. The WDEQ soil suitability criteria specifies that clay (CL), silty clay (SiQ, and sand (S) textures are marginally suitable soils. Based on soil texture, 55 samples, or approximately 70 percent (or 146 feet) of the overburden is considered suitable, whereas 21 of the samples (7 clay and 14 silty clay), or 30 percent (or 63 feet), is considered marginal. In accordance with the 1978 Selective Placement of Coal Strip Mine Overburden in Montana, III Spoil Mixing Phenomena, published by the Montana Agricultural Experiment Station, after dragline mixing the resultant spoil will continue to show areas of marginal soils suitability but these areas are expected to be smaller in extent with mixing to improve suitability. Based on experience with mining and spoil mixing of similar materials on the existing mine areas, the remaining marginal spoil materials are not expected to inhibit reclamation. Sodium Adsorption Ratio (SAR). SAR is a widely accepted index for characterizing soil sodicity, which describes the proportion of sodium to calcium and magnesium in soil solution. When SAR is greater than 13, the soil is called a sodic soil. Excess sodium in sodic soils causes soil particles to repel each other, preventing the formation of soil aggregates. This results in a very tight soil structure with poor water infiltration, poor aeration and surface crusting, which makes tillage difficult and restricts seedling emergence and root growth. The PR -7 Mine Expansion Area overburden SAR values ranged from 0.4 to 50, and averaged 18.97 ± 14.42. Approximately 42 percent (or 88 feet) of the overburden samples have suitable SAR values less than 12; the remaining 58 percent (or 121 feet) of the overburden samples have unsuitable SAR values greater than 12. However, the suitability of soils with unsuitable SAR values is further evaluated considering saturation percentage and clay content as discussed below. The approved Trapper Mine Permit topsoil and overburden SAR suitability criteria listed in Table 2.7-2a specifies that suitable spoil material has a SAR value less than 12, a saturation percentage less than 80, and a clay content less than 40 percent. These criteria are similar to those found in Table 5 in Williams 2-391 w revision: Approved: ___