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and Schuman (1987), which specifies a SAR less than 20, a saturation percentage less than 90, and clay <br />content less than 35 percent. <br />Application of these criteria indicate that approximately 99 percent (or 207 feet) of the overburden is <br />suitable. Only Sample 30, an approximately 3 foot thick carbonaceous shale interval, would be classified <br />as unsuitable. Sample 30 is located in the dozer spoil zone, which would be thoroughly mixed during <br />stripping and spoiling off the highwall. Application of the five -percent dragline mixing rule suggests that <br />all of the unsuitable overburden/spoil values would likely be mixed so that they would be considered <br />suitable with respect to potential sodicity. <br />Evidence of the assumption that mixing of potentially inhibitory sodium concentrations to acceptable <br />levels is documented in the historic regraded spoil data found in the 1989 through 2000 Annual <br />Reclamation Reports, where 110 regraded spoil samples are reported to have an average SAR of 0.65 <br />and ranged up to a maximum of 7.1, which is significantly less than the average SAR value of 18.97 <br />obtained from the overburden samples in this evaluation. Based on the result, the probable sodicity <br />values of all of the overburden in the PR -7 Mine Expansion Area would likely lie within the suitable <br />category after mixing during mining and reclamation. <br />Saturation Percentage (SP). Saturation percentage is the amount of water in a saturated soil paste on a <br />dry weight basis. It is an indicator of soil -water holding capacity and swelling potential. Overburden SP <br />ranged from 22.6 to 113 percent and averaged 50.3 ± 23.4 percent. Approximately 85 percent (or <br />177 feet) of the overburden samples are classified as suitable with a WDEQ SP range of 25 to <br />80 percent. The remaining 15 percent (or 32 feet) of the overburden has marginally suitable SP values <br />less than 25 or greater than 80 percent. <br />Four of the marginally suitable SP values are associated with coal seams (Sample 11, Sample 21, Sample <br />35, and Sample 54) which will be mined and hauled to the power plant. This indicates that 10.4 feet or <br />33 percent of the marginal SP overburden will be removed by mining. SP was not studied in the <br />Montana dragline mixing study; however, the five -percent mixing rule applies and suggests that the <br />remaining marginally suitable overburden would be sufficiently mixed to achieve a suitable SP. SP by <br />itself is not usually a concern with respect to overburden suitability. As discussed in the SAR section, the <br />primary SP concern is its interaction with soil sodicity and these data show no such interaction or <br />concern. <br />Verification of this assumption is found in the 1988 through 2000 Annual Reclamation Reports, where a <br />total of 110 regraded spoil SP values averaged 42.5 percent, which is consistent with the average <br />overburden SP value of 50.3 percent found in this evaluation. Based on these results, the likely SP value <br />of the overburden in the PR -7 Mine Expansion Area would be considered suitable after mixing during <br />mining and reclamation. Further evidence to support this assumption is found in the SP values <br />associated with the H overburden, which are less than 53.4 percent, and will ultimately form the <br />surface overburden materials. <br />Water Soluble Selenium (Sel. The importance of water-soluble Se in coal mine reclamation relates to its <br />potential contamination of mine spoils and subsequent uptake by plants or leaching to groundwater or <br />surface water. Overburden Se concentrations ranged from 0.0006 to 0.298 milligrams per kilogram <br />(mg/kg) or parts per million (ppm), and averaged 0.064±0.062 mg/kg. All of the PR -7 overburden Se <br />concentrations are generally low and are less than the WDEQ topsoil and overburden suitable Se <br />concentration of 0.3 ppm. <br />2-391x <br />=:_vision: PP -65) <br />r proved: <br />