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surface water. Overburden extractable As concentrations ranged from less than the 2 mg/kg analytical <br />reporting limit to 3 mg/kg and averaged 2.01 ± 0.12 mg/kg. Samples 1, 22, and 40 were the only <br />samples with detectable As concentrations of 2, 2, and 3 mg/kg, respectively. Ninety-eight percent (or <br />206 feet) of the PR -7 Mine Expansion Area overburden extractable As concentrations satisfy the WDEQ <br />extractable As suitability criterion of less than 2 ppm. The remaining 2 percent (or 3.6 feet) of <br />overburden are considered marginally suitable and exceed the WDEQ extractable As criterion of <br />greater than 2 ppm. <br />Extractable As was not studied in the Montana dragline mixing study, however, the five percent mixing <br />rule applies and suggests that the remaining marginally suitable overburden would be sufficiently <br />mixed after mining and reclamation to achieve extractable As suitability. <br />Nitrate/Nitrite as Nitrogen (N). The deficiency of plant -available N can be a limiting factor in the <br />successful revegetation and long-term stability of land disturbed by surface mining. Deficiencies arise <br />because disturbance alters the flow of N through the soil -plant -microbial ecosystem. The overburden <br />nitrate/nitrite (as N) concentrations ranged from 2 to 27 mg/kg and averaged 2.75 ± 3.65 mg/kg. All of <br />the overburden sample nitrate/nitrite (as N) concentrations are less than the WDEQ topsoil and <br />overburden suitability criterion of 50 ppm. <br />Summary <br />This evaluation finds that the overburden materials within the PR -7 Mine Expansion Area are suitable <br />for reclamation and are essentially free of constituents that pose a threat to plant growth, surface <br />water, and groundwater. Most of the unsuitable materials are associated with the ten coal seams <br />(Samples 11, 18, 21, 26, 35, 54, 61, 64, 70 and 72) and will be removed during mining and not <br />incorporated in the overburden. Once the overburden materials are mixed via mining and further <br />mixed via regrading and reclamation operations, they will be at least of an equal, if not better, physical <br />and chemical quality than the topsoil currently found on this site. <br />2-391 z <br />oproved: 3 <br />