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Geochemistry of selenium release from the Elk River Valley coal mines Lussier, Christine

Abstract

Elevated levels of selenium (Se) were detected downstream from the five open pit coal mines in the Elk River Valley, British Columbia. Se is an essential nutrient but, in excessive amounts, it may cause teratogenic deformities and reproductive failure in fish and birds. To provide mine operators in the Elk River Valley with the information needed to assess the risk of Se release from waste rock and plant refuse a study of Se's modes of occurrence in the strata disturbed by mining and the geochemical mechanisms of its release was conducted. The mineralogical associations of Se were studied in 16 samples representative of the different types of material at the sites. Methods used to characterize sample mineralogy include X-ray diffraction, scanning electron microscopy, sequential extractions and heavy liquid separation. Se had both organic and inorganic associations in all lithologies tested, but sulphides, in particular pyrite, were indicated as the main Se-bearing component in the studied lithologies. The amount of organic matter in the materials appeared to play a role in determining the degree of Se enrichment in sulphides, with materials high in organics containing sulphides with less Se substitution. Humidity cells were used to determine the rate of Se release from coal, interburden, foot wall, parting and coarse refuse. The rate of Se release was not proportional to the total amount of Se in the sample, suggesting that mineralogical factors, such as texture, pyrite liberation and porosity, determine the rate of Se oxidation. A strong positive correlation between the amount of Se and sulphate in leachate from the humidity cells, suggested that sulphide oxidation is likely the source of Se being released into tributaries of the Elk River.

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