The geochemistry of selenium associated with coal waste in the Elk River Valley,Canada

Selenium (Se) concentrations more than 12 times greater than the provincial freshwater quality guideline (2 µg/L) were detected in the Elk River downstream from the five open-pit coal mines in southeastern British Columbia's Elk River Valley. To identify possible sources of Se to the Elk River, samples from the coal-bearing Mist Mountain Formation were studied using X-ray diffraction, elemental and oxide analyses, sequential extractions and heavy liquid separation. Between 2.5 and 21.3% of the total Se in the five types of materials studied is water-soluble and 1.0 to 10.6% is associated with hydrous ferric and manganese oxides. Se associated with sulphides and organic material varies between 60 and 84% of the total Se and Se in the silicate structure varies from 5.9 to 24.7%. The ratio of sulphides to the total of organic carbon is well-correlated with the amount of Se in materials closely associated with coal seams containing less than 6 mg/kg of Se (r=0.916). This may suggest that the amount of organic matter present during deposition affects the amount of Se incorporated into sulphides.