CONCEPTUAL MODEL OF SELENIUM RELEASE FROM SHALE UNITS WITHIN THE MEADE PEAK MEMBER OF THE PHOSPHORIA FORMATION

From the body of experimental work on the behavior of selenium in the Meade Peak Member of the Phosphoria Formation in Southeast Idaho a conceptual model of selenium mobility has been developed. Column leaching and batch extraction tests supporting recent Environmental Impact Statements for phosphate mines provide data by which the conceptual model is calibrated. The greatest mass of soluble selenium from column tests occurs in the first few pore volumes. The conceptual model suggests compound release mechanisms that include both oxidation of primary sulfide minerals as well as desorption reactions from products of historic oxidation processes. The mass of soluble selenium is typically greater from unsaturated columns compared to columns of like material conducted under saturated columns. Additionally, sulfate concentrations in the first few pore volumes of the unsaturated columns are greater than from saturated columns suggesting the occurrence of sulfide oxidation in the unsaturated columns. As expected, sulfate concentrations are greater from unaltered Center Waste Shale relative to altered shale. The notation of altered verses unaltered is a field observation describing the degree of weathering that has occurred over geologic time.

The conceptual model described herein proposes that the selenium leached from the saturated column is primarily selenate desorbed from ferric hydrous. The incremental increase of selenium from unsaturated columns compared to saturated columns can be attributed to releases from oxidation of sulfide minerals containing selenium. Some of the unsaturated columns also show a slight decrease in pH over the first few pore volumes which is consistent with sulfide oxidation. Column tests run for ten or more pore volumes show that selenium concentrations generally decline to less than 0.1 mg/l and remain constant after about pore volume three. This model of selenium release including both desorption and oxidation of sulfide minerals is relevant to designing column tests and the selecting number of pore volumes flushed through the column.