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Paper No. 4
Presentation Time: 9:10 AM

INTEGRATING LAB AND FIELD DATA TO UNDERSTAND SELENIUM GEOCHEMISTRY IN THE IDAHO PHOSPHATE MINING DISTRICT


WILLIAMSON, Mark A., Geochemical Solutions, LLC, 1943 Lakewood Drive, Loveland, CO 80538, mark@geochemical-solutions.com

The Cretaceous Phosphoria formation that occurs in the phosphate mining district of southeast Idaho contain selenium which, upon mining disturbance, may be released to water resources. Selenium associated with waste rock has contributed to the death of livestock and significant loading to surface water resources and occurs in waste rock springs and seeps at concentrations on the order of 2-8 mg/L.

Laboratory characterization of selenium leaching from waste rock (obtained from exploration drill core) has been conducted by several workers using standard batch and column tests as well as column tests modified to simulate oxidizing geochemical environments. Additional determination of total selenium has been conducted. Overall, leach testing indicates only a small percentage of selenium is leachable, prompting characterization of dumps to address potential in place weathering (e.g. oxidation of seleniferous pyrite).

Field studies have been conducted to obtain waste rock core from facilities of differing ages, styles of construction and of reclamation. Leach testing of recovered core yields data for selenium release that are consistent with all previous laboratory testing despite the variety of waste dump settings. Bore holes were used to install PVC stems to facilitate oxygen monitoring, which revealed a range of degrees of oxygen availability with relatively low temperatures. Seep chemistry from hydrologically closed-system waste rock dumps is shown to be consistent with strong preferential flow within waste.

On integrating field and laboratory data, the release of selenium from waste rock is shown to be driven by the release of water-soluble selenium that is derived from weathering of seleniferous pyrite over geologic time, with very limited current oxidation of reduced selenium phases present in the matrix. The rate of ultimate release to water resources is limited by hydrodynamic characteristics of waste rock, which becomes the focus of reclamation design.

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