MINERALOGICAL RESIDENCE OF ALPHA-EMITTING CONTAMINATION AND IMPLICATIONS FOR MOBILIZATION FROM URANIUM MILL TAILINGS

The rate and magnitude of contaminant release from mill tailings to groundwater are known to depend on the form and mineralogy of the host grains. Using samples from three uranium mill sites in the western United States, we identified four types of alpha-emitting host grains - those containing barium-strontium sulfates, authigenic siliceous material, uranium minerals, and iron-titanium-vanadium oxides. These four grain types constitute a classification scheme for the tailings. Each milling process (acid or alkaline) produces distinct types of grains. In acid-milled tailings, such as those at Slick Rock, Colorado, the dominant source of alpha-emissions is from barium-strontium sulfate. The barium-to-strontium ratio covers the entire solid-solution range between barite and celestite. In alkaline-milled tailings, alpha-emissions come predominantly from siliceous composite grains, which are interpreted as grains from the mill feed that have been altered during milling. In the siliceous composite grains, radionuclides are encased by siliceous material resembling chalcedony. Other alpha-emitting grains appear to be unrelated to milling; some uranium minerals and iron-titanium-vanadium oxides appear to have passed through the milling process relatively unaltered. The classification scheme identified in this study reflects the geochemical reactivity of the tailings with groundwater. Our findings can be used to improve confidence levels when predicting; (1) source loading to a groundwater system; (2) health effects from inhaled radioactive dust; and (3) long-term performance of uranium tailings containment cells.