US GEOLOGICAL SURVEY-RESEARCH ON THE ENVIRONMENTAL FATE OF URANIUM MINING AND MILLING WASTES

Studies by the US Geological Survey (USGS) of uranium mill tailings (U MT) have focused on characterizing the forms in which radionuclides ar e retained and identifying factors influencing the release of radionuc lides to air and water. Selective extraction studies and studies of ra dionuclide sorption by and leaching from components of UMT showed alka line earth sulfate and hydrous ferric oxides to be important hosts of radium-226 (Ra-226) in UMT. Extrapolating from studies of barite disso lution in anerobic lake sediments, the leaching of Ra-226 from UMT by sulfate-reducing bacteria was investigated; a marked increase in Ra-22 6 release to aqueous solution as compared to sterile controls was demo nstrated. A similar action of iron(III)-reducing bacteria was later sh own. Ion exchangers such as clay minerals can also promote the dissolu tion of host-phase minerals and thereby influence the fate of radionuc lides such as Ra-226. Radon release studies examined particle size and ore composition as variables. Aggregation of UMT particles was shown to mask the higher emanating fraction of finer particles. Studies of v arious ores and ore components showed that UMT cannot be assumed to ha ve the same radon-release characteristics as their precursor ores, nor can Ra-226 retained by various substrates be assumed to emanate the s ame fraction of radon. Over the last decade, USGS research directed at offsite mobility of radionuclides from uranium mining and milling pro cesses has focused on six areas: the Midnite Mine in Washington; Ralst on Creek and Reservoir, Colorado; sites near Canon City, Colorado; the Monument Valley District of Arizona and Utah; the Cameron District of Arizona; and the Puerco River basin of Arizona and New Mexico.