Abdelouas reported in 2006 that, worldwide, the underground and surface mining of uranium has generated 938 × 106 m3 of mill tailings. The radioactivity of these tailings depends on the grade of ore mined and varies from less than 1 Bq/g to more than 100 Bq/g. The most common mode of disposal is near-surface impoundment in the vicinity of the mine or mill. The principal radiation risks from uranium tailings are gamma radiation, essentially from radium decay; windblown radioactive dust dispersal; and radon gas and its radioactive progeny, which are known to cause lung cancer. Uranium mill tailings are also often associated with elevated concentrations of highly toxic heavy metals, which are a major source of surface and groundwater contamination. Due to their high sulfide content (a few to tens of wt%), tailings may acidify groundwater, accelerating the release of radioactive and hazardous elements.
The author concluded that uranium mine and mill tailings are hazardous due to radioactive and toxic elements that remain from the ore deposits. Most of these “inherited” elements are associated with Fe- and other metal-oxyhydroxides formed during the neutralization process in acid mill tailings. However, these elements may also form separate mineral phases, including autunite (uranium) and Ca arsenate (arsenic). Catastrophic or continuous release of contaminants can have a substantial impact on the environment. The mobility of radionuclides and toxic metals is enhanced by acidification of tailings due to sulfide oxidation catalyzed by microbial activity.
Tailings impact ecosystems by negatively affecting the growth of the microbial community, plants, and animals. Adequate disposal of tailings—e.g. by using bottom liners and caps on tailings designed to last for 200 to 1,000 years, in compliance with strictly established regulations— efficiently reduces the release of radionuclides and hazardous elements. While surface remedial action mainly consists of stabilizing the tailings onsite, active remediation of contaminated groundwater is required for complete remediation of certain mill tailings sites (e.g. Tuba City site, Arizona). Technologies including in situ bioremediation are still being developed to improve the remediation of mill tailings sites.
In all cases, groundwater monitoring is required for tens of years. Future research should focus on the role of microorganisms on the behavior of radionuclides and metals in tailings under disposal conditions. A better understanding of the entire microbial community at tailings sites is crucial to the development of effective remediation strategies. Hence, while iron- and sulfur-oxidizing bacteria may cause radionuclide concentrations in surface and groundwater to rise through acidification, metal- and sulfate-reducing microorganisms may help precipitate radionuclides, such as uranium, via reduction of U6+ to U4+.
[None of the above environmental issues are present in the in situ mining of uranium. See (here).]
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