Worldwide, the 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.