The leach residue still contains most of the radioactive decay products of uranium. The solids are known as tailings. The tailings are normally transferred in a slurry and historically were simply dumped into nearby depressions or lakes, depending on nature to do its best to cope with the environmental devastation. The uranium ore of Elliot Lake contains the sulphide mineral pyrite, which can oxidize into sulphuric acid. Acidic drainage is a major environmental problem with uranium tailings in Elliot Lake.
Acidity also mobilizes some metals such as radium, copper, zinc, nickel and lead.
In addition to the problem of acidic drainage, uranium wastes have the problem of radiological contamination. The amount of any radioactive element decreases by a factor 1,000 in ten half lives. Thus, in 760,000 years, one gram of thorium-230 will be reduced to a milligram. For human purposes, radiological contamination from uranium tailings is a problem forever. It was only following public attention in the 1960s that regulatory attention began to be paid to uranium tailings in the 1970s, looking at long-term radiological and acidification impacts.