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At much higher concentrations, which may be relevant for testing environmental contamination such as mine tailings, the accuracy is within 4 percent.

Rare earth metals are extracted by digging enormous pits and dissolving rocks in caustic acids, sometimes leaving behind toxic and radioactive tailings.

The complex accumulated some 40 million tons of tailings—leftovers of milling uranium—and other foul residues before it closed in 1992.

The judge concluded that it was not a “practical and workable” way to render the tailings nonreactive or to keep water out of them over time.

Nearly a third of the world's mine tailings are stored within or near protected conservation areas, University of Queensland research has found.