tailings

"The tailings release and breach was promptly brought under control within hours of detection," the firm said in a statement on 3 September.

The spill happened when a tailings dam, used to store toxic waste and heavy metals, collapsed into the Kafue River, a key drinking water source, following heavy rain.

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.