The contaminated site lies just a few kilometres from a pumping facility that supplies the town, home to an estimated 300,000 people.
"We have broadcast messages on radio telling police and local authorities to prevent people drinking or washing in this river and not to give its waters to their livestock," said Environment Minister Didace Pembe, who travelled to Likasi as part of a team of senior government officials investigating the contamination.
Law enforcement officers early in November intercepted four trucks laden with radioactive material. Some 17 tonnes of this confiscated material was reported to greatly exceed permitted radiation levels.
Although a court ordered the safe disposal of the seized cargo, provincial authorities said those charged with doing this instead dumped the waste into the Mura.
The man responsible for ordering the toxic cargo to be dumped has been arrested, according to Pembe. He said criminal charges would soon be filed against the suspect, who he declined to identify.
"The minerals belong to a Chinese company, Magma-Lubumbashi, which did not ask for the waste to be dumped in a river. Rather, it was a Congolese official whose name I do not want to reveal because the enquiry is continuing," he said.
Pembe said other mining companies had been asked to provide vehicles so that the waste could be removed from the contaminated site.