There is a strong possibility that the radiation detected in Ottawa biowaste last month came from the waste of medical patients, but the city’s director of Water and Waste­water Services said it’s impossible to know for sure.

The dose of radiation received from the biosolids was extremely small — less than 1/10,000 of what is normally received annually from natural sources, said Dixon Weir.

The radioactive material is very likely the medical isotope iodine-131.

Similar situations have occurred in other cities in North America and Weir said the research points to “normal excreta that comes from people undergoing medical treatments.”

“They do not represent a risk to our works, the public or the environment,” said Weir.

On Jan. 29, two truckloads of biosolids from the R.O. Pickard Environmental Centre were turned away at the U.S. border after radioactive material was detected.

Two more truckloads were discovered the next day at the plant.

An investigation failed to identify the source and no other biosolids have tested positive since then.

Weir said many trucks crossed the border without incident since then.

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