The presence here of plutonium — the most toxic of radio isotopes — is attributed to two sources. Finding either one is like looking for a needle in a haystack.  

The Department of Energy doesn’t know where the plutonium is.

In March 2001, the DOE reported that the nuclear facility in Weldon Spring handled recycled uranium for years.  DOE investigators reported that 70,000 metric tons of recycled uranium passed through the plant between 1957 and 1966, when the Mallinckrodt Chemical ran the operation for the Atomic Energy Commission. The investigation calculated that 2.4 grams of plutonium would have present in this amount.

Recycled uranium is hotter because it has been irradiated in a nuclear reactor. At the time, it was estimated that exposure to one-millionth of an ounce of plutonium could cause cancer.

But the recycled uranium may not be the only source of potential plutonium contamination in the St. Louis region.

That’s because the Belgian Congo pitchblende that Mallinckrodt processed to make the first atomic bombs contains small amounts of plutonium, according to the Agency for Toxic Substance and Disease Registry.

Waste byproducts from the pitchblende processing is known to have contaminated a several sites in the St. Louis area, including Coldwater Creek and West Lake Landfill.