Plutonium in freshwaters: sources and behaviour in the Ottawa River basin

Plutonium has been input into the Ottawa River basin by human activities since 1945. The sources of Pu inputs consist of global weapons' fallout from tests performed far afield, releases from nuclear research and operations at Chalk River Laboratories (CRL) and discharges from the Nuclear Power Demonstration reactor facility (NPD). Cumulative Pu fallout estimated from soil cores is 55 Bq m-2 and agrees with estimates derived from global maps of Pu deposition. The residence time of fallout Pu in the drainage basin soils is very long. Although small lakes in the catchment trap Pu in their sediments, 30-70% of the Pu input flows out of the lakes. The Pu input from CRL is much less than the amount of Pu input to the river from erosion and leaching of Pu fallout from weapons' tests. The Pu input from operations at CRL have a unique isotopic signature because the isotopic ratio of 238Pu:239,240Pu is much greater than that in fallout. This provides an independent validation of the conclusion that the releases from reactors are very small. Pu concentrations in Ottawa River water are very similar to those measured in other freshwater systems. The radiological risk to the public from exposure to Pu in river water is much less than that from exposure to natural background radioactivity. (author)