Time-dependent behaviour of radiocaesium: A new method to compare the mobility of weapons test and Chernobyl derived fallout

Environmental radiocaesium (Cs-137) originates primarily from two sources, atmospheric nuclear weapons testing, and the Chernobyl accident. It has not , to our knowledge, previously been possible statistically to compare chang es in the environmental mobility of Cs-137 from these two sources since the weapons test fallout varied in a complex manner over a number of years. A novel technique is presented for curve-fitting measurements with a time-dep endent input function such as that for weapons test fallout. Different mode ls were fitted to measurements of both: pre- and post-Chernobyl Cs-137 acti vity concentrations in five: major Finnish rivers. It was shown that there was no significant difference in the temporal changes in Cs-137 mobility fr om these two sources during the years after fallout. Transport parameters d erived from:weapons test measurements gave good predictions of the long-ter m contamination of these rivers by Chernobyl fallout. Changes in Cs-137 act ivity concentrations in rivers after Chernobyl have previously been shown t o decline as a result of slow sorption to clay minerals in catchment soils. It is shown that weapons test fallout also exhibited this slow decline ove r time. Rates of decline in Cs-137 activity concentrations 10 years after f allout correspond to effective ecological half-lives (T-e f f) in the range 10-30 years. Removal of activity from the catchment was found to have no s ignificant effect on the long-term decline in Cs-137 activity concentration s in these rivers. (C) 2000 Elsevier Science Ltd. All rights reserved.