Long-term behaviour of plutonium in air and deposition and the role of resuspension in a semi-rural environment in Germany

Despite a long while since the last surface nuclear weapons test (China, 1980), and since the Chernobyl accident (1986), very small concentrations of plutonium in air and deposition are still observable. The present paper reports and discusses results from our sampling site at Munich-Neuherberg from 1970 to the end of 1991, emphasizing the period since 1985. The methods of sample collection, preparation and radiochemical plutonium analysis are indicated in brief. To obtain Pu values above the detection limit, after 1985 our regular monthly samples were combined to yearly ones, except during the Chernobyl period. The annual mean values of 239+240Pu air concentration (e.g. 6-2 nBq m-3 between 1987 and 1991) and annual sums of total deposition (e.g. 9-2 mBq m-2 between 1987 and 1991) are reported as well as the Pu specific activities in the top soil layers (0-1, 1-2, 2-5 cm) in 1987 and 1993. The temporal variation of these quantities and the role of resuspension in interpreting the presently observed Pu concentrations are discussed. A practically constant resuspension factor K, 1.20±0.05·10-9 m-1, is obtained for 239+240Pu from 1989 through 1991, provided the decrease of Pu in the top 0-1 cm soil layer is taken into account