Distribution of radionuclides in forest soils of varying pedochemistry

The aim of the present work is to evaluate the influence of pedochemical parameters on the distribution of radionuclides in forest soils. Investigations were carried out by low-level gamma spectrometry on profiles sampled at intensively monitored sites (Level II program) in forestry. These monitored sites are part of the european forestry environmental program exploring the conditions and development of forestry ecosystems. An extensive data base is available for these places collected over more then twenty years . For the measurements sampling sites of different soil types and local vegetation located in Saxony, Germany were chosen. At each profile layers of 30 x 30 cm2 area and 1 cm thickness were taken from the surface litter down to about 50 cm depth. The analysed radionuclides included artificial nuclides from the fallout after nuclear weapons tests in the fifties and sixties and after the Chernobyl accident ( 137Cs, 134Cs, 125Sb, 241Am - daughter of 241Pu - and 207Bi), the cosmogenic 7Be as well as members of the natural decay series of 238U (e.g. 238U, 226Ra, 210Pb) and 232Th ( 228Ra and 228Th). From the measurements the following conclusions can be drawn: Regardless of the geological background the nuclides 241Am, 207Bi and 125Sb and the excess 210Pb remain in soil layers with high organic content (O- horizon, Ah-horizon). Their distributions can be used for age determinations in these horizons. Comparison between the results of the different methods are presented and discussed. The mobility of 134Cs and 137Cs is apparently higher. Whereas the Cs nuclides originating from the Chernobyl accident are detectable mainly in the Of-Oh horizons, 137Cs from the nuclear weapons fallout is found from the A horizons down to the C-horizon. Uptake of Cs by plants controls its behaviour in the surface layers. Disequilibria between Uranium and Radium are obvious in the Of, Oh- as well as in the Bs horizons. They reflect uranium accumulation by degraded plant material and Radium transport, respectively. On the other hand, varying ratios between 226Ra ( 238U-series) and 228Ra ( 232Th-series) are detected and discussed. (author)