Geographical trends in 137Cs fallout from the Chernobyl accident and leaching from natural surface soil in Norway

In order to follow the turnover of 137Cs in natural soils and estimate future trends in exposure of livestock, samples of natural surface soils were collected at 0-3 cm depth at 464 sites in 1995 and 463 sites in 2005 covering the country. In both cases the geographical pattern observed was similar to the original distribution from 1986, but the decline of 137Cs activity in the surface soil was not the same everywhere. In 1995 the 137Cs reduction since 1986 was found to be considerably greater in coastal areas than farther inland. The main reason for this appears to be the much greater deposition of marine cations such as Mg2+ and Na+ in the coastal areas, replacing Cs ions fixed on soil particle surfaces. This cation exchange appeared to be particularly strong near the southern coast where deposition of NH4+ from transboundary air pollution is evident in addition to the marine cations. During 1995-2005 the 137Cs decline in the surface soil was more uniform over the country than in the preceding 10-year period but still significantly higher in coastal areas than inland. Differences in precipitation chemistry may have influenced the uptake of 137Cs in terrestrial food chains.