Uptake of radionuclides by plants growing on Brazilian soil: the effect of soil ageing

The behaviour of radionuclides in soil is governed by several mechanisms that can vary significantly according to the specific reactivity of each element and soil properties. The 137Cs is one of radionuclides that generally reduces with time its mobility and phytoavailability due to irreversible fixation in high activity clay mineral such as illite, vermiculite and montmorilonite. A long-term experimental essay using Brazilian soils was done in order to determine the effect of ageing of contamination on 137Cs mobility in soils and transfer to plants. To perform this study, 4 different soils with different properties were contaminated with 137Cs at different period: The older contamination refers to an urban soil contaminated at the Goiania accident (1987). A similar type of Goiania's soil (Ferralsol rich in Gibbsite) was artificially contaminated with 137Cs in 1993. A subtropical class of soil (Nitisol) was contaminated in 1996 and two other tropical soils were contaminated in 2000 (Acrisol and Ferralsol rich in Goethite). The time's effect was studied by characterizing the evolution of soil properties and the changes in the distribution of radionuclides between phases till 2006. In addition, the phytoavailability was evaluated by carrying out experiments in lysimeters where radish was sowed at different periods:1996, 2000 and 2004. These results showed that the phytoavailability changed with time only in 2 situations: after changes in some soil properties such as pH or due to Cs fixation in high activity clay mineral when it was present in the soils even as trace mineral. The 137Cs distribution in soil showed that Fe oxides are the main sink for this element in all type of soil and 14 years after contamination, the 137Cs was still available for plants in the Ferralsol Gbbiste rich. In the Nitisol, 5 years after contamination, the 137Cs was not detected as in the slightly acidic phase of sequential extraction neither detectable in radish roots or leaves, indicating strong sorption mechanism, only explained by the presence of traces of vermiculite in these soils. Older contamination or younger contamination than contamination in Nitisol did not reduced plant uptake in that extreme way. These results suggest that soils originated from highly weathered areas, identified by the absence of high activity clay minerals and low fertility, the 137Cs uptake by plants can remains high for a long period. (author)