Estimation of rooting depth of 137Cs uptake by plants

Understanding the soil-to-plant transfer process of Cs-137 is essential for predicting the contamination levels of plants in contaminated areas. The rooting depth is considered one of the key factors explaining the difference in the activity concentration of Cs-137 in different plant species. In this study, the distributions of Cs-137 and Cs-133 in soils and plants were investigated, and the plants' rooting depth of Cs-137 uptake was estimated using the Cs-137/Cs-133 ratios in exchangeable fractions of soils and biological samples. The results showed that different plant species accumulate different levels of Cs-137 and Cs-133. The Cs-137/Cs-133 ratios were fairly constant in plants of the same species. The average Cs-137/Cs-133 ratios in bamboo grasses and ferns were 0.015 +/- 0.009 (n = 5) and 0.13 +/- 0.04 Bq ng(-1) (n = 10) in Yamakiya, respectively. The percentage of Cs-137 in the exchangeable fraction of the uppermost soil layer was lower than that in the deeper soil layers. The activity concentrations of Cs-137 in the soil profiles decreased sharply with depth, whereas the depth distributions of Cs-133 were uniform. Therefore, the Cs-137/Cs-133 ratios were driven mainly by the Cs-137 activity concentrations in soil. The plants' rooting depths of Cs-137 uptake were estimated on the basis of the relationships between the averaged Cs-137 /Cs-133 ratio in the soil layer and the Cs-137 /Cs-133 ratio in the plant. The results indicate that the deeper-rooted species such as bamboo grasses have a lower accumulation of Cs-137 than the superficial-rooting species such as ferns. The soil-to-plant transfer factors would be determined using rooting depth by calculating the averaged activity concentration of Cs-137 within the estimated rooting depth.