Goor François, Thiry Yves
Radiation Protection Research Unit, Belgian Nuclear Research Centre (SCK CEN), Boeretang 200, 2400 Mol, Belgium.
Sci Total Environ. 2004 Jun 5;325(1-3):163-80. doi: 10.1016/j.scitotenv.2003.10.037.
In a large forested area affected by the Chernobyl radioactive fallout, especially in CIS, the lasting recycling of radiocaesium (137Cs) by the trees is a source of long-term contamination of woody products. The quantitative description of the 137Cs dynamics in contaminated forest is a prerequisite to predictive modelling and further management of such territories. Three even-aged mono-specific Scots pine stands (17, 37 and 57 years old) were selected in a contaminated woodland in southeastern Belarus to constitute an adequate chronosequence. We determined the potassium and radiocaesium annual fluxes involved in the biological cycling in each stand using a well-documented calculation methodology. Qualitatively, 137Cs was shown to be rapidly recycled in trees through the same pathways as K and to redistribute similarly between the tree components. Compared to K, a higher fraction of 137Cs, corresponding to about the half of the annual uptake, is immobilised in perennial organs. With tree development, trunk wood and bark become prevailing sinks for 137Cs since they represent an increasing pool of biomass. In the pine chronosequence, the current root absorption, respectively, mobilizes 0.53, 0.32 and 0.31% year(-1) of the total 137Cs pool in soil. Variations in the 137Cs uptake do not reflect differences in the 137Cs balance between stands. In the two older stands, 51 and 71% of the current tree contamination are related to earlier accumulation subsequent to the initial fallout interception and recycling. The soil is the dominant source of long-term tree contamination. A simple modelling based on the measured 137Cs fluxes indicates that, for young stands, radioactive decay-corrected contamination would stabilize after reaching a maximum of 25 years after the 137Cs deposition. Stemwood presents a maximum of 15 years after the deposition and decrease afterwards mainly through radioactive decay. In the older stands, the decontamination is constant without local maximum of 137Cs level in the wood. The 137Cs contamination of tree components is the result of different influential processes like root uptake, internal translocation and immobilisation. For more accurate predictions, the calibration of existing models would be benefited by comparing with the 137Cs annual fluxes instead of the simple transfer factor coefficients. In the perspective of other applications, there is a need of such data for other radionuclides as well as for heavy metals.
在受切尔诺贝利放射性沉降物影响的大片林区,尤其是独联体地区,树木对放射性铯(137Cs)的持续再循环是木质产品长期受污染的一个来源。对受污染森林中137Cs动态进行定量描述是对此类区域进行预测建模和进一步管理的前提条件。在白俄罗斯东南部一个受污染的林地中,选择了三个同龄的单一树种苏格兰松树林分(树龄分别为17年、37年和57年),以构成一个合适的时间序列。我们使用一种记录完备的计算方法,确定了每个林分生物循环中涉及的钾和放射性铯的年通量。定性地说,137Cs在树木中通过与钾相同的途径迅速再循环,并在树木各部分之间以类似方式重新分布。与钾相比,137Cs中更高比例(约占年吸收量的一半)被固定在多年生器官中。随着树木生长,树干木材和树皮成为137Cs的主要汇,因为它们代表着不断增加的生物量库。在松树时间序列中,当前根系吸收分别调动了土壤中137Cs总量的0.53%、0.32%和0.31%(每年)。137Cs吸收量的变化并未反映林分之间137Cs平衡的差异。在两个较老的林分中,当前树木污染的51%和71%与初始沉降物拦截和再循环之后的早期积累有关。土壤是树木长期受污染的主要来源。基于实测的137Cs通量进行的简单建模表明,对于幼龄林分,经放射性衰变校正后的污染在137Cs沉积后达到最大值25年后将趋于稳定。树干木材在沉积后15年达到最大值,之后主要通过放射性衰变而减少。在较老的林分中,去污是持续的,木材中137Cs水平没有局部最大值。树木各部分的137Cs污染是根系吸收、内部转运和固定等不同影响过程的结果。为了进行更准确的预测,通过与137Cs年通量而非简单的转移因子系数进行比较,将有利于对现有模型进行校准。从其他应用的角度来看,也需要其他放射性核素以及重金属的此类数据。