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空气中放射性铯污染后针叶树中放射性铯再循环的实验定量研究:田间损失动态、转运及最终分配

Experimental quantification of radiocesium recycling in a coniferous tree after aerial contamination: Field loss dynamics, translocation and final partitioning.

作者信息

Thiry Y, Garcia-Sanchez L, Hurtevent P

机构信息

National Radioactive Waste Management Agency (Andra), Scientific Division, 1-7, rue Jean Monnet, 92298 Châtenay-Malabry Cedex, France.

French Institute for Radiological Protection and Nuclear Safety (IRSN), PRP-ENV/SERIS/L2BT, CE Cadarache bt 183, BP 3, 13115 Saint-Paul-lez-Durance, France.

出版信息

J Environ Radioact. 2016 Sep;161:42-50. doi: 10.1016/j.jenvrad.2015.12.017. Epub 2016 Jan 13.

Abstract

After foliar interception of radioactive atmospheric fallout by forest trees, the short-term recycling dynamics of radiocesium from the tree to the soil as well as within the tree is a primary area of uncertainty in the modeling of the overall cycle. The partitioning of radiocesium transfers in a spruce tree exposed to aerial deposits was investigated during one growth season to reveal the dynamics and significance of underlying processes. The rate of radiocesium loss resulting from foliage leaching (wash-off) was shown to have a functional dependence on the frequency of rainy episodes in a first early stage (weathering 60% of initial contamination during 70 days) and on the amount of precipitation in a second stage (weathering 10% of initial deposits during the following 80 days). A classical single exponential decay model with offset and continuous time as predictor lead to a removal half-life t1/2 of intercepted radiocesium of 25 days. During the growth season, the similar pattern of the internal (134)Cs content in new shoots and initially contaminated foliage confirmed that radiocesium was readily absorbed from needle surfaces and efficiently translocated to growing organs. In the crown, a pool of non-leachable (134)Cs (15-30%) was associated with the abiotic layer covering the twigs and needle surfaces. At the end of the growth season, 30% of the initial deposits were relocated to different tree parts, including organs like stemwood (5%) and roots (6%) not directly exposed to deposition. At the scale of the tree, 84% of the residual activity was assimilated by living tissues which corresponds to a foliar absorption rate coefficient of 0.25 year(-1) for modeling purposes. According to the significant amount of radiocesium which can be incorporated in tree through foliar uptake, our results support the hypothesis that further internal transfers could supply the tree internal cycle of radiocesium extensively, and possibly mask the contribution of root uptake for a long time.

摘要

森林树木对放射性大气沉降物进行叶面截留后,放射性铯从树木到土壤以及在树木内部的短期再循环动态,是整个循环建模中一个主要的不确定领域。在一个生长季节内,对暴露于大气沉降物中的云杉树中放射性铯转移的分配情况进行了研究,以揭示潜在过程的动态和意义。结果表明,在第一阶段早期(70天内风化掉初始污染的60%),树叶淋溶(冲刷)导致的放射性铯损失速率与降雨事件的频率呈函数关系,在第二阶段(接下来的80天内风化掉初始沉积物的10%),与降水量呈函数关系。一个以偏移量和连续时间为预测变量的经典单指数衰减模型得出,截留的放射性铯的去除半衰期t1/2为25天。在生长季节,新梢和最初受污染树叶中内部(134)铯含量的相似模式证实,放射性铯很容易从针叶表面吸收,并有效地转移到生长器官中。在树冠中,一部分不可淋溶的(134)铯(15%-30%)与覆盖嫩枝和针叶表面的非生物层有关。在生长季节结束时,30%的初始沉积物转移到了不同的树体部位,包括未直接暴露于沉降物的树干(5%)和根系(6%)等器官。在树体尺度上,84%的剩余活度被活组织吸收,为建模目的,这对应于0.25年-1的叶面吸收速率系数。根据通过叶面吸收可纳入树木的大量放射性铯,我们的结果支持这样的假设,即进一步的内部转移可以广泛地为树木内部的放射性铯循环提供补给,并可能在很长一段时间内掩盖根系吸收的贡献。

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