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

Published Date
September 2016, Vol.161:42–50, doi:10.1016/j.jenvrad.2015.12.017

Special Issue: Radioactive contamination in forest ecosystems: From Chernobyl to Fukushima

Title 

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

• Author 

• Y. Thiry ^a,
• L. Garcia-Sanchez ^b,
• P. Hurtevent ^b,,

• aNational Radioactive Waste Management Agency (Andra), Scientific Division, 1-7, rue Jean Monnet, 92298 Châtenay-Malabry Cedex, France
• bFrench Institute for Radiological Protection and Nuclear Safety (IRSN), PRP-ENV/SERIS/L2BT, CE Cadarache bt 183, BP 3, 13115 Saint-Paul-lez-Durance, France

Received 28 April 2015. Revised 18 December 2015. Accepted 19 December 2015. Available online 13 January 2016.

Highlights

• •
  ¹³⁴Cs leaching dynamic of the spruce canopy depended on the frequency of rainy episodes before day 70 and on precipitation amount afterwards.
• •
  Rainfall events better predicted field loss than time, with a leaching rate of 7.5%/event.
• •
  Radiocesium was readily absorbed from needles surface and efficiently translocated to the growing organs.
• •
  Calculated foliar absorption rate coefficient was 0.25 year⁻¹.
• •
  Internal transfers could supply the tree internal cycle of radiocesium extensively and possibly mask the contribution of root uptake for a long time.

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 t_1/2 of intercepted radiocesium of 25 days. During the growth season, the similar pattern of the internal ¹³⁴Cs 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 ¹³⁴Cs (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⁻¹for 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.

Keywords

• Radiocesium
• Spruce tree
• Leaching
• Foliar uptake
• Translocation

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• ∗ 
  Corresponding author.
  
  

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