Differences in leaching characteristics of dissolved radiocaesium and potassium from the litter layer of Japanese cedar and broadleaf forests in Fukushima, Japan

https://doi.org/10.1016/j.jenvrad.2020.106417Get rights and content

Highlights

  • We conducted serial leaching tests on 137Cs and K from forest litter.

  • Leaching speed of 137Cs and K decreased as leaching proceeded.

  • Leachable fraction of 137Cs decreased with increasing decomposition degree.

  • Leachable fraction of 137Cs was less than one-tenth of the leachable fraction of K.

Abstract

Cesium is an element that belongs to the same group as K, and is known to show similar behaviour to that of K in plants. In this study, we conducted a serial leaching test for 120 h to compare the leaching characteristics of dissolved 137Cs and K in forest litter, obtained from Japanese cedar and deciduous broadleaf forests located 40 km from the site of the 2011 Fukushima Dai-ichi nuclear power plant accident. The litter was collected in 2018 and was divided into three groups according to the decomposition level. The cumulative leachable fraction of 137Cs at 120 h ranged from 0.3% to 3.3%, suggesting that most of the 137Cs in the litter was hardly leachable in water. The leachable fraction of 137Cs generally decreased with the decomposition level of the litter, implying that the easily leachable 137Cs eluted into the water during the first stage of decomposition. Meanwhile, the cumulative leachable fraction of K at 120 h was approximately 10 times greater than that of 137Cs and ranged from 22.7% to 54.8%. The leaching speeds of 137Cs and K decreased suddenly with elapsed time regardless of the tree species, decomposition degree, or element. Our findings contribute to the long-term understanding of the 137Cs cycle in forest ecosystems.

Introduction

Radioactive contamination in forest areas following a nuclear accident is difficult to control using techniques, such as decontamination, compared with other ecosystems such as an agricultural land and an urban area. Studies conducted after the Chernobyl nuclear power plant accident in 1986 have shown the prolonged contamination of forest ecosystems and products (IAEA, 2010). Food monitoring for radioactive substances after the Fukushima Dai-ichi nuclear power plant accident in 2011 have also shown that wild foods from forests still have higher concentrations of 137Cs than agricultural crops (Uekusa et al., 2015). An appropriate understanding of the migration of 137Cs in the forest ecosystem is required for the long-term forest management, resumption of forest industry and utilisation, and radiation protection of forest users after a nuclear accident.

On a long-term basis, forest litter is one of the most important components of the 137Cs cycle in a forest ecosystem. Cesium-137 deposited on a forest floor shortly after a nuclear accident was observed to migrate into the soil for a while through litter decomposition, and the fraction absorbed by plants was returned to the forest floor as litterfall (Shaw, 2007). As 137Cs leached from the litter layer mainly exists in a dissolved form (Takada et al., 2016) that is highly bioavailable (Nakanishi et al., 2013), a portion of 137Cs continues to circulate in the forest ecosystem (Goor and Thiry, 2004). Therefore, contribution of the litter layer to the 137Cs cycle in a forest ecosystem needs to be examined thoroughly.

Several studies have been conducted focusing on the migration of 137Cs from litter using litterbags and lysimeters (Rafferty et al., 1997; Huang et al., 2016; Kurihara et al., 2018a, 2018b; Nakanishi et al., 2014; Koarashi et al., 2016). These studies were conducted under actual environmental conditions and quantitative results for downward migration of 137Cs were obtained; however, the information obtained is very site-specific and does not particularly address the leaching mechanism. Some leaching tests have been conducted in laboratories, however most tests were adapted to the natural conditions similar to a field experiment. For example, Sakai et al. (2015) conducted a leaching test in laboratory to examine the adsorption of 137Cs and K on soil particles in river water, Saito et al. (2015) studied the leaching of 137Cs from well-decomposed litter (FH layer) in addition to the soil layers by conducting a leaching test, and Saito et al. (2017) adopted leaching tests to litter-bags installed in an actual environment. These studies did not evaluate the fundamental leaching characteristics such as leaching capacity and the relation with litter decomposition, and the mechanisms were not examined. The laboratory experiments focusing on the fundamental leaching characteristics will provide an important knowledge.

Potassium is an element that belongs to the same group as Cs and is known to show similar behaviour in plants (Kruglov et al., 2005). Like K, plants absorb Cs via roots (Broadley and Willey, 1997), and 137Cs shows almost the same distribution as K among all the tree components (Goor and Thiry, 2004). While studies on the similarity of K and Cs in a plant have been conducted, knowledge on the similarity of leaching characteristics of K and Cs eluted from a dead plant (e.g., litter) is still insufficient. As K has long been known to leach easily from litter (Lousier and Parkinson, 1978; Osono and Takeda, 2004), several studies indicated that 137Cs in litter does not have similar properties as K. Saito et al. (2015, 2017) reported that 137Cs is difficult to elute from litter by conducting a leaching test; however, they did not evaluate K. Sakai et al. (2015) also observed that the concentrations of both 137Cs and K in litter decreased by putting litter in water, and found that the decrease in K was greater than that of 137Cs. A litterbag experiment after the Chernobyl accident also showed that the decrease of K in the litter was faster than that of 137Cs (Rafferty et al., 1997). Knowledge on the similarity or non-similarity between the leaching characteristics and mechanisms of 137Cs and K in litter can support the long-term prospect of 137Cs dynamics in a forest ecosystem.

In this study, we conducted a serial leaching test with reference to the tank leaching test (NEN 7345, 1994; Malviya and Chaudhary, 2006; Sakanakura et al., 2003) to examine the leaching characteristic of dissolved 137Cs from litter. We also compared the leaching characteristics of 137Cs and K.

Section snippets

Litter samples

The litter samples for the leaching test were collected from two types of typical forest in Kawamata Town, Yamakiya District, located approximately 40 km northwest of the Fukushima Dai-ichi nuclear power plant (FDNPP): a Japanese cedar (Cryptomeria japonica) forest (37°35′ N, 140°41′ E; 550 m asl) and a deciduous broadleaf forest (37°36′ N, 140°40′ E; 580 m asl), dominated by konara oak (Quercus serrata). The total atmospheric deposition of 137Cs from the FDNPP accident was estimated to be 470

Properties of litter samples

Table 1 shows the activity concentrations of 137Cs (kBq/kg-dry) and K content (mg/kg-dry) of the litter samples. The C/N ratio and the loss on ignition of the litter samples collected in December 2017 are also shown as a reference. The criterion of the litter layers (L1, L2, and F layers) is same regardless of the sampling season. The activity concentrations of 137Cs range from 0.6 to 57.4 kBq/kg, indicating that the activity concentration increases with the degree of litter decomposition in

Discussion

This study evaluates the leachable fraction of 137Cs and K in litter by conducting a leaching test in a laboratory. The leaching speed of 137Cs and K was 1/100–1/1000 of the initial state after 120 h of the leaching test (Fig. 3), and it is unlikely that leaching would significantly increase beyond this time, suggesting that 137Cs was mostly leached in the present study. However, additional tests are required to discuss leachable fraction of 137Cs and K over 120 h and the leachable capacity.

Conclusions

The present study focused on the contribution of the litter layer on the 137Cs cycle in two forest ecosystems. This is the first study to examine the fundamental leaching characteristics, i.e. the leachable fraction of 137Cs and K in litter and their relation to the degree of litter decomposition. The cumulative leachable fraction of 137Cs at 120 h was 0.3–3.3%, indicating that most 137Cs in litter was hardly leachable in water. This trend was particularly strong for the well-decomposed litter.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank Dr. J. Takahashi at the University of Tsukuba and Dr. K. Tagami at the National Institutes for Quantum and Radiological Science and Technology for their support and useful comments regarding the analysis and discussion sections of the paper. We thank Dr. Y. Iwasaki at the National Institute of Advanced Industrial Science and Technology for his support regarding the statistical analysis. We also thank Ms. K. Fujii and Ms. Y. Agawa for their support with the measurements.

References (28)

  • M. Takada et al.

    Simplified measurement method for dissolved radio-Cs in litter and soil seepage water using copper-substituted Prussian blue

    Chemosphere

    (2016)
  • IAEA

    Handbook of Parameter Values for the Prediction of Radionuclide Transfer in Terrestrial and Freshwater Environments, Technical Report Series No. 472

    (2010)
  • T. Ichikawa et al.

    Effects of the conversion of the forest management type from natural deciduous broadleaved forests to artificial Japanese cypress (Chamaecyparis obtusa) and Japanese cedar (Cryptomeria japonica) forests on soil microbial flora and mineralization characteristics of organic carbon

    Jpn. J. For. Environ.

    (2003)
  • T. Ichikawa et al.

    Changes in the concentration and amount of radioce-sium in decomposing litters in satoyama consisting of deciduous broad-leaved forest

    J. Jpn. For. Soc.

    (2015)
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