Contribution of radioactive 137Cs discharge by suspended sediment, coarse organic matter, and dissolved fraction from a headwater catchment in Fukushima after the Fukushima Dai-ichi Nuclear Power Plant accident

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Highlights

  • The 137Cs concentrations of suspended sediment were 2.4–49 kBq/kg.

  • The 137Cs concentrations of coarse organic matter were 0.85–14 kBq/kg.

  • The annual proportions of contribution of 137Cs discharge by suspended sediment were 96–99%.

  • The total annual 137Cs discharge from the catchment was 0.02–0.3% of the deposition.

Abstract

Radiocesium (137Cs) migration from headwaters in forested areas provides important information, as the output from forest streams subsequently enters various land-use areas and downstream rivers. Thus, it is important to determine the composition of 137Cs fluxes (dissolved fraction, suspended sediment, or coarse organic matter) that migrate through a headwater stream. In this study, the 137Cs discharge by suspended sediment and coarse organic matter from a forest headwater catchment was monitored. The 137Cs concentrations in suspended sediment and coarse organic matter, such as leaves and branches, and the amounts of suspended sediment and coarse organic matter were measured at stream sites in three headwater catchments in Yamakiya District, located ∼35 km northwest of Fukushima Dai-ichi Nuclear Power Plant (FDNPP) from August 2012 to September 2013, following the earthquake and tsunami disaster. Suspended sediment and coarse organic matter were sampled at intervals of approximately 1–2 months. The 137Cs concentrations of suspended sediment and coarse organic matter were 2.4–49 kBq/kg and 0.85–14 kBq/kg, respectively. The 137Cs concentrations of the suspended sediment were closely correlated with the average deposition density of the catchment. The annual proportions of contribution of 137Cs discharge by suspended sediment, coarse organic matter, and dissolved fraction were 96–99%, 0.0092–0.069%, and 0.73–3.7%, respectively. The total annual 137Cs discharge from the catchment was 0.02–0.3% of the deposition.

Introduction

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident following the earthquake and tsunami disaster on 11 March 2011 resulted in substantial release of radionuclides to the environment. The distribution of deposited radiocesium (137Cs) in the area has been investigated in numerous studies (e.g. Hirose, 2013, Mikami et al., 2015a, Mikami et al., 2015b, Saito et al., 2014). Approximately 70% of the area where 137Cs was deposited was forested (Calmon et al., 2015).

In forests, it is known that 137Cs is absorbed by trees and surface soil. Studies on the interception of 137Cs by coniferous forest canopies (Kato et al., 2012a) and the vertical distribution of aerial dose rate and its temporal change (Kato and Onda, 2014) found evidence of 137Cs retention by trees. Kato et al. (2015) and Loffredo et al. (2014) reported transfer of canopy-intercepted 137Cs to the forest floor by measuring throughfall, stemflow, and litterfall. The 137Cs profiles in the soil showed that most of the 137Cs was located in the upper 5 cm (Kato et al., 2012b, Matsunaga et al., 2013, Takahashi et al., 2014).

Because 137Cs in soil is strongly absorbed into clay minerals (Cremers et al., 1988, Walling, 1998), subsequent migration of the deposited 137Cs occurs through the physical movement of soil particles by soil erosion (Yoshimura et al., 2014a) and possibly by soil erosion into rivers by surface runoff (Bonnett, 1990, Fukuyama et al., 2005). Based on the estimate by Yamashiki et al. (2014) that 84–92% of total 137Cs transport was by particulate matter in the Abukuma River, this matter appears to be the main medium by which 137Cs was transported through rivers (Nagao et al., 2013, Ueda et al., 2013, Sakaguchi et al., 2014). A similar result was also obtained from stream observations in a forested catchment (Ohte et al., 2012). Discharge of dissolved 137Cs was also monitored, and it was shown that dissolved 137Cs concentration increased over time during a rainfall event (Iwagami et al., 2015). However, the mechanism of the increase has not been well understood. One possibility is that 137Cs is leached from suspended sediment or coarse organic matter. Leaching of 137Cs from smectite is possible through a cation-exchange reaction (Yamashina and Fukushi, 2013). On the other hand, the inhibitive reaction of clay particle organization on cesium desorption has also been reported (Fukushi et al., 2014). From multipoint observations in a stream in a small forested catchment, it was suggested that desorption of 137Cs from litter to stream water can occur at low flow rates in a litter-rich environment (Yasutaka and Nakamura, 2013). A leaching study conducted in the laboratory (Sakai et al., 2015) demonstrated that 137Cs was leached from litter samples to water with long stirring times.

Numerous studies have observed 137Cs concentrations in suspended sediments and dissolved fraction samples. However, there have been few reports of 137Cs migration through rivers by organic matter such as leaves and branches, whereas studies have shown high 137Cs concentration in litter in forest sites (Hisadome et al., 2013, Kato et al., 2015, Loffredo et al., 2014). The study of 137Cs migration from headwaters in forested areas provides important information as the output from forest streams subsequently enters various land-use areas and downstream rivers (Freed et al., 2004, Garcia-Sanchez, 2008, Evrard et al., 2015). Thus, it is important to determine the composition of 137Cs fluxes (dissolved, suspended sediment, or coarse organic matter) that migrate through a headwater stream. To this end, the objective of this study was to examine the contribution of 137Cs discharge by suspended sediment, coarse organic matter, and dissolved fraction from a headwater catchment in Fukushima.

Section snippets

Study area

Stream water discharge, turbidity, and precipitation were monitored, and suspended sediment and coarse organic matter were sampled in three small headwater catchments in Yamakiya District, Nihonmatsu City, Fukushima Prefecture, Japan: Koutaishi-yama (KOU, 0.54 km2; 94% forested: 72% conifer, 22% broad-leaf), Iboishi-yama (IBO, 0.17 km2; 71% forested: 44% conifer, 27% broad-leaf), and Ishidaira-yama (ISH, 0.075 km2; 92% forested: 13% conifer, 79% broad-leaf). The catchments are located ∼35 km

Sampling of suspended sediments and coarse organic matter

Suspended sediment (SS) and coarse organic matter (Org) were sampled approximately monthly from August 2012 to September 2013. The configurations of the SS sampler and Org sampling net along the stream are shown in Fig. 2. The SS was sampled using a time-integrated SS sampler (Phillips et al., 2000). Through field trials, Phillips et al. (2000) confirmed that the collected samples provided statistically representative values of the particle size and carbon content in ambient water based on

137Cs concentrations of SS and Org

The 137Cs concentrations of the SS and Org samples collected from August 2012 to September 2013 are shown in Table 1 together with related data. The time series of discharge, 137Cs concentrations of SS and Org, and 137Cs concentrations of the dissolved fractions at KOU, IBO, and ISH are shown in Fig. 3. Some of the discharge and turbidity data were underestimated because of missing data due to interruptions in the logger measurements by low temperatures or overflow.

At KOU, the 137Cs

Conclusions

Hydrometric observations, suspended sediment sampling, coarse organic matter sampling, and associated 137Cs analysis were conducted to investigate 137Cs concentrations in suspended sdediment and coarse organic matter, and their contributions to 137Cs discharge from a headwater forested area in Fukushima. The observations were carried out in three headwater catchments in Yamakiya District, Nihonmatsu City, Fukushima Prefecture, Japan, from August 2012 to September 2013. The 137Cs concentrations

Acknowledgments

This research was conducted as a commissioned study from the Japanese Atomic Energy Authority (JAEA) as a part of the MEXT-funded FY2012 project titled “Establishment of the grasp method to determine the long-term effects caused by radioactive materials from the Fukushima Dai-ichi Nuclear Power Plant accident.”

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