Radionuclide Analysis on Bamboos following the Fukushima Nuclear Accident

In response to contamination from the recent Fukushima nuclear accident, we conducted radionuclide analysis on bamboos sampled from six sites within a 25 to 980 km radius of the Fukushima Daiichi nuclear power plant. Maximum activity concentrations of radiocesium 134Cs and 137Cs in samples from Fukushima city, 65 km away from the Fukushima Daiichi plant, were in excess of 71 and 79 kBq/kg, dry weight (DW), respectively. In Kashiwa city, 195 km away from the Fukushima Daiichi, the sample concentrations were in excess of 3.4 and 4.3 kBq/kg DW, respectively. In Toyohashi city, 440 km away from the Fukushima Daiichi, the concentrations were below the measurable limits of up to 4.5 Bq/kg DW. In the radiocesium contaminated samples, the radiocesium activity was higher in mature and fallen leaves than in young leaves, branches and culms.


Introduction
On March 11, 2011, several large earthquakes hit the northeast coast of Honshu, Japan, followed by a massive tsunami along the east coastline of Japan. The tsunami that damaged the Tokyo Electric Power Company (TEPCO) Fukushima Daiichi nuclear power plants caused huge releases of radionuclides into the environment of Japan [1]. However, the accident also had global effects as suggested by the detection of radioxenon ( 133 Xe) in Washington, USA [2] and Vancouver, Canada [3], and of radioiodine ( 131 I) and radiocesium ( 134 Cs, 137 Cs) in California and Washington, USA [4], Thessaloniki, Greece [5], Bremen, Germany [6], Huelva, Spain [7] and Krasnoyarsk, Russia [8]. The spread and effects of the 134 Cs and 137 Cs radionuclides have become of prime interest because of their dose and long half-lives of 2 and 30 years, respectively. Indeed, we are faced by the serious problem of radiocesium attachment to and/or uptake by agricultural plants [9].
Bamboo is a fast-growing renewable biomass that is widely distributed throughout Asia. In Japan, the above-ground biomass of Phyllostachys pubescens has been estimated at 116.5 t dry matter ha 21 for culms and with a gross annual soil respiration of 52.3 t dry CO 2 ha 21 [10]. The large quantities of bamboo leaves that are often used for bamboo grass tea or as cattle food are indicative of their huge environmental impact. Also, the fallen leaves that are spread in the environment might enter the human food chain via soil decomposer. Here, we report the results of the radionuclide analysis on bamboos following the Fukushima nuclear accident.

Ethics Statement
No specific permits were required for the described field studies: a) no specific permissions were required for these locations/ activities; b) location are not privately-owned or protected; c) the field studies did not involve endangered or protected species.

Plant samples
We sampled leaves and branches of Phyllostachys nigra var.  (Fig. 1). New leaves described in this report were estimated to have emerged approximately two weeks before collection. The heights of all bamboo at collection were 3 to 5 m. Before radioactive measurements, all samples were dried at 60uC (dry oven, SANYO, MOV-112S) for 24 hours.

Radioactive measurements
The samples were analyzed by gamma spectrometry, equipped with a high purity germanium detector (Princeton Gamma-Tech, IGC-30180) and a multi-channel analyzer (Canberra, DSA-1000). The detector was shielded with 5 cm of lead to reduce background contributions from the surroundings. For determination of 134 Cs activity concentrations, gamma-ray energies of 604.70 and 795.85 keV were used. The 137 Cs activity concentrations were determined from the 661.66 keV peak energies. The samples were measured for a period of 7,200 seconds.
We cite data of the air dose rate 1 m above the ground surface for Minamisouma, Fukushima, and Aizuwakamatsu cities, opened by the Japanese Ministry of Education, Culture, Sports, Science   and Technology (MEXT) [11]; the data for Kashiwa city, opened by The University of Tokyo [12]; the data for Toyohashi city, opened by Aichi Prefecture [13]; and the data for Beppu city, opened by Oita Prefecture [14]. All air dose rates were measured using portable survey meters from late June to mid-August, 2011.

Results
Typical gamma-ray spectra from mature leaves are shown in Figure 2A and B. The 134 Cs and 137 Cs were clearly detected as peaks of gamma-ray energy in the samples obtained on 24 th July from Minamisouma city ( Fig. 2A), but were below the measurable limits in the 20 th August samples from Toyohashi city (Fig. 2B). Unfortunately, we could not determine the quantity of 40 K because of its low signal to noise ratio (Fig. 2), as 40 K was not derived from the nuclear accident but from its natural occurrence at 0.0117 percent, and has an in vivo behavior similar to that of stable potassium. Table 1 shows the activity concentrations of bamboo leaves and branches from the various sampling sites from around early July to early September, 2011. Strikingly high activity concentrations were obtained in samples from Minamisouma and Fukushima cities near the Fukushima Daiichi nuclear plants. The highest 134 Cs and 137 Cs activities were 71.1 and 79.1 kBq/kg dry weight (DW) in mature leaves sampled in Fukushima city, respectively. In Aizuwakamatsu and Kashiwa cities, radiocesium activities were one order lower than those in Minamisouma and Fukushima cities. In Toyohashi and Beppu cities, radiocesium activities were below the measurable limits of up to 4.50 Bq/kg DW. The radiocesium activities tended to increase with the rise in air dose rates at the sampling sites (Fig. 3).
In the radiocesium contaminated samples, leaves showed higher activities than branches (Table 1) and culms ( Table 2) and, among these, mature and fallen leaves showed higher activities than young leaves (Table 1).
To check whether the bamboos absorbed the radiocesium or not, we split the outer layer of the culms with a cutter knife to separate them into inner and outer culm layers. The activities within the inner layers were comparable to those in the outer layers (Table 2), suggesting radiocesium uptake.

Discussion
In this study, we report the radiocesium contamination of bamboos sampled within a 25 to 980 km radius of the Fukushima Daiichi nuclear power plant. It has recently been reported that the 134 Cs and 137 Cs activity concentrations of azalea leaves collected in Chiba city, 220 km away from the Fukushima Daiichi, were 3.35 and 3.78 kBq/kg fresh weight, respectively [15]. These levels are comparable to our bamboo samples taken from Kashiwa city, 195 km away from the Fukushima Daiichi. Our results indicated that radiocesium contamination in mature and fallen leaves were higher than young leaves. It is possible that young leaves had not yet emerged at the time of the accident so that direct deposition from radiocesium fallout would have been negligible. On the other hand, the activity within the inner culm layers suggests the uptake of radiocesium by bamboo in Kashiwa, 195 km away from the Fukushima Daiichi (Table 2). Cesium is an alkaline metal, and is present as a monovalent cation in all soils. Because of its similarity with its nutrient analogue, potassium, a considerable amount of radiocesium is thought to have been taken up by the bamboos from contaminated soil. A recent study reported the uptake of radiocesium by emerged leaves of 14 plant species collected 220 km away from the Fukushima Daiichi, and suggested that the translocation velocity of radiocesium varied among the species tested [15]. Our results may be in accord with results from experimental field-grown rice plants that showed a lower translocation velocity of 133 Cs than potassium [16].
The mature leaves of bamboo are often used to prepare bamboo grass tea. The radiocesium activity concentrations of the bamboo leaves from Minamisouma, Fukushima, Aizuwakamatsu and Kashiwa were over 500 Bq/kg, a temporary regulatory value decided by the Japanese Ministry of Health, Labour and Welfare (MHLW). As the effective dose coefficients for ingestion of 134 Cs and 137 Cs are 0.019 and 0.013 mSv/Bq, the effective doses from the 200 mg amounts that are roughly used for a cup of tea prepared from milled tea powder from Fukushima and Kashiwa cities would be 0.476 and 0.0243 mSv, respectively. If just six cups of the bamboo grass tea from Fukushima city are consumed every day, the radiation dose from the tea would be in excess of 1 mSv/ year. Therefore, we consider it preferable to avoid the regular drinking of bamboo tea from highly contaminated areas surrounding the Fukushima nuclear plant.