Keywords

7.1 Introduction

The nuclear power plant accident that occurred in Fukushima Prefecture in March 2011, caused a wide spread of radioactive nuclides over various parts of Japan. This raised the immediate concern that agricultural products might become directly contaminated by radioactive nuclide fallout. Therefore, an investigation of the contamination of vegetables by radiocesium (134Cs and 137Cs) was initiated about 2 months after the accident (Oshita et al. 2011, 2013).

For this investigation, vegetable samples were collected from the fields at the Institute for Sustainable Agro-ecosystem Services, Graduate School of Agricultural and Life Sciences, The University of Tokyo, located in Nishitokyo City, Tokyo. Samples were also collected from fields near a mountain village in Fukushima Prefecture.

Another possible problem, which could have developed over time in addition to direct contamination, was the transfer of radioactive nuclides through root absorption (indirect contamination). Therefore, the transfer of radioactive nuclides from the soil was studied by intentionally spreading high-concentration radioactive nuclides onto culture media. Previous studies examined rice plants (Tensho et al. 1959; Yonezawa and Mitsui 1965; Tsumura et al. 1984), grapes (Zehnder et al. 1995), mushrooms (Ban-nai et al. 1994) and field crops (Uchida et al. 1987; Ban-nai et al. 1995; Ban-nai and Muramatsu 2002). Another study assessed the effects of environmental radioactivity on foodstuffs (Schwaiger et al. 2004). To examine radionuclide transfer coefficients, 137Cs at an extremely high concentration was used in many cases such as the paper by (Ban-nai et al. 1995) [490 kBq/pot (about 163 kBq/kg, as the one pot was filled with 3.0 kg of the soil)] and that by (Broadley and Willey 1997).

In the results reported here, the amounts of radiocesium (134Cs + 137Cs) transferred from the field soil to vegetables at a low concentration in the soil of ≤1,235 Bq/kg dry weight were measured in order to determine the transfer of radiocesium in vegetable-­producing fields.

7.2 Materials and Methods

7.2.1 Vegetable and Soil Samples

Samples were obtained from two different locations. One was the agricultural research field at the Institute for Sustainable Agro-ecosystem Services located in Nishitokyo City, Tokyo (altitude approximately 60 m above sea level; this site is referred to as “Nishitokyo City”). This site was about 230 km away from the Fukushima Daiichi nuclear power plant. Potatoes (Irish Cobbler) and cabbage (YR Rakuzan) that grew in this field and the soil around these vegetable-producing fields were collected on 16 May 2011 (40 days after the settled cabbage planting and 47 days after potato planting), on 27 May 2011 (51 days after the settled cabbage planting and 58 days after potato planting) and on 20 June 2011 (75 days after the settled cabbage planting and 82 days after potato planting). Soil samples, mostly consisting of surface soil (depths of 0–5 cm), were sampled at the time of vegetable sampling. In addition, on 11 June 2011, more soil samples were collected at 5-cm intervals and at depths as great as 35 cm below the ground surface.

Potato (Kita-Akari), cabbage (Shikidori) and lettuce (Berkley) samples were collected from a mountain village (altitude approximately 420 m above sea level) located in the southeastern part of Fukushima Prefecture and about 60 km away from the Fukushima Daiichi nuclear power plant. They were sampled on 18 July 2011, 99 days after potato planting, 78 days after the settled cabbage planting and 49 days after the lettuce planting. Soil was sampled from each field at 5- or 10-cm intervals and at depths as low as 30 cm below the ground surface.

7.2.2 Radioactive Nuclide Concentration Measurements

We measured the concentrations of 134Cs and 137Cs, both of which have a long half-­life, as well as naturally occurring 40K using a germanium semiconductor detector (ORTEC, SEIKO EG&G Co. Ltd.). For the samples collected on 16 May 2011, the measurement time was set to 50,000 s for the vegetable samples and 18,000 s for the soil samples. For all other samples, the measurement time was set to 3,600 s.

7.3 Results and Discussion

7.3.1 Concentrations of Radioactive Nuclides in Vegetables and Soil (Tokyo)

Table 7.1 shows the measured concentrations of the radioactive nuclides 134Cs, 137Cs and 40K in samples of potato leaves (both washed and unwashed), roots, tubers and soil. Among the potato samples collected 47 days after planting, an extremely small amount of radiocesium was detected in the washed (137Cs was detected but 134Cs was not) and unwashed (both 134Cs and 137Cs were detected) leaf samples. However, radiocesium (134Cs and 137Cs) was not detected in the unwashed leaf sample of potatoes collected 58 days after planting, although a very small amount was detected in the washed sample.

Table 7.1 Concentration of 134Cs, 137Cs and 40K of potato and soil in Tokyo
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These results contradicted our predictions. This could have been because the detection limit for the unwashed leaf sample was about twice as high as that for the washed leaf sample, as indicated in Table 7.1. Because the amount of the unwashed leaf sample was less than that of the washed leaf sample, we suspected a greater detection limit for the unwashed leaf sample.

No radiocesium was detected in the root samples. With regard to tubers, which are the edible parts of plants, 137Cs was detected at a concentration of 2.4 Bq/kg wet weight in the sample collected 58 days after planting. Although the concentration of 134Cs was lower than the detection limit, the measurement is provided as a reference only. 134Cs and 137Cs were not detected in the sample collected 82 days after planting.

To explain these results for tubers, we considered potassium (K) to be an important factor for tuber growth. Translocation and storage of carbohydrates occurs in staple crops rich in starch, such as potatoes, and potassium ions (K+) move around in these plants. A report on daikon radish described that cesium behaved like potassium in plants, in that the potassium concentrations in root tubers decreased after a certain stage of plant growth and cesium exhibited the same tendency (Ban-nai and Muramatsu 2002).

Comparison of the concentrations of 40K based on this hypothesis showed that the concentrations in the samples collected 82 days after planting were about 45% of those in the samples collected 58 days after planting. We estimated that the normal 40K concentration in potato tubers was 124.6 Bq/kg wet weight. This was based on the assumption that the potassium concentration in potato tubers is 410 mg/100 g (Standard Tables of Food Composition in Japan, http://www.mext.go.jp/b_menu/shingi/gijyutu/gijyutu3/toushin/05031802.htm) and the radioactivity of 1 g of potassium is 30.4 Bq/g, also considering the natural abundance of 40K.

The value of 124.6 Bq/kg wet weight was almost 82% of the value of 151.2 Bq/kg wet weight measured in the sample collected 82 days after planting. This indicated that the measured concentration of 40K corresponded closely to the potassium concentration in tubers. Because the potassium concentration would naturally decrease to about 45% in the plants collected 58 days after planting, as with 40K, we considered that the concentration of radiocesium, which behaves like potassium, would be reduced by half in the sample collected 82 days after planting and thus become undetectable.

Table 7.2 shows the concentrations of the radioactive nuclides 134Cs, 137Cs and 40K in the samples of cabbage leaves (edible parts and outer leaf, washed and unwashed), roots and soil. Radiocesium was detected only in the samples of the outer leaf (non-edible parts) collected at 40 days and 75 days after settled planting. Because the concentrations were lower in the samples collected 75 days after planting than in the samples collected 40 days after planting, we assumed deposition of radioactive fallout.

Table 7.2 Concentration of 134Cs, 137Cs and 40K of cabbage and soil in Tokyo
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It should be noted that the measurements obtained for the samples collected 75 days after planting indicated that the concentration of 137Cs was lower than that of 134Cs, although the detection sensitivity should have been higher for 137Cs. One reason for these results was assumed to be the low concentrations in the samples that were close to the detection limit. However, this needs to be clarified. The major reason for radiocesium becoming undetectable in the outer leaf samples collected 51 days after planting was believed to be the six times higher detection limit for these samples than that for samples collected 75 days after planting.

Figure 7.1 shows the vertical distributions in the soil for radioactive 134Cs, 137Cs and 40K. The “depth” axis in this figure shows negative values because depth was measured using the bottom of the ridge as the reference point (zero) when collecting soil samples from the potato field. In the soil samples from the potato and cabbage fields, distribution of radiocesium was observed in the plow layer, which was 15 cm below the ground surface. The concentrations were particularly high in the soil samples from the surface and ridges. Tables 7.1 and 7.2 show that the radiocesium concentration in the surface soil varied depending on the day of sampling. We assumed that this indicated a two-dimensional concentration distribution.

Fig. 7.1
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Concentration of 134Cs and 137Cs at each depth in soil observed on 9 June 2011 at Institute for Sustainable Agro-ecosystem Services in Nishitokyo City

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7.3.2 Concentrations of Radioactive Nuclides in Vegetables and Soil (Fukushima Prefecture)

Table 7.3 shows the concentrations of the radioactive nuclides in samples of potato tubers (washed) and aerial parts (leaf and stem, washed). Table 7.4 shows the concentrations in samples of edible cabbage parts (unwashed and washed), outer leaves (washed) and roots (washed). The concentrations in the samples of edible lettuce parts (unwashed and washed) and roots (washed) are shown in Table 7.5. Figure 7.2 shows the distributions of radioactive 134Cs and 137Cs concentrations in the soil samples collected at different depths in each field.

Table 7.3 Concentration of 134Cs, 137Cs and 40K of potato in Fukushima Prefecture (99 days after planting)
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Table 7.4 Concentration of 134Cs, 137Cs and 40K of cabbage in Fukushima Prefecture (78 days after planting)
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Table 7.5 Concentration of 134Cs, 137Cs and 40K of lettuce in Fukushima Prefecture (49 days after planting)
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Fig. 7.2
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Concentration of radiocesium (134Cs + 137Cs) at each depth in soil observed on 18 July 2011 in Fukushima Prefecture

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An extremely small amount of radiocesium (134Cs and 137Cs) was detected in the potato tuber sample (Table 7.3). In this case, 134Cs and 137Cs in the soil at depths from 0 to 5 cm were detected at 568.6 and 666.4 Bq/kg dry weight (total of 1,235 Bq/kg dry weight), respectively (Fig. 7.2). With regard to the transfer of radiocesium, one study investigated the transfer rate from plant roots to organs in hydroponic conditions (Uchida et al. 1987).

They found that in root crops and leaf vegetables, the transfer rate of 137Cs, determined by (activity of plant sample/day)/(mean activity of culture solution), to roots is faster by about one order of magnitude than that to other plant organs located above the ground. In contrast, in root vegetables (daikon radish) growing in soil culture, the transfer factor for leaves is higher than that for tubers, with a ratio of 4.1 for 137Cs (Ban-nai and Muramatsu 2002). Regardless of which hypothesis is adopted, the concentration of radiocesium in the aerial parts shown in Table 7.3 was thought to be too high to have resulted from uptake through root absorption. Thus, the high concentration was assumed to be due to the deposition of radioactive fallout.

The transfer factor from soil to potato tubers was calculated for reference. Based on the concentrations of 137Cs in the tuber and soil samples (583 Bq/kg dry weight as a mean value for vertical positions from −10 to 10 cm; not shown in Fig. 7.2), the transfer factor was estimated to be 0.0057 using the following equation: (137Cs concentration in tuber; fresh vegetable)/(137Cs concentration in soil; dry soil). This value was smaller than the index value of 0.067 (http://www.maff.go.jp/j/press/syouan/nouan/110527.html) provided by the Japanese Ministry of Agriculture, Forestry and Fisheries (MAFF), which was derived by the same equation. When the sum of the 134Cs and 137Cs concentrations was used in this calculation, the transfer coefficient was 0.0061.

In cabbage, no radiocesium was detected in samples of the outer leaf (washed) and root (washed). In lettuce, a very small amount of 137Cs was detected in a root sample but not in any other plant organ sample. In these cases, the maximum concentrations of 134Cs and 137Cs in the soil were found at depths ranging from 0 to 5 cm of 291.8 and 359.4 Bq/kg dry weight (total of 651.2 Bq/kg dry weight), respectively (Fig. 7.2). Data provided by MAFF in 2011 yielded the transfer factor of radiocesium from soil to vegetables in which the concentrations in soil were close to the values observed in Fukushima in our investigation (Ministry of Agriculture, Forestry and Fisheries, http://www.s.affrc.go.jp/docs/nogyo_gizyutu/pdf/3_1.pdf). Although we did not observe the transfer of radiocesium from the soil to cabbage or lettuce, it will be important to continue obtaining data on the transfer of radiocesium from soil with low contamination for further study.

The measured concentrations of radioactive nuclides in the soil showed a vertical distribution pattern within the plow layer for 134Cs and 137Cs, with surface soil having a high concentration level (Fig. 7.2). These results corresponded with the results obtained in Nishitokyo City (Fig. 7.1).

7.4 Conclusion

We investigated the transfer of radiocesium from soil with low contamination to vegetables. An extremely small amount of 137Cs was detected in the tuber samples of potatoes collected 58 days after they were planted in a dry field with a 137Cs concentration of 50–100 Bq/kg dry weight. However, radiocesium was not detected in potato samples collected 82 days after planting. Potassium is known to be transported in plants to adjust the osmotic pressure in cells. We suspected that cesium behaved in much the same way as potassium, which resulted in a lower concentration in potatoes at harvesting season.

In potatoes grown in a dry field with a 137Cs concentration of 530–670 Bq/kg dry weight, a very small amount of 137Cs was transferred to the tubers. This transfer coefficient was calculated to be 0.0057, although this should be considered as a reference value only. This value is smaller than the index value provided by the Japanese MAFF. However, it should be noted that this index value was determined from data in the range of 0.00047–0.13. In this investigation, any detectable transfer of cesium from the soil to the edible parts of cabbage or lettuce was observed. Because the transfer factor varies depending on soil characteristics, it will be necessary to collect more data for soil with a low contamination level and conduct more studies on the transfer of radiocesium from soil to vegetables in the fields where agricultural production is continuing.

Our investigation of soil did indicate a vertical distribution pattern for radiocesium concentrations, with a higher concentration in surface soil. We anticipate that by making the concentrations of radiocesium more uniform (reduced) by land cultivation, deep plowing or other soil mixing methods, the transfer of radiocesium will be reduced.