Effects of Radioactive Cesium from Suspended Matter and Fallout on Agricultural Products

Abstract

After the accident at the Tokyo Electric Power Company’s Fukushima Daiichi Nuclear Power Station, agriculture field decontamination has been considered to produce safe agricultural products. However, secondary contamination of agricultural products via fallout and suspended matter has not been contemplated in these counter measures. Therefore, we examined secondary contamination of crops in the area affected by the FDNPP accident. Radioactive cesium concentrations in Komatsuna cultivated using noncontaminated soil and water were affected by location of cultivation, height of cultivation, and whether the plants were washed or not. These results suggest that, depending on the region, secondary contamination of crops occurs due to the attachment of soil particles that rebounded from the ground by rainfall, and that radioactive cesium on the surface can be taken into the crop. By understanding the environmental and meteorological factors that increase the risk of contamination and the sources of secondary contamination, it is believed that future risks can be reduced.

4.1 Introduction

The disastrous accident at the Tokyo Electric Power Company’s Fukushima Daiichi Nuclear Power Plant (FDNPP) in 2011 caused the release of significant amounts of radioactive waste, resulting in radiological contamination in the surrounding forests, agricultural fields, and residential areas. Although ¹³¹I and ¹³²I were the main radionuclides found immediately after the accident, the environmental radiation was primarily derived from radioactive cesium (Cs), particularly ¹³⁴Cs and ¹³⁷Cs from 10 days after the accident (United Nations Scientific Committee on the Effects of Atomic Radiation 2014; Yoshimura et al. 2020). Radioactive Cs was released either in gaseous or insoluble particulate form, and the gaseous form was considered to be transported with sulfate-suspended matters in the atmosphere, deposited on the ground, and attached to soil particles or vegetation (Kaneyasu et al. 2012). Radioactive Cs activity in the atmosphere has gradually decreased since the FDNPP accident (Abe et al. 2021). However, it can still be detected in fallout and atmospheric suspended matter, especially in areas around the FDNPP, including the decontaminated fields (Nuclear Regulation Authority, Japan 2019a, b). One of the contributing factors could be a resuspension of radioactive Cs deposited on the ground (Ishizuka et al. 2017).

Agriculture is the main industry in the Fukushima Prefecture. To produce safe agricultural products, field decontamination, potassium fertilization to prevent absorption of radioactive Cs via roots, and other counter measures have been considered (Fukushima Prefecture 2014; Ministry of the Environment 2013). However, secondary contamination of agricultural products by radioactive cesium via fallout and suspended matter has not been contemplated in these counter measures. Therefore, it is important to understand the effect of radioactive cesium from fallout and suspended matter on the secondary contamination of agricultural products.

4.2 Effects of Radioactive Cs from Suspended Matter and Fallout on Japanese Mustard Spinach (Komatsuna)

To determine the effects of radioactive Cs from suspended matter and fallout on the secondary contamination of agricultural products, Japanese mustard spinach (Brassica rapa L. var. perviridis, Komatsuna) was cultivated in the area affected by the FDNPP accident (Nihei et al. 2018). Briefly, Komatsuna was cultivated using noncontaminated soil and water. Pots containing Komatsuna were placed 30 cm (hereafter referred to as “below”) or 120 cm (referred to as “above”) above the ground surface (Fig. 4.1). The pots were placed at two locations in the Fukushima Prefecture: Site A, approximately 35 km from the FDNPP with an air dose rate of approximately 0.4 μSv h⁻¹ (measured in August 2018), and Site B, approximately 3.5 km from the FDNPP with an air dose rate of 10.0 μSv h⁻¹. Site A was in a zone that was evacuated until March 2016 and subsequently decontaminated. Site B was also within an evacuation zone, but was not yet decontaminated. Komatsuna were cultivated during three periods: August 16–September 9, 2018 (Period 1), September 9–October 6, 2018 (Period 2), and October 6–November 23, 2018 (Period 3). After harvesting part of the Komatsuna, half of the plants were not washed (hereafter, “unwashed plants”) and the other half were washed with water (“washed plants”). To collect the fallout, basins filled with water were placed at each site during each cultivation period. Radiocesium in the fallout was collected as dissolved and particulate fractions, which were separated by filtration. Activity of ¹³⁷Cs in the sample was measured using a Ge-semiconductor detector.

Fig. 4.1

Komatsuna cultivation system installed at the test site. Pots filled with noncontaminated soil were placed at heights of 30 cm and 120 cm above the ground. Basins filled with water were placed to collect fallout at the same heights as the pots

Figure 4.2 shows the ¹³⁷Cs concentrations in the unwashed and washed Komatsuna samples cultivated at each study site. The measurement results are expressed as activity concentration of ¹³⁷Cs per kg of dry weight (Bq kg⁻¹). The dry weight-base concentration is approximately ten times higher than the wet weight-base concentration, which is applied to official monitoring inspections of agricultural products. The concentration of ¹³⁷Cs in Komatsuna cultivated at Site A ranged from 8 to 73 Bq kg⁻¹, while that at Site B showed higher values ranging from 18 to 171 Bq kg⁻¹, with secondary contamination depending on the ¹³⁷Cs level in the fields. If we consider the wet weight-base concentration, the values were less than the standard limit of radioactive Cs concentration allowed for agricultural products (100 Bq kg⁻¹) for shipping and ingestion.

Fig. 4.2

Radioactive Cs concentration of Komatsuna cultivated using noncontaminated soil in each site

The ¹³⁷Cs concentrations of Komatsuna cultivated at different heights showed that closer proximity to the ground generally related to higher concentrations of ¹³⁷Cs (Fig. 4.2). Similarly, Fig. 4.3 shows increased ¹³⁷Cs concentrations in the fallout collected in lower basins than that in higher basins. This difference in the ¹³⁷Cs concentration depending on installation height suggests that deposition of soil particles that rebounded from the ground due to rainfall contributed to secondary contamination of the Komatsuna.

Fig. 4.3

Deposition of ¹³⁷Cs in the water samples from the basins installed at Sites A and B

Although the ¹³⁷Cs concentration in washed plants was significantly lower than that in unwashed plants, the washed plants showed detectable activity. This suggests that the secondary contamination was caused not only by external adhesion to the leaf surface but also internal absorption of ¹³⁷Cs from the suspended matter or fallout. Assuming that the ¹³⁷Cs concentration in the unwashed plants was a result of both external adhesion on the leaf surface and internal absorption, and that the ¹³⁷Cs concentration in washed plants was derived from internal absorption only, the proportion of internal absorption to total ¹³⁷Cs was estimated as the ratio between the ¹³⁷Cs concentrations of washed and unwashed plants. The average proportions were estimated to be 62% at Site A and 71% at Site B, with variation depending on the cultivation period.

4.3 Secondary Contamination Due to Fallout

There are two possible pathways for secondary contamination of agricultural products due to fallout: root uptake of radioactive Cs deposited on the soil in the pots, or foliar absorption of radioactive Cs adhered to the plant surface. To investigate the root uptake, the pots were filled with noncontaminated soil and left for 31, 77, and 161 days in Site B. After each period, the soil in the pot was used to cultivate Komatsuna in a phytotron. The ¹³⁷Cs concentrations of the soil at 31, 77, and 161 days were 3700, 4500, and 7000 Bq m⁻², respectively, and the ¹³⁷Cs concentrations of the Komatsuna grown in the respective soils were 9, 21, and 34 Bq kg⁻¹. These results indicate that root uptake of radioactive Cs from the contaminated soil due to fallout is a legitimate pathway for secondary contamination of agricultural products. However, the fact that the radioactive Cs concentration in Komatsuna was below the allowable limit for shipping and ingestion, despite the ¹³⁷Cs concentration in soil reaching 4500 Bq kg⁻¹ by staying in the evacuation zone, suggests that the impact of this pathway on secondary contamination is minimal.

4.4 Characteristics of Radioactive Fallout

The ¹³⁷Cs fallout at each site showed that approximately 90% of the ¹³⁷Cs was detected in the particulate fraction, while the remaining 10% was in soluble form (Fig. 4.3). Part of the dissolved Cs could present in a particulate form during suspension in the atmosphere and then dissolved in the water of basins during the fallout-collection periods. This dissolved fraction is bioavailable and could be absorbed from the attached leaf surface or the roots.

To analyze the fallout particulate matter in detail, the particulates in the basin at Site B were collected on membrane filters and analyzed using autoradiography. The autoradiographic images of the membrane filter showed black dots, which represent the presence of radioactive particles (Okumura et al. 2019) in these samples (Fig. 4.4). One of the particles had a diameter of approximately 2 μm and radiation of approximately 0.5 Bq. Particles were investigated using a scanning electron microscope with an energy-dispersive X-ray spectrometer (EDS). The peaks of Si and O suggest that the matter consists mainly of silicate. Additionally, Fe, Zn, K, Cl, and Cs were detected. These characteristics of radioactive particles are similar to those of the Cs-bearing microparticles reported by Adachi et al. (2013). These particles containing high concentrations of radioactive Cs rarely increase the radioactive Cs in agricultural products, because it is difficult to leach the cesium from them. The other particles had approximate diameters of 60–120 μm and approximate radiation levels of 0.4–1.2 Bq. EDS spectra showed an abundance of Si, Al, and O, suggesting that these are basically soil particles with fine clay minerals. Since radioactive Cs that is stably fixed in the clay layer is difficult to dissolve, the radioactive Cs in these particles is likely only minimally absorbed by plants. However, in addition to the glassy or clay-fixed form observed in this study, radioactive Cs in suspension also adsorb onto organic matter (Okumura et al. 2019). Organic matter could decompose over time and release radioactive Cs. Therefore, further investigation is necessary regarding the bioavailability of radioactive Cs in organic particles and their effect on secondary contamination of plants.

Fig. 4.4

Autoradiograph image of radioactive materials collected from water samples and detected with imaging plate. Black dots indicate presence of radioactive materials

4.5 Relationship Between Fallout Adhesion and Meteorological Factors

The concentration of ¹³⁷Cs in Komatsuna fluctuated depending on the cultivation period within the same area. Factors such as wind speed and precipitation were examined to assess their effect on the varying ¹³⁷Cs concentrations observed during the growth periods of Komatsuna (Fig. 4.5). No clear relationship between these factors and ¹³⁷Cs concentration was observed for Site A, but higher precipitation and faster maximum wind speeds resulted in higher ¹³⁷Cs concentrations in Site B for Komatsuna. Particles containing radioactive Cs deposited on soil and vegetation are considered to have been carried by wind and precipitated by rain (Nuclear Regulation Authority, Japan 2019a). These meteorological factors, in addition to the types and growth stage of plants, multiply the effects on the adsorption and detachment of radioactive Cs during cultivation, likely causing the inconsistent results between Sites A and B. Therefore, further research is needed to quantitatively evaluate the effects of these factors on the activity of radioactive Cs in agricultural products.

Fig. 4.5

Correlation analysis between precipitation, maximum wind speed, and radioactive Cs levels in cultivated Komatsuna

4.6 Conclusions

To determine the effects of radioactive Cs in suspended matter and fallout on the secondary contamination of agricultural products, Komatsuna was cultivated using noncontaminated soil and water at sites affected by the FDNPP accident. Komatsuna cultivated at the decontaminated site had lower concentrations of radioactive Cs than those cultivated at the radioactive-contaminated site. Komatsuna cultivated closer to the ground showed higher levels of radioactive Cs than those cultivated at greater heights. This result suggests that the radioactive Cs of Komatsuna grown in noncontaminated soil is affected by suspended matter as well as the deposition of soil particles that rebounded from the ground due to rainfall. The levels of radioactive Cs decreased when the plants were washed with water. This result suggests that the secondary contamination of Komatsuna was derived from both absorption into the plant and adhesion to the plant surfaces. The fallout contained both dissolved and particulate forms of radioactive Cs, and it was assumed that a portion of these contributed to secondary contamination. To reduce the risk in the future, secondary contamination can be mitigated through the understanding of environmental and meteorological factors that increase the contamination risk, secondary contamination sources, and contamination differences by crop type.