Keywords

9.1 Compost Derived from Grass Silage Contaminated with Radioactive Cesium

The Fukushima Daiichi Nuclear Power Plant (Tokyo Electric Power Co. Ltd.) accident in 11 March, 2011, released enormous amounts of radionuclides, causing radioactive contamination of many plants, animals, soil, rivers, oceans, and other environments. More than 10 years have passed since that accident, but grass silage contaminated with Cs-134 (134Cs) and Cs-137 (137Cs) was left untouched in pastures and farmlands immediately after the accident. While some of the low-containing grass silage (less than 400 Bq/kg) has been plowed into the soil, the highly contaminated grass silage (approximately 8000 Bq/kg or less) is still left untreated.

In our previous studies, we have attempted to utilize microbial methods of organic waste reduction to convert highly contaminated grass silage into compost (Manabe et al. 2016; Yoshii et al. 2019) through aerobic ultrahigh temperature (more than 110 °C) compost fermentation techniques (Oshima and Moriya 2008; Moriya et al. 2011). The contaminated grass of about 2700 Bq/kg was converted to 297 Bq/kg of compost. The compost made by aerobic ultrahigh temperature fermentation was applied to crops grown in the laboratory, but no radioactive cesium was transferred to the plants.

In the present study, we investigated the transfer of radioactive cesium to pasture grasses and its effects on the environment, including soil, space, and rivers, when compost is used on actual pastures.

9.2 Pasture Cultivation Using Compost Derived from Radioactively Contaminated Grass Silage

The experimental farmland in Kurihara-city, Miyagi-prefecture, Japan, located 150 km north of the Fukushima Daiichi Nuclear Power Plant, averaged 320 Bq/kg of radioactive cesium in June 2017 after radioactive decontamination (Fig. 9.1). This farmland originally was used for rice cultivation but was converted to pasture after the Fukushima Daiichi Nuclear Power Plant accident. The 1200 m2 farmland was divided into 6 plots of 200 m2 each, and 4 of these plots were applied with 2, 5, 10, and 30 kg of radioactively contaminated grass silage compost (297 Bq/kg, made by aerobic ultra-high temperature fermentation techniques) per m2 using manual spreader or excavator, and the remaining 2 plots were fertilized with chemical fertilizers or cattle manure as control (Fig. 9.2). After those plots were plowed, pasture Orchard grass (Dactylis glomerata) was sown and grown for about 2 months starting in June 2017. No fertilizer or pesticide was applied. In the first 3 weeks after sowing, the 30 kg/m2 compost application caused the growth disturbance in the pasture, but the pasture recovered after that. The other test plots showed good growth throughout the test period (Fig. 9.3). Soil pH and exchangeable potassium increased with increasing compost application. Sufficient potassium in the soil is thought to compete with crop absorption of radiocesium (134Cs + 137Cs). The pastures were cut at 37, 49, and 68 days after sowing at 5 cm above the ground surface. The concentrations of radioactive cesium in the grasses were measured with a germanium semiconductor detector (Mirion Technologies Canberra, Tokyo, Japan). As a result, radioactive cesium levels in the leaves and stems of the grasses were not detected in any of the test plots (total levels of 134Cs and 137Cs were less than 20 Bq/kg). Genetic analyses of the cut grasses by polymerase chain reaction targeting the 5.8S ribosomal ITS region were performed and confirmed to be Japanese millet (Echinochloa esculenta) or Orchard grass. These weeds such as Japanese millet were thought to be species that were originally established in the test plots.

Fig. 9.1
A photo of an aerial view of pastureland in Kurihara city, Japan. It highlights the central 3 plots of the pastureland. A map presents the distance between Kurihara city, Miyagi prefecture and Fukushima Daiichi, nuclear power plant as 150 kilometers.

The testing site (1200 m2 pastureland, average 320 Bq/kg) in Kurihara-city, Miyagi-prefecture, Japan (150 km away from Fukushima Daiichi nuclear power plant) https://earth.google.com/web/@38.86985053,140.98021213,135.46996306a,211.35840595d,35y,358.53244018h,0t,0r

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Fig. 9.2
3 photos. Photo 1 presents an aerial view of plots of pastureland. It highlights the boundaries of 6 adjacent plots, a to f. Photo 2, has a tractor in the field. Photo 3, has a Proclain spreading manure on the land.

Overview of test plots. Control plots were fertilized with chemical fertilizers (a) or cattle manure (b). The amounts of fertilizer applied were 2 kg (c), 5 kg (d), 10 kg (e), and 30 kg (f) per m2 of radioactively contaminated grass compost (297 Bq/kg). Compost application by manure spreader (b–d) or excavator (e, f). Location of measurement of air dose rate (①–④) and river water radioactivity (① upstream, ③ outlet of drainage) during the cultivation experiment

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Fig. 9.3
6 photos a through f present good growth of grass in the plots.

The pastures at 2 months after radioactively contaminated grass compost (made by aerobic ultra-high temperature fermentation) application. (a–f) Soil pH and exchangeable potassium increased with increasing compost usage. Grass growth was best in plot (e) where 10 kg/m2 of grass compost was applied

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Each soil radioactivity did not change significantly before and after compost application (Table 9.1). Air dose rates at 1 m from the ground in four corners around the test sites before and after application of the radioactively contaminated grass silage compost were measured using a scintillation instrument (PA1000, Horiba, Kyoto, Japan). The results showed that the air dose rates before and after compost application ranged from 0.06 to 0.12 μSv per hour, with no significant changes due to the application of radioactively contaminated silage compost (Table 9.2).

Table 9.1 Radioactive cesium concentrations in soil before and after application of contaminated grass compost made by aerobic ultrahigh temperature fermentation
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Table 9.2 The pH, radiocesium, and total nitrogen concentrations in river water upstream and the outlet of the test plots during the application of contaminated grass compost made by aerobic ultrahigh temperature fermentation
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Water samples were collected from upstream of the river adjacent to the test site and directly below the test site, just six times. No radioactive cesium was detected in any of the river waters. There was also no significant change in the total nitrogen content of river water before and after application of radiocesium contaminated grass silage compost (Fig. 9.4).

Fig. 9.4
A line graph plots fluctuating lines for 1, 2, 3, and 4. The lines start between 0.060 and 0.120 of the y-axis and end between 0.060 and 0.100 of the y-axis. The line for 3 has the lowest values.

Air dose rate around the test plots in 2017 (see Fig. 9.2) during pasture cultivation test using grass compost contaminated with radioactive cesium

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9.3 Conclusion

The pastures were grown with 2, 5, 10, and 30 kg of compost produced from contaminated grass silage applied per square meter, and the grass and weeds were undetectable for radioactivity. As the amount of contaminated grass compost applied increased, the pH in the soil increased and the amount of potassium also increased. Therefore, it may be expected to have a soil improvement effect to inhibit the absorption of radioactive cesium. No effect was observed on air dose rate or river radioactivity in the surrounding area where contaminated grass compost was applied. The results of this study confirm that crop cultivation using contaminated grass compost is safe in open field cultivation.