[A-GE03_30PO1] Subsurface Mass Transport and Environmental Assessment

[AGE03-P14] Cs migration to rice crop from soil after stripping the contaminated top soil at Iitate Village in Fukushima Prefecture.

*Junko NISHIWAKI¹, Naomi ASAGI¹, Masakazu KOMATSUZAKI¹, Masaru MIZOGUCHI², Kosuke NOBORIO³ (1.College of Agriculture, Ibaraki University, 2.Graduate school of Agricultural and Life Sciences, The University of Tokyo, 3.School of Agriculture, Meiji University)

Keywords:stripping top soil off, rice crop, caesium

Iitate Village is at about 40 km northwest from a Fukushima Daiichi nuclear power plant. An agricultural fertile layer in agricultural fields was contaminated by radionuclides, e.g., 134-Cs, 137-Cs, and 90-Sr, just after the accident of the Fukushima Daiichi Nuclear Power Plant in 2011. The decontamination work is an important subject for villagers to return to a village and live there again. Three decontamination methods are proposed by a Ministry of Agriculture, Forestry and Fisheries. They are 1) Stripping the top soil off, 2) Removal of fine particles after soil and water mixing, and 3) Tillage reversal. By the report of the Ministry of Agriculture, Forestry, and Fishery, 90% of radioactive contaminant has been removed by the method of stripping the contaminated top soil off. In this time, we examine the Cs migration to rice crop from soil after stripping the contaminated top soil off. We used ~4*20 m paddy field at Iitate Village in Fukushima Prefecture. At first we decontaminated the site using the method of stripping 5 cm top soil off. After that potassium chloride (KCl) was put in all area as basal fertilizer on June 8, 2013. We comparted the area and made three kinds of treatments such as (1) mixed with rice straw that was harvested last year here, (2) only decontaminated, and (3) mixed with farmyard manure, and transplanted rice crop (rice cultivar is hitomebore) on June 9, 2013. We had sampled top soils at three points from each plot twice a month and the 134-Cs, 137-Cs, and 40-K concentration in soils were analyzed using a Ge semiconductor detector. The concentration of 134-Cs, 137-Cs, and 40-K in rice crop was analyzed by a NaI scintillation counter after harvest.As a result, 134-Cs is about half of 137-Cs. Since the half-life of 34-Cs is two years, it has become approximately a half. Changes of Cs and K concentrations in soil were not observed during a rice cultivation period. The concentrations of radionuclides in mixed rice crop were 572.93±8.05 Bq/kg-dry / Cs-134, 1089.35±11.41 Bq/kg-dry/ Cs-137, and 127.29±27.59 Bq/kg-dry/ K-40. Although these values were comparatively high, soil did not show the high dose. The reason of that would be the volume of mixed rice straw was small and migration of caesium from the rice straw to the soil was hardly happened. The soil dose mixed with manure had been high through the whole cropping period. It might be the original manure dose was high, but the analysis of the manure has not completed yet. The concentration of radionuclides in rice crop below a detection limit of the NaI scintillation counter and it is below the regulation value defined in our country.