Weathering half-life of radioactive cesium for winter rye (Secale cereale L.) and Italian ryegrass (Lolium multiflorum Lam.) directly contaminated by the 2011 Fukushima Daiichi Nuclear Power Station accident

Abstract

Changes in radioactive cesium (radiocesium) concentrations in winter rye (Secale cereale L.) and Italian ryegrass (Lolium multiflorum Lam.) grown in fields contaminated by the accident at the Fukushima Daiichi Nuclear Power Station of the Tokyo Electric Power Company were examined to clarify the half-lives of weathering as an environmental process that removed radiocesium during the early stages after its deposition. The radiocesium concentrations in the rye plants that resulted from direct deposition of fallout in 2011 decreased greatly from 140.5 kBq kg–1 dry weight (DW) on March 31 to 13.2 kBq kg–1 DW on April 27, during the 27 d between the vegetative growth and heading stages. The amounts of rye radiocesium per unit area decreased simultaneously, from 69.8 kBq m–2 to 12.6 kBq m–2. The radiocesium concentrations in rye were half and a quarter of those in Italian ryegrass during the vegetative and heading stages, respectively, although the radiocesium concentrations in Italian ryegrass also decreased with plant growth. The values of the weathering half-life of 8.0 and 11.0 d for the radiocesium concentration per unit DW and amount per unit area in rye were approximately half those of Italian ryegrass, at 14.1 and 23.1 d, respectively. Therefore, less radiocesium-contaminated material seemed to be retained on the plant surface of rye than on Italian ryegrass. The radiocesium concentrations of rye harvested in the same experimental fields in the year following the accident were below the value defined in the February 2012 provisional regulation for cattle roughage (100 Bq kg–1 at a water content of 0.8 kg kg–1).