Vertical Concentration Profiles of Radioactive Caesium and Convective Velocity in Soil in a Paddy Field in Fukushima

Abstract

Concentration profiles of ¹³⁴Cs and ¹³⁷Cs were obtained in an undisturbed paddy field before plowing in Fukushima prefecture on May 24th, 2011, about two month after explosions of Fukushima Daiichi nuclear power plant. Top soil of 0 - 15cm was sampled, by separating it into 6 layers of 1 - 5cm thick. Resulting concentration profile indicates that 88% of ¹³⁴Cs+¹³⁷Cs existed within 0 - 3cm and 96% within 0 - 5cm layer. However, although most of radioactive Cs stays within a few cm surface layer, a part of it already reached 10 - 15cm layer. Mean travel distance of ¹³⁴Cs and ¹³⁷Cs were calculated from the profile to be 1.74cm;on the other hand, mean travel distance of water molecules during the 70days was also estimated as [precipitation - evaporation]/[volumetric water content] to be about 20cm. Therefore, convective velocity of Cs was 1/10 of water due to sorption of Cs on soil. However, this ratio of Cs velocity is 2 - 3 order greater than that calculated from soil-soil solution distribution coefficients(Kd) for soils of Japanese paddy field shown in a literature, indicating that the movement of Cs in the field occurs in condition far from sorption equilibrium.