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

Abstract

Concentration profiles of１３４Csand１３７Cs 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 sepa-rating it into6layersof1―5cm thick. Resulting concen-tration profile indicates that 88%of１３４Cs＋１３７Cs existed within 0―3cmand 96%within 0―5cm layer. However, although most of radioactiveCs stays within a few cm sur-face layer, a part of it already reached 10―15cm layer. Mean travel distance of１３４Csand１３７Cs were calculated from the profile to be 1.74cm;ont he other hand, mean travel dist ance of water molecules during the 70 days was also estimated as［precipitation―evaporation］/［ volumet-ric watercontent］tobe about 20cm.The refore, convec-tive velocity ofCswas 1/10 of water due to sorption of Cs onsoil. However, this ratio ofCsvelocity is 2― 3order greater than that calculated from so il-soil solution distri-bution coefficients（Kd）fors oils of Japanese paddy field shown in a literature, indicating that the movement of Cs in the field occurs in condition far from sorption equilib-rium. RADIOISOTO