Organic Colloid and the Migration of Radioactive Cesium in Polluted Soil in Fukushima, Japan.

The attachment/detachment of organic colloid originated from litter and surface soil plays an important role in the fate and transport of radioactive cesium in soil. The issue has drawn great attention after 2011 Japanese nuclear power plant accident (NPP) in Fukushima where forests cover more than 80% of ground surface. The surface soil was partly covered by thick litter layer and has relatively high organic carbon content ranging from 3 to 50% by mass. Up to date, the mechanism of cesium migration associated with organic colloid is still not well understood. In this study, we aim to (i) monitor the cesium downward migration and organic matter in polluted soil in Fukushima based on field survey, (ii) examine the distribution of cesium in polluted litter and fractionated organic colloid suspension, and (iii) investigate the facilitation of organic colloid on downward migration of cesium in 10-cm soil columns. Field survey in 2013-2015 at an abandoned forest located 40 kilometers northwest of the Fukushima Daiichi NPP, shows that cesium is highly correlated with organic carbon content in top 30 cm of surface soil. Although cesium mainly deposited in litter and top few cm of surface soil, it can be still detected (30 – 366 Bq/kg) at 30-cm deep soil in many locations. Cesium concentration in fractionated organic colloid is 2.8 times higher than the original polluted litter. Organic colloid tends to be originated from a light fraction of organic matter in the litter. Average particle density of organic colloids is 1.1 g/cm³. The column experiments show that the migration of cesium in soil matrix greatly varies and strongly depends on the migration of mobile organic colloids. After loading 20 - 33 mm of cesium-organic-colloid suspension, 10 – 68% of cesium can pass through 10-cm soil columns, however the release of cesium from these soil columns after a similar amount of subsequent pure water and dilute electrolyte solution is insignificant. The flocculation/detachment of organic colloid in soil matrix is likely a main factor to control the downward migration of cesium.