Vertical migration of radiocesium fallout in soil in fukushima

Abstract

The vertical migration of radiocesium fallout in the soil was monitored for 1 year at several locations in Fukushima after the nuclear power plant explosion. We determined the vertical gamma ray intensity profiles in boreholes in the soil using a scintillation survey meter with a lead collimator to restrict the incoming radiation, only allowing horizontal detection. The average migration distances of radiocesium at two time points were accurately determined based on the difference in the depth of the centroids of two gamma ray intensity profiles. The results showed that although the convective velocity of radiocesium was unexpectedly as high as 1/10th of the velocity of the infiltrating rainfall water 2-3 months after the nuclear plant accident, the velocity decreased to 1/100th-1/200th of that of the water after 6-12 months. This indicated that strong fixation of radiocesium to clay particles occurred during the initial 2-3 months. Radiocesium uptake by plant roots may have decreased remarkably along with the mobility of radiocesium in the soil.