Influence of drying period on migration behavior of radionuclides in aerated soil layer

In an aerated soil layer under natural condition, water that infiltrates into it flows discontinuously because of repeated cycles of rainfall and drying. Therefore, column tests, in which 2,000 ml of deionized water was fed into sandy soil layer dried for 3∼90 days after it is contaminated by 60Co, 85Sr and 137Cs aqueous solution, were carried out to examine influence of the drying period on migration behavior of those nuclides. All the radionuclides showed both a steeply decreasing part corresponding to their cationic form and a gradually decreasing part corresponding to their particulate form in the concentration distribution curve along the soil column depth. In the vicinity of the top surface of soil layer, no influence of drying on 60Co and 137Cs concentrations was observed, but 85Sr moved a little into a deeper part from the top surface with longer drying period. Such moving tendency of 85Sr was found to be connected with the Ca2+ concentration in the interstitial water. In the deeper soil layer, no influence of drying on 85Sr concentration was observed, but the 60Co and 137Cs concentrations were increased with the drying period. This is considered to be caused from that each product of the movable 60Co(OH)2 and the 137Cs adsorbed on the fine particle increased with the rise of pH and the fine particle concentration, respectively, during the drying period. Thus, this study has suggested possible causes affecting on the radionuclide migration under discontinuous flow condition. (author)