Effects of radiocesium fixation potentials on Cs retention in volcanic soil profiles of Fukushima forests.

Radiocesium is well-known to be stabilized by clay minerals in soils, while volcanic soils could typically be poor in micaceous clays that fix Cs effectively. We investigated Cs fixation potentials [radiocesium interception potential (RIP)] and depth distribution of Cs stocks in volcanic soils to analyze effects of clay content and mineralogy on soil retention and migration of Cs after the Fukushima nuclear accident. Clay minerals of the volcanic soils were dominated by hydroxy-interlayered vermiculite (HIV) and short-range-order minerals, irrespective of bedrocks. The soil RIPs were positively correlated with mass of clay fraction among the HIV-dominated volcanic soils, but RIP per clay content in the HIV-dominated volcanic soils were lower than in the soils rich in illite or vermiculitic clays. The small RIPs in the organic horizons resulted in rapid Cs migration from the organic horizons and their accumulation in the upper 0-5 cm mineral soil. Although RIPs of the studied volcanic soils fall within the lower class among major soil types, vertical Cs migration in the mineral soil is small unless RIP is less than 300-400 mmol kg due to the low clay contents and shortage of illite or vermiculite.