Vertical migration of radiocesium and clay mineral composition in five forest soils contaminated by the Fukushima nuclear accident

In forest soils contaminated by radiocesium (Cs-134 and Cs-137), deposition from the Fukushima nuclear accident, clay minerals might play important roles in long-term cesium (Cs) dynamics through sorption. To determine whether radiocesium can be retained within the organic layer and the upper mineral soil layers in the Fukushima region, we investigated the vertical distribution of Cs-134 and Cs-137 and the clay mineral composition in five soil profiles of varying radiocesium deposition levels and vegetation types. X-ray diffraction analyses and oxalate extraction suggested that hydroxy-interlayered vermiculites and short-range-ordered aluminum (Al) and iron (Fe) compounds (i.e, allophane and ferrihydrite) were major clay mineral species of the upper soil layers. The vertical soil distribution of Cs-134 and Cs-137 suggested that most of them were retained in the organic layer and upper mineral soil layer under different levels of deposition. Within 1.5 years after the accident, both Cs-134 and Cs-137 were leached from the organic layer, and most of these (59-73%) were accumulated in the upper soil layer (0-5cm). The proportion of Cs-137 (or Cs-134) leaching from the organic layer was greater at sites receiving greater amounts of precipitation. The substantial accumulation of Cs-137 in the upper soil layer, irrespective of the Cs-137 deposition level or clay mineral composition, suggests that sorption capacities of clays and organic matter are sufficiently high to retain Cs-137 in the surface soil during at least the initial stage of contamination.