Vertical migration of radio-caesium derived from the Fukushima Dai-ichi Nuclear Power Plant accident in undisturbed soils of grassland and forest

• Vertical distribution of radiocaesium in undisturbed soils affected by the Fukushima NPP accident was studied.
• The ratio 137Cs : 134Cs ≈ 1 shows that Fukushima fallout is the dominant source.
• Chemical and physical characterization of soils was performed.
• Organic matter and silicate mineral contents in soil appeared to have influential controls on 137Cs retention.
• Compartmental and analytical models were fitted to the measured Cs profiles.

The vertical distribution of radio-caesium (137Cs and 134Cs) in undisturbed soil profiles of grassland and forest soils, derived from the Fukushima-Daiichi Nuclear Power Plant (FDNPP) accident that occurred on 11 March 2011, was studied. Surface soil and depth profile soil samples were collected from six locations within the 20 km zone of FDNPP, during November 2012 and June 2013. The activity ratio for 137Cs and 134Cs was found to be almost constant about 1 within the soil profiles as well as in the surface soil, indicative of FDNPP accident origin. From soil depth profile distribution of Cs activity, it is observed that Cs is strongly bound to soil materials, which slows Cs migration. > 90% of the activity was found to be retained within the upper 5 cm layer. Retardation of Cs movement has been quantified by measuring sorption of Cs in soil in terms of distribution coefficient (Kd) using the laboratory batch method. Faster migration has been observed in case of forest land soil compared to grassland soil. The empirical migration velocity of Cs radio isotope was estimated from the depth profile Cs concentration and found to vary from 1.1 to 1.7 and 0.85 to 3.5 cm y− 1 in grassland and forest soil, respectively. The residential half life for Cs isotopes was found to be 1.03–7.75 y and 1.18–4.67 y for grassland and forest land respectively using a compartmental model. In addition to the empirical analysis of the profiles, analytical models were fitted to the data which may help elucidate the physical nature of the transport of trace elements.