Depth distribution of 137Cs, 134Cs, and 131I in soil profile after Fukushima Dai-ichi Nuclear Power Plant Accident.

Soil, vegetation and other ecological compartments are expected to be highly contaminated by the deposited radionuclides after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident triggered by a magnitude 9.0 earthquake and the resulting tsunami on 11 March 2011. However, there is no field measurement data on the depth distributions of radiocaesium and (131)I concentrations in soil profile. In this study, the depth distribution of the deposited radionuclides in the cultivated soil profile was investigated in one of the most contaminated area after FDNPP accident. The result of this study demonstrated that greater than 86% of total radiocaesium and 79% of total (131)I were absorbed in the upper 2.0 cm in the soil profile. The relaxation mass depth (h(0)) derived from the depth distribution of radiocaesium and (131)I in the soil profile at the study site were 9.1 kg m(-2) and 10.4 kg m(-2), respectively. The h(0) of (137)Cs in the studied soil profile was greater than those for the cultivated soils nearby the Chernobyl NPP. The positive relationship was found between clay content of topsoil and the h(0) of (137)Cs. However, further analysis is required to clarify the effect of clay content on the initial penetration depth of deposited (137)Cs in soil profile.