Depth profiles and mobility of 129I originating from the Fukushima Dai-ichi nuclear power plant disaster under different land uses

Abstract

The long-term environmental mobility of anthropogenic 129I (physical half-life: 1.57 × 107 y) is of increasing concern from the view point of a low-level radiation risk assessment. A large amount of 129I and 137Cs (physical half-life: 30.1 y) were introduced into the environment from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) disaster. The apparent downward migration rates of the FDNPP-129I and -137Cs were estimated from their respective depth profiles on several land uses (upland field, empty lot, ground) in Fukushima.The total deposition amounts of the FDNPP-129I and -137Cs varied from 10.2 to 720 mBq/m2 and from 18 to 1393 kBq/m2, respectively, which swamped the regional 129I and 137Cs background levels in the Fukushima area. The mass depth (g/cm2) is employed in this study that is an indicator of the depth to which the nuclide penetrated into the soil and accounts for soil bulk density. The expectation mass depths and apparent diffusion coefficients (g2/cm4/d), derived from several depth profiles on different land uses, were 0.750–3.55 g/cm2 and 0.090–4.8 g2/cm4/d for the FDNPP-129I, and 0.965–4.96 g/cm2 and 0.12–5.1 g2/cm4/d for the FDNPP-137Cs, respectively. The expectation mass depth values of the FDNPP-129I were mostly greater than those of -137Cs at these sites. However, several anomalous cases have been found with higher expectation mass depth values of the FDNPP-137Cs than that of -129I in 4 upland fields and an empty lot. The expectation mass depth values of the FDNPP-129I decreased with an increase of the carbon content (%) in the topmost layer of the soil core. On the other hand, there is a weaker correlation between the FDNPP-137Cs and carbon content. This indicates that 129I mainly combined with organic materials and 137Cs is mainly combined with other soil components. The apparent diffusion coefficients D of FDNPP-129I and -137Cs were calculated using an unsteady state diffusion equation. The mobility of FDNPP-129I and -137Cs are rapidly decreased by combining with the soil components. However, some results have high D values of 129I and 137Cs in the soil. The difference in the soil components can produce such anomalous cases.