MODELING OF TEMPORAL AND VERTICAL CHANGES IN 137Cs CONCENTRATION IN LAKE ONUMA

According to the observed data of the activity concentration of $^{137}$Cs in Lake Onuma on Mount. Akagi, located in Gunma Prefecture, the decrease of the activity concentration slowed down since 2 years after the accident of Fukushima Daiichi Nuclear Power Plant. In the present study, we applied the time fractional diffusion equation to reproduce long-term changes in the time series of the activity concentration. In addition, we also reproduced the vertical distribution of the activity concentration with the parameter values, estimated by the time series data. We found that the model fit the observed data well for both time series and vertical distribution.