A modeling approach to estimate the 137Cs discharge in rivers from immediately after the Fukushima accident until 2017

Abstract

We developed a simple model to evaluate and predict the ¹³⁷Cs discharge from catchments using a tank model and the L-Q equation. Using this model, the ¹³⁷Cs discharge and discharge ratio from the Abukuma River and 13 other rivers in the Fukushima coastal region were estimated from immediately after the Fukushima accident up to 2017. The ¹³⁷Cs discharge (and discharge ratio to the deposition inventory in the catchment) of the Abukuma River and 13 other rivers in the Fukushima coastal region during the initial six months after the accident were estimated to be 18 TBq (3.1%) and 11 TBq (0.79%), respectively. These values of ¹³⁷Cs discharge ratio were 1-2 orders of magnitude higher than those observed after June 2011 in previous studies (Ueda et al., 2013; Tsuji et al., 2016; Iwagami et al., 2017a), indicating that the initial ¹³⁷Cs discharge from the catchments through the rivers was significant. The simulated initial ¹³⁷Cs discharge rates for the initial six months after the Fukushima accident were about 9-30 times larger in each catchment than those after that point until 2017, though initial ¹³⁷Cs concentration in river water was derived from an extrapolation of data based on a two exponentially decreasing fitting. However, it was found that the impact on the ocean from the initial ¹³⁷Cs discharge through the rivers can be limited because the ¹³⁷Cs discharge from the Abukuma River and the 13 other rivers in the Fukushima coastal region (29 TBq) was two orders of magnitude smaller than the direct release from Fukushima Dai-ichi Nuclear Power Plant (FDNPP) into the ocean (3.5 PBq) and from atmospheric deposition into the ocean (7.6 PBq) (Kobayashi et al., 2013). This model is expected to be useful to evaluate and predict ¹³⁷Cs discharge from catchments in future water management and in the estimation of ¹³⁷Cs discharge into reservoirs and the ocean.