Applicability of K_d for modelling dissolved ~(137)Cs concentrations in Fukushima river water: Case study of the upstream Ota River

摘要

A study is presented on the applicability of the distribution coefficient (K-d) absorption/desorption model to simulate dissolved Cs-137 concentrations in Fukushima river water. The upstream Ota River basin was simulated using GEneral-purpose Terrestrial Fluid-flow Simulator (GETFLOWS) for the period 1 January 2014 to 31 December 2015. Good agreement was obtained between the simulations and observations on water and suspended sediment fluxes, and on particulate bound Cs-137 concentrations under both base and high flow conditions. By contrast the measured concentrations of dissolved Cs-137 in the river water were much harder to reproduce with the simulations. By tuning the K-d values for large particles, it was possible to reproduce the mean dissolved Cs-137 concentrations during base flow periods (observation: 0.32 Bq/L, simulation: 0.36 Bq/L). However neither the seasonal variability in the base flow dissolved Cs-137 concentrations (0.14-0.53 Bq/L), nor the peaks in concentration that occurred during storms (0.18-0.88 Bq/L, mean: 0.55 Bq/L), could be reproduced with realistic simulation parameters. These discrepancies may be explained by microbial action and leaching from organic matter in forest litter providing an additional input of dissolved Cs-137 to rivers, particularly over summer, and limitations of the K-d absorption/desorption model. It is recommended that future studies investigate these issues in order to improve simulations of dissolved Cs-137 concentrations in Fukushima rivers.