5:15 PM - 6:45 PM
[MAG34-P06] Reproduction of radiocesium dynamics in a dam lake by a numerical model considering desorption from bottom sediment
Keywords:radiocesium, dam lake, desorption from sediment, numerical model
In the dam lakes affected by nuclear disasters, desorption of radiocesium (137Cs) from lake sediments slows down the decrease rate of the dissolved 137Cs concentration in the discharge water. The desorption rate of 137Cs from sediment is strongly affected by the oxygen and temperature in the lake bottom environment. Therefore, a numerical model that solves heat and water transfer in the lake water and the sediment would effectively reproduce and predict the 137Cs load by dam discharge. In this study, we constructed a model to simulate the 137Cs dynamics in the Yokokawa Dam (in Fukushima Pref.), based on our monitoring data in the inflow, lake water, and discharge water, and also the results obtained from the incubation experiments using the bottom sediments.
In the lake water model, water temperature, flow rate, and water quality data including 137Cs concentration observed at the dam inflow point were inputted, and water flows were calculated based on the diffusion equation according to the temperature distribution in the lake water. In addition, the heat balance of the lake surface was solved based on meteorological data from the JMA AMeDAS and the Fukushima District Meteorological Observatory, and the heat diffusion equation was solved using the inflow water temperature to simulate the temperature distribution. In the bottom sediment module, the model calculates the concentration of NH4+, obtained from the dissolved oxygen concentration and the organic matter decomposition rate in the sediment from the lake water flow computation, and also calculates the 137Cs desorption flux from the lake bottom by solving 137Cs diffusion in the pore water of sediment, taking into account the NH4+ dependency of the distribution coefficient of 137Cs. This model could generally reproduce the seasonal variation and long-term decay of dissolved 137Cs in dam discharge water. Still, it is necessary to improve the reproducibility of 137Cs concentration fluctuations after intensive runoff events and when the water storage rate remarkably declines by refining such as soil particle behaviors.
In the lake water model, water temperature, flow rate, and water quality data including 137Cs concentration observed at the dam inflow point were inputted, and water flows were calculated based on the diffusion equation according to the temperature distribution in the lake water. In addition, the heat balance of the lake surface was solved based on meteorological data from the JMA AMeDAS and the Fukushima District Meteorological Observatory, and the heat diffusion equation was solved using the inflow water temperature to simulate the temperature distribution. In the bottom sediment module, the model calculates the concentration of NH4+, obtained from the dissolved oxygen concentration and the organic matter decomposition rate in the sediment from the lake water flow computation, and also calculates the 137Cs desorption flux from the lake bottom by solving 137Cs diffusion in the pore water of sediment, taking into account the NH4+ dependency of the distribution coefficient of 137Cs. This model could generally reproduce the seasonal variation and long-term decay of dissolved 137Cs in dam discharge water. Still, it is necessary to improve the reproducibility of 137Cs concentration fluctuations after intensive runoff events and when the water storage rate remarkably declines by refining such as soil particle behaviors.