5:15 PM - 7:15 PM
[AHW30-P02] Analysis of a dam lake water flow using the seasonal fluctuation waveform of dissolved cesium-137 concentration
Keywords:caesium-137, dam lake, seasonal fluctuation, averaged residence time
According to surveys in the area near the Fukushima Daiichi Nuclear Power Plant, the dissolved 137Cs concentration in river water generally shows a behavior linked to water temperature and when the seasonal fluctuations in the concentration of dissolved 137Cs and water temperature are approximated by a cosine function, the phases mostly correspond in rivers (Tsuji et al., 2023). However, a significant lag in the phase of the dissolved 137Cs concentration relative to water temperature was observed in dam lakes (Tsuji et al., under review). Assuming that the contribution of generation and disappearance within the lake to the dissolved 137Cs concentration in the lake water is limited, this phase difference can be considered to correspond to the average flow time until the inflow water reaches the discharge point of the dam lake. In lakes with several meters in depth, the inflowing water flows only through the surface layer during the stratification period, so the actual flow time should be shorter than the “average residence time” obtained from the amount of water in the dam lake and the discharge flow rate. Therefore, dissolved 137Cs can be used as a tracer to estimate the actual residence time of lake water and the thickness of the surface layer that contributes to the actual lake water flow. In this study, we propose a method for estimating the average annual residence time of lake water and the thickness of the surface layer that contributes to lake water flow, using the results of observations of the dissolved 137Cs concentration in inflowing and discharge water in a dam lake near the Fukushima Daiichi Nuclear Power Plant.
The analysis target was the Yokokawa Dam, located in Fukushima Prefecture. Since 2014, the dissolved 137Cs concentration and water temperature in the main inflow water (2 locations, total catchment area is 79% of the entire dam lake) and discharge water was observed monthly. As the dissolved 137Cs concentration in the inflow and discharge water showed significant seasonal variation, these concentrations were approximated by a cosine function formula to the date in the year, including the annual decay effect (environmental half-life). In addition, the flow rate and dissolved 137Cs concentration in the tributary catchment area (no observation data) were estimated from the catchment area and the average catchment deposition of 137Cs (as of July 2011), and the waveform of the main inflow and tributary inflow was superposed to reproduce the dissolved 137Cs concentration in the discharge water.
The identification of an approximation equation using the least squares method revealed that the phase of the dissolved 137Cs concentration in the discharge water lagged by 18 days to the water temperature and that there was a 25-d lag to the dissolved 137Cs concentration in the main inflow water. The average residence time obtained from the average annual water storage volume and discharge volume was 74 d, suggesting that the lake water in the deep layer hardly contributes to the entire flow of the lake water. By multiplying the annual average discharge by the phase difference (18 d) and dividing by the area of the lake surface, it was estimated that the thickness of the surface water layer in which the lake water flows was approximately 3 m on average per year.
However, there was a case where the waveform of the dissolved 137Cs concentration in the discharge water deviated from the cosine function due to the desorption of 137Cs from the lake bed. Therefore, to clarify the conditions under which this method is effective, it is necessary to accumulate the observation data of various dam lakes.
The analysis target was the Yokokawa Dam, located in Fukushima Prefecture. Since 2014, the dissolved 137Cs concentration and water temperature in the main inflow water (2 locations, total catchment area is 79% of the entire dam lake) and discharge water was observed monthly. As the dissolved 137Cs concentration in the inflow and discharge water showed significant seasonal variation, these concentrations were approximated by a cosine function formula to the date in the year, including the annual decay effect (environmental half-life). In addition, the flow rate and dissolved 137Cs concentration in the tributary catchment area (no observation data) were estimated from the catchment area and the average catchment deposition of 137Cs (as of July 2011), and the waveform of the main inflow and tributary inflow was superposed to reproduce the dissolved 137Cs concentration in the discharge water.
The identification of an approximation equation using the least squares method revealed that the phase of the dissolved 137Cs concentration in the discharge water lagged by 18 days to the water temperature and that there was a 25-d lag to the dissolved 137Cs concentration in the main inflow water. The average residence time obtained from the average annual water storage volume and discharge volume was 74 d, suggesting that the lake water in the deep layer hardly contributes to the entire flow of the lake water. By multiplying the annual average discharge by the phase difference (18 d) and dividing by the area of the lake surface, it was estimated that the thickness of the surface water layer in which the lake water flows was approximately 3 m on average per year.
However, there was a case where the waveform of the dissolved 137Cs concentration in the discharge water deviated from the cosine function due to the desorption of 137Cs from the lake bed. Therefore, to clarify the conditions under which this method is effective, it is necessary to accumulate the observation data of various dam lakes.