Effect of 137Cs desorption from sediment on the formation of dissolved 137Cs concentrations in dam discharge water

In the region affected by the Fukushima nuclear accident in 2011, some freshwater fish shipments continue to be suspended owing to radioactive contamination (mainly ¹³⁷Cs) of the aquatic environment. In predicting the future ¹³⁷Cs contamination of aquatic organisms, investigations must focus on the dynamics of ¹³⁷Cs in dissolved form, which is highly bioavailable and abundant in the environment. In particular, dam lakes that deposit large amounts of sediment contaminated with ¹³⁷Cs and have a long residence time of water can substantially influence the dynamics of ¹³⁷Cs in rivers, as suggested by prior research. This study focuses on the effect of desorption of ¹³⁷Cs from lake sediment on the formation of dissolved ¹³⁷Cs concentrations in dam discharge water, using the results from monitoring surveys at two dam lakes located near the Fukushima Daiichi Nuclear Power Plant.

We collected inflow and discharge water from the Matsugabo and Yokokawa dams in Fukushima Prefecture every month from 2014 and measured the concentration of dissolved and particulate ¹³⁷Cs in the water using the cartridge filter method. On the basis of these results, combined with flow data from the dam lakes, we estimated the annual budgets of ¹³⁷Cs (inflow/outflow) in the dam lakes. For the particulate form, annual ¹³⁷Cs inflow into the lakes decreased by more than 80% in most years, indicating that most of the inflow particles sedimented. For the dissolved form, the annual discharge of ¹³⁷Cs was higher than the annual inflow of ¹³⁷Cs, concurring with results from a neighboring dam lake. This increment suggests ¹³⁷Cs desorption from the sediment.

According to the monthly monitoring data, the dissolved ¹³⁷Cs concentration in the dam discharge water at some periods showed a higher value than the peak value from the previous year. This phenomenon was observed when the reservoir storage rate of the dam lake fell below approximately 30%. To determine the main source of dissolved ¹³⁷Cs in the dam lake, we investigated the horizontal distribution of the dissolved ¹³⁷Cs concentrations at several points in Yokokawa dam lake and the vertical distribution of the dissolved ¹³⁷Cs concentration at the center of the lake in August 2023, when the water level was very low. The concentration of dissolved ¹³⁷Cs in the lake water was found to increase in the inlet part of the lake, while the concentration remained almost the same in the downstream direction from the site. The concentration of dissolved ¹³⁷Cs at the center of the lake was almost unchanged vertically. This trend was different from the increase in the concentration of dissolved ¹³⁷Cs in bottom water, previously observed at the same location (Tsuji et al., 2022). These results indicate that ¹³⁷Cs desorption from sediment in the inlet area mainly led to the increase in the dissolved ¹³⁷Cs concentrations in the lake water, in part owing to the low volume of flowing water.