Evaluation of radioiodine (129I) dissolution from sediment of a brackish lake beside a spent nuclear fuel reprocessing plant in Japan

Flux and pathway of iodine dissolution from brackish lake sediment in the northeast of Japan

Sediment is one of the most important entities controlling the environmental dynamics of iodine. We previously evaluated the dissolution flux of iodine from aquatic sediment to its overlying water in a brackish lake through an incubation experiment with a sediment core sample, reporting the regulation factors of this flux, such as the temperature and oxygenic conditions. In this study, factors controlling the seasonal variation in the dissolution flux of iodine were investigated via an incubation experiment using the sediment core samples collected in autumn and summer. The dissolution flux in this study was significantly smaller than that reported in our previous study. The iodine concentration detected in the overlying water of the sediment during incubation in this study was positively correlated with the concentrations of some inorganic ions, such as Na⁺, Cl^-, and SiO₄^2-, while these correlations were not confirmed in our previous study. As the dissolution of sedimentary iodine includes two pathways, which are the diffusion of sedimentary pore water and degradation of organic matter on surface sediment, correlations potentially indicate that the dissolution flux estimated in this study has a larger contribution from sedimentary pore water than that in our previous study. In addition, the higher flux estimated in our previous study was considered to be caused by the larger contribution from iodine derived from the degradation of phytoplanktonic organic matter on surface sediment. Assuming that the dissolution fluxes estimated in the previous and present studies are representative of the fluxes during and excluding the high productivity season in Lake Obuchi, respectively, we estimated the annual flux at 2.7 g y^-1 m^-2, which is comparable with our previous estimation.